A WORD ABOUT VACUUM PUMPS
For this discussion we will use the example of an old style Welch Model 1405, although this information may be applied to any rotary vane, oil sealed mechanical vacuum pump, whether belt drive or direct drive. The old style 1405 was rated at 60 L/min. (2.1 CFM) with a guaranteed ultimate vacuum of 0.1 micron (0.1 millitorr or mtorr). Many mom & pop backyard-type wholesale neon shops as well as small retail shops used these pumps because they were quality pumps, relatively inexpensive, small, compact, quiet and readily available from most sign supply distributors and vacuum equipment suppliers. However, this does not mean they were the best choice.
First, it is important to understand that an oil sealed mechanical vacuum pump, whether belt drive or direct drive, is not efficient at pumping water vapor. It is also important to understand that at a bombarding temperature of 200°C roughly 70-80% of the total gas load from a unit is water vapor. However, the ratings of a vacuum pump, that is the capacity in liters per minute (L/min.) or cubic feet per minute (CFM) and the ultimate pressure (vacuum) capability, are relative to pumping dry air. These numbers are often misunderstood as to how they apply to real-world conditions: In this case, processing luminous tubing.
PUMPING SPEED RATING
Mechanical vacuum pumps of this type are rated for their free air displacement capability. This is generally stated as L/min. or CFM*, although some pumps manufactured offshore are rated for cubic feet per hour when sold to the U.S. market.
Free air displacement is defined as: The volume of air passed through the pump per unit of time when the pressure at the intake and exhaust of the pump is equal to atmospheric pressure (ATM). In other words, how much air the pump can displace when the intake and exhaust are wide open to ATM. In our example the free air displacement of the old 1405 is 60 L/min. as previously stated.
Free air displacement is also the pumping speed of the pump under the above conditions; i.e., when the intake and exhaust are at ATM. However, the pumping speed changes as the pressure at the intake of the pump decreases. Therefore, pumping speed is defined as: The volume of air per unit of time that the vacuum pump is able to remove from the system.
When the pump is connected to a vacuum system and begins to pull a vacuum the pumping speed changes. As the pressure in the system is reduced and the vacuum gets better the ability of the pump to displace air (or other gas load) progressively gets worse. For example, the same 1405 pump that has a free air displacement of 60 L/min. at ATM only has a pumping speed of about 35 L/min. at a vacuum level of 10 microns. A vacuum of 10 microns is not an adequate vacuum for neon work and the pumping speed of the pump has already fallen off to almost half of its free air displacement. At a vacuum of 5 microns, which is considered by most experts in the field to be the bare minimum for neon work, the pumping speed is roughly 20 L/min. As the pressure is reduced further the pumping speed continues to get worse until the pump reaches its blank-off pressure, or ultimate vacuum capability. (In real-world situations the actual blank-off pressure is much higher than the advertised “guaranteed ultimate vacuum”). At the blank-off pressure the pumping speed is only a few liters per minute.
Even though it is roughly ⅓ of the free air displacement, the pumping speed of 20 L/min. at the 5 micron vacuum level may sound adequate. However, this equates to just ⅓ liter per second. At a bombarding temperature of 200-300°C an average size 15mm unit can release several liters per second of gas load. Therefore, the ⅓ liter per second pumping speed is not adequate to remove the gas load fast enough, before the unit cools to the critical re-absorption temperature of 175°C. Further, the pumping speed for water vapor is much worse than it is for air. Although it would be very difficult to calculate the percentage decrease in pumping speed for water vapor for a given pump due to the variables involved, such as tubing diameter, length, coated or uncoated, how the glass has been stored, etc., it is certain that the performance of the pump falls off considerably when pumping water vapor as opposed to dry air. The majority of gas load in this situation is water vapor, so the ability of the pump to remove the gas load is greatly reduced.
Larger pumps can be used in an attempt to compensate for the lack of performance of smaller pumps in the low micron range. To some extent this may be helpful as a larger capacity pump will rough pump the unit quicker, down to a certain point. But regardless of how big a mechanical vacuum pump is it still suffers from the same inability to pump water vapor in the low micron range, though it will do it marginally better than a smaller pump.
* To convert CFM to liters per minute multiply the CFM by 28.32. To convert liters per minute to CFM multiply the liters per minute by 0.0353.
ULTIMATE PRESSURE / VACUUM RATING
This heading may sound confusing to some. The correct terminology is “ultimate pressure”. However, most think of anything less than atmospheric pressure as being a vacuum, which is also correct. The terms seem to contradict each other. But for our purposes, the terms ultimate pressure and ultimate vacuum are the same thing.
The reasoning behind the term ultimate pressure: If there were a perfect vacuum, which does not exist, it would be absent of all gas molecules. This is considered absolute zero, or zero pressure. If even one molecule of gas existed it would have the ability to exert a pressure inside a sealed chamber – no matter how small of a pressure. Therefore, the chamber would have in it a pressure above zero.
The reasoning behind the term ultimate vacuum: A device that has the ability to remove some of the air or other gas from a chamber is considered a vacuum pump, mechanical or otherwise. Once the pump begins to remove the gas from the chamber and the pressure inside that chamber falls below ATM a vacuum is created. The pump will continue to remove gas molecules from the chamber until the pumps ability to remove gas and the rate of outgassing and/or continued gas load caused by leaks reaches equilibrium. The vacuum inside the chamber does not improve regardless of how long the pump continues to pump. This level of vacuum is the best, or the ultimate, that the pump can achieve. Therefore the term ultimate vacuum applies.
Ultimate pressure is defined by one vacuum pump manufacturer as: “The lowest attainable pressure in a vacuum system; The lowest attainable pressure by a vacuum pump; Ultimate pressure is limited by the pumping speed of the vacuum pump and the vapor pressure of the vacuum pump sealing fluid, among other factors.”
The advertised ultimate pressure (or vacuum) for 2 stage rotary vane, oil sealed high vacuum mechanical pumps is typically between 0.1 and 0.5 microns. However, this number is extremely misleading in real-world situations. To understand why one must know how the pump manufacturers test their pumps.
First, pumps are tested in a laboratory setting and not in a manufacturing plant somewhere. The lab will have closely controlled ambient air conditions. A temperature of 68-70°F with relative humidity as close to zero as possible is not uncommon. Prior to testing the test pump is meticulously cleaned, much more so than an “off the assembly line” pump is. The highest quality high vacuum pump oil available is used. The pump is run with this oil for extended periods of time and flushed as many as 5 or 6 times to remove any remaining volatiles and contaminants from the pump. For the final “vacuum reading” test the pump is fitted with an LN2 trap (liquid nitrogen trap) directly on the intake of the pump, which prevents any oil vapor from backstreaming to the vacuum gauge. The pump will run for at least 48 hours before the vacuum reading is accepted. The vacuum gauge used for the test will be the most favorable for the desired indicated vacuum. For example, when a McLeod gauge takes a pressure measurement it compresses the gas in the instrument in order to take the reading. Therefore, this type of gauge cannot measure condensable vapors, as do electronic gauges. If condensable vapors such as water vapor or oil vapor are present in the test chamber (in this case the vacuum pump intake) the indicated level of vacuum is different than the actual vacuum when using a McLeod gauge. Therefore, this type of instrument may be used if it indicates a better vacuum than the actual vacuum under the test conditions.
IN CONCLUSION:
Immediately following bombarding the unit has to be evacuated to the lowest possible pressure before the glass cools to 175°C as this is the temperature that the released gases and vapors will begin to condense and redeposit on the walls of the tubing. Referring to the preceding information on mechanical pump performance we know that a mechanical pump cannot accomplish this by itself. Therefore, in order to achieve the required vacuum in the time frame necessary a secondary vacuum pump must be used in conjunction with the mechanical vacuum pump. It is our strong belief from our many years in the neon equipment business that the overall best choice of secondary vacuum pump for our purposes is the single stage glass oil Diffusion Pump. They have no moving parts, are isolated from electrical ground, are easy to maintain in-house and provide fast pumping speeds in the low micron range. A reasonably sized mechanical vacuum pump will provide sufficient fore-pressure for the diffusion pump to function properly. The type and size of mechanical pump necessary is a two stage oil sealed rotary vane pump with a free air displacement of at least 100 L/min., but preferably larger.
RECOMMENDED BOMBARDING PROCEDURE
(Internal bombarding method)
© COPYRIGHT 2006 Mark A. Snyder/SVP Neon Equipment
FOREWORD
Proper processing is essential to consistently obtain high quality, long life luminous tubing. To accomplish this requires specific equipment and an established procedure. The ‘bare bones’ pumping system of days gone by, which consisted of a greased stopcock manifold, 30-year-old 25 L/min. vacuum pump, 7.5 kva 15,000 volt bombarder capable of only 650 mA. and using newspaper as a “temperature gauge”, never produced consistent results – and it never will. Add to this the seat-of-the-pants approach without any instruments other than an oil or mercury manometer (instruments that would otherwise give the technician information and control over the operating parameters) and the procedure automatically became guesswork at best.
The procedure described herein will provide a sound, controlled approach to processing and allow the processing technician to obtain consistent, repeatable results and the ability to produce top quality tubes that will give years of trouble free operation.
EQUIPMENT
For this website version of our Recommended Bombarding Procedure it will be assumed that one of our equipment packages is being used with an appropriately sized bombarder and matching choke. Information on each individual piece of equipment, such as manifolds, vacuum pumps, the various instruments, etc., may be found elsewhere on this website.
GENERAL BOMBARDING PROCEDURE
It must be understood that the bombarding procedure is not an exacting science. There are many variables to consider such as tubing diameter and length, coated or un-coated tubing (and if coated, which coating), electrode manufacturer and type of emission coating, size of electrodes, number of bends in a unit, moisture content, etc. However, there are certain parameters that must be followed in order to obtain the desired results. Therefore, the following procedure should be considered a guideline, but one that establishes these parameters.
For the procedure basis I will use the example of a 15mm diameter tube, with 8 feet of glass (not including the electrodes) and coated 6500 white with 15mm, 80mA electrodes. In this case one electrode will be a tubulated electrode. It will be found that when following this procedure, using a tubulated electrode on one end will produce the same consistent results as using two non-tubulated electrodes with a side tubulation attached to the unit. Further, if the glass worker prefers to side tubulate the unit, as some do and as is necessary sometimes, it will also be found that it is not necessary to do so in the middle of the unit to achieve even heating of the electrodes and glass, provided a properly constructed and manufactured electrode is used. Electrodes that heat inconsistently and/or unevenly are more a function of the electrode rather than the procedure used.
STEP 1
To begin, the vacuum gauge should be turned off and the stopcock leading to it should be closed. This will isolate the gauge tube (sensor) from an inrush of air that would otherwise temporarily contaminate the gauge tube. Although this will not permanently damage it, it will require a considerable amount of time to degas before an accurate reading can be expected. Vent the manifold to atmosphere by slowly opening the air vent/blow hose stopcock. Attach the unit(s) to be processed to the manifold.
RECOMMENDATION: Do not use rubber, vinyl or Teflon tubing, etc. for this purpose. These materials exhibit tremendous outgassing properties and some produce a phenomenon known as molecular permeation. Because of this phenomenon, the material will never stop outgassing no mater how long it is pumped and satisfactory vacuum levels may never be realized.
STEP 2
Close the air vent stopcock and slowly open the main stopcock completely. Allow the tube to evacuate for 20~30 seconds.
STEP 3
During STEP 2, connect the bombarder leads to the electrodes, and the temperature gauge lead to the neon unit.
NOTE: The temperature gauge lead should be attached at least 6" to 8" from either electrode but does not have to be in the middle of the unit. Our tests have shown that the unit will heat quite evenly along its entire length with the exception of tight bends and the glass jacket surrounding the metal electrode shells.
CAUTION!
Check to make sure that the vacuum gauge is turned OFF and the STOPCOCK for it IS CLOSED.
Set the bombarder choke control to the minimum setting, preferably ~150 mA.
STEP 4
Close the U-gauge stopcock. Using the air vent stopcock, admit 3~4mm of air into the manifold as indicated by the pressure scale on the U-gauge. This can easily be done by placing your index finger over the open end of the side arm of the air vent stopcock. Opening and closing the stopcock will allow a small amount of air into the manifold. It may be necessary to repeat the procedure a couple of times to get the desired pressure.
Bombard the tube using 150-200 mA of current until a glass temperature of 100ºC is reached as indicated by the temperature gauge. Do not allow the pressure to rise above 4~5mm. (If manifold pressure reaches 5mm before the glass temperature reaches 100ºC, turn off the bombarder and reduce the pressure to 3mm by slowly opening and closing the main stopcock, then begin bombarding again until 100ºC is reached before proceeding.)
Turn off the bombarder and open the main stopcock completely. Let the tube pump down for 30~60 seconds, depending on the overall size of the unit (diameter x length), in this case (15mm single coated tube 8 ft. long) about 30 seconds is sufficient.
WARNING! WHENEVER THE MAIN STOPCOCK IS GOING TO BE OPENED (for whatever reason) THE BOMBARDER MUST BE OFF! FAILURE TO DO THIS CAN RESULT IN A BOMBARDER FLASHBACK (DISCHARGE) THROUGH THE MANIFOLD AND MAIN STOPCOCK. THIS IS VERY DANGEROUS AND CAN ALSO DAMAGE MANIFOLD AND DIFFUSION PUMP COMPONENTS.
THIS IS VERY IMPORTANT! ALWAYS TURN OFF THE BOMBARDER BEFORE OPENING THE MAIN STOPCOCK!
This “pre heat” procedure eliminates excessive moisture in the tube that develops from storing the glass and during bending and splicing. It is especially helpful when processing fluorescent blues and whites. It removes the moisture that often causes premature discoloration in coated tubes during bombarding. Although not entirely necessary, it also helps in processing clear red tubes by reducing the chance of dark spots developing and the metal electrode shell oxidizing during the intense heating period at the end of the bombarding procedure.
STEP 5
Following the pre-heat and pre-evacuation procedure, close the main stopcock and U-gauge stopcock. Using the air vent stopcock, admit 2mm of air into the manifold as indicated by the pressure scale on the U-gauge.
The length and diameter of the tube being processed and the capacity of the bombarder usually determine the amount of pressure used to begin bombarding. For example, a 10mm tube less than 4 feet long should have an initial bombarding pressure of ~1mm. This is so the glass will not overheat before the electrodes start to get hot. On the other hand, a 15mm tube 12 feet long should have an initial pressure of ~3mm if possible. This is determined by whether or not your particular bombarder can light up a tube this large with this much pressure in it. In our example we will use an initial pressure of 2mm.
NOTE: For proper bombarding the glass should get hot first and then the electrodes. If the electrodes become red hot before the glass reaches a temperature of at least 175ºC, the contaminants released from the electrodes may deposit themselves on the cooler surface of the glass. As the glass gets hotter some of the contaminants may cause discoloration in fluorescent tubes as they are burned off the surface of the fluorescent powder, thereby leaving dark spots and/or residue. The discoloration may or may not get worse over time.
STEP 6:
After establishing the correct pressure, leave the U-gauge stopcock closed in order to monitor the pressure inside the manifold while processing the tube. Turn the bombarder on and adjust the current to ~200 mA. Continue to bombard until the glass temperature reaches ~150ºC as indicated by the temperature gauge.
It is necessary to monitor the pressure inside the manifold as indicated by the U-gauge scale. As the glass becomes hotter the pressure will begin to increase due to gases and vapors being released from the surface of the glass and electrodes. In this example, any time the pressure increases to more than 1-2mm above the initial setting (up to 3-4mm), reduce the pressure to the initial setting of 2mm by slightly opening the main stopcock (with the bombarder off) until the initial pressure is again obtained.
STEP 7:
At 150ºC adjust the pressure back to 2mm if necessary and increase the current to 300 mA. (If smaller tubing is being processed, such as 12mm, it may not be necessary to increase the current at this point). Remember to maintain the correct pressure range while proceeding.
STEP 8:
Continue to bombard until the glass temperature reaches ~200ºC. Reduce the pressure to 2mm and increase the current to 400 mA and continue bombarding. (On smaller diameter tubing reduce the pressure to 2mm and increase the current to 250-300 mA).
The electrodes should now be getting hotter - a dull red-orange color - and the glass will continue to increase in temperature.
STEP 9:
Maintain 2-3mm of pressure until the glass temperature is ~250ºC. At 250ºC reduce the pressure to 1 ½ to 2mm and increase the current to 10-15 times the electrode current rating. For example, for an 80 mA electrode increase current to 800-1,200 mA. For a 30 mA electrode increase current to 300-450 mA, etc. For our example increase current to 800-850 mA.
NOTE: The maximum allowable current recommended by the electrode manufacturer should be used if it is provided with the electrodes.
STEP 10:
Continue to bombard at 1 ½ to 2mm pressure at the maximum current for the particular electrode used (in our case, 800-850 mA) until the entire metal shell of the electrode is a bright, incandescent, almost translucent, light shade of orange (sort of an illuminated pumpkin orange) the entire length of the shell. This is very important! Electrodes that are not completely processed will cause discoloration in mercury tubes and cause red tubes to go dead prematurely. Depending on electrode size, this final heating of the electrodes may take 10-20 seconds. However, it should be done in the shortest time possible without excessive current to achieve the desired results.
If applied for too long of a time period, the high currents used for processing the electrodes may damage the electrodes by sputtering them. These high currents may also damage the stability of the fluorescent powder causing decreased light output and premature discoloration in mercury tubes.
By the time the electrodes are completely processed the glass temperature should be 275-300ºC. However, care should be taken that the glass temperature does not exceed ~310ºC.
NOTE: As a rule of thumb to determine whether or not the electrodes were heated to a high enough temperature (technically ~1,200ºC), they should exhibit some visible glow (though diminishing as the electrodes begin to cool) for 15-20 seconds after the bombarder is turned off. If they cannot do this, they were probably not heated to a high enough temperature.
STEP 11:
Turn the bombarder off and completely open the main stopcock to evacuate the tube. Open the U-gauge stopcock. Turn on the vacuum gauge, but wait a few moments (~10 seconds) before opening the vacuum gauge stopcock.
NOTE: With O-ring type stopcocks, “completely open” means to unscrew and retract the white Teflon stopcock plug from the glass barrel as far as possible without the upper most O-ring loosing its seal inside the glass barrel.
STEP 12:
Immediately following Step 11, gently but thoroughly heat the tubulation glass with the tipping torch between the neon unit and the manifold with special attention given to the area where the final seal-off will be made. However, if it is a mercury unit do not heat the trap bubble that contains the mercury; heat the tubulation to within ~½” of the bubble.
This step is done for several reasons: To prevent contaminant gases that are being pumped out of the neon unit from condensing on an otherwise cold tubulation, to burn off impurities that are present on the interior surface of the tubulation glass, which would otherwise be liberated when the final seal-off is made, and to prevent the mercury from picking up contaminants as it is rolled through the tubulation into the neon unit.
STEP 13:
While the unit is being evacuated, unhook the bombarder leads from the electrodes and monitor both the temperature gauge and vacuum gauge to confirm that the following minimum pump down criteria is met:
• By the time the glass cools to 200°C the pressure should be below 10 µ (microns).
• By the time the glass cools to 175°C the pressure should be below 5 µ.
• By the time the glass cools to 150°C the pressure should be at or below 1 µ.
• Final vacuum before filling should be 1 µ or less.
NOTE: The above pump down speeds should be considered bare minimums. Faster pump down times should be strived for.
More satisfactory pumping speeds are:
• By the time the glass cools to 200°C the pressure should be below 5 µ.
• By the time the glass cools to 175°C the pressure should be below 1 µ.
• By the time the glass cools to 150°C the pressure should be below 0.5 µ.
• Final vacuum before filling should be below 0.5 µ, but preferably 0.1 µ or less.
CAUTION!
Do not “flash” the bombarder to check for adequate vacuum. This can damage the electrodes, the emission coating inside the electrodes and the fluorescent powder. The damage may not be evident until the unit has been in service for some time. “Flashing” the bombarder to check for adequate vacuum may also cause a bombarder flashback through the manifold, which may damage manifold and vacuum pump components.
STEP 14:
When the tube has cooled to 70-80°C, and the pumping speed criteria has been met and a vacuum of 1 micron or less (preferably less) has been obtained, the unit is ready for filling:
• Remove the temperature gauge lead.
• Close the vacuum gauge stopcock and turn the vacuum gauge off.
• Close the main stopcock.
• Close the U-gauge stopcock.
• Fill the unit with the desired gas to the correct pressure for the size of tubing used.
• Immediately OPEN the U-gauge stopcock.
• Seal the unit off from the manifold.
AGING:
The finished unit should be ‘aged’ or ‘burned-in’ at 1½ - 2 times the normal operating current of the electrodes used. For example, 30 mA electrodes should be aged at 45-60 mA and 60 mA electrodes aged at 90-120 mA, etc.
Units filled with red gas should be the correct color immediately, but should never take more than a few minutes to come up to full color. Mercury units should be run for a few minutes before the mercury is inserted to examine gas color, electrode firing, to insure a stable discharge and to clean up any residue impurities. After this is done, turn the unit off and allow the electrodes to cool. Insert the mercury and roll the mercury from one electrode to the other, making sure some mercury sticks to each electrode. Make the final seal-off of the tubulation and age the unit by running it at the previously stated burn-in currents until the mercury has vaporized throughout the entire length of the unit and it is up to full color.
AGING CURRENT:
There will undoubtedly be some disagreement with the listed recommendations. However, electrode manufacturers commonly use excessive currents to do accelerated life tests on electrodes. They have found that a properly processed tube should be able to operate at 3 to 4 times the normal operating current for several hours with no adverse effects to the glass or electrodes. If a unit cannot do this it simply was not processed correctly and/or inferior materials were used to fabricate the unit.
PUMPING SPEEDS & FLUSHING:
If the pumping speeds listed previously are obtained it should not be necessary to use any type of flushing gas or procedure. However, if a flushing procedure is desired a simple additional step that can further “clean” the tube follows:
After bombarding is complete and the unit has cooled to ~200°C and there is absolutely no glow to the electrodes, admit a few millimeters of neon gas into the tube and evacuate immediately. The theory is that once a certain level of vacuum has been reached, say 1 micron (1x10-3Torr), all the remaining molecules of gas in the tube are impurities, such as water vapor, carbon dioxide, etc. If an inert gas such as neon is introduced, and the pressure is again reduced to 1 micron, 50% of the remaining gas inside the tube will now be neon gas and the other 50% impure gas. Therefore, the remaining impure gas has been reduced by half. (Specifically, at a pressure of 1 micron there are 40 trillion molecules of gas left in every cubic centimeter of ‘space’).
Actually, the end results are slightly more favorable than this because the more common gases are pumped out more easily, thereby leaving a higher percentage of inert gas as part of the remaining gas load. The remaining inert gas will obviously not affect the operation of the tube.
When performing this step always remember to close the vacuum gauge stopcock before flushing. Exposing the gauge tube to inert gas will not permanently harm the gauge tube, but it will require a period of time to degas before accurate readings can be obtained.
COMMENTS:
Bombarding is a procedure that should not be rushed. A slow, gentle approach will be found to be more effective than a fast, intense one in terms of maximum light output, electrode life and the elimination of contaminants that would otherwise effect the operation of the tube. For example, if a piece of tubing is sustained at a temperature of 200°C for a period of 2 minutes, more impurities will be liberated than if the same temperature were sustained for only a few seconds. As well, if the electrodes are heated gradually the metal will be more thoroughly degassed and the emission coating will be more completely converted than if the procedure were rushed. Moreover, a slow ‘cook’ time is more desirable than a fast one.
Relative to processing the electrodes, a heating time of 5 to 6 minutes (depending on the particular electrode) with a final shell temperature of 1,000°C will result in a shell that is roughly 90% degassed. The shorter the bombarding time, the less the shell will be degassed. At a bombarding time of 2 minutes and a shell temperature of 650°C (dull, dark red) the shell is only ~45% degassed. Generally speaking, the less that the shells are heated and degassed, the less the emission coating is processed and converted. Emission coating that is not processed and converted will gradually contaminate the tube over a period of time causing discoloration and tube failure.
I have visited shops where the ‘pumper’ tries to bombard units as fast as possible, then have to let the unit pump down for 3-4 minutes in an attempt to obtain a satisfactory vacuum as determined by ‘flashing’ the bombarder. The overall processing time for the unit ends up taking at least 5-6 minutes, mainly due to the length of time required attempting to outgas improperly heated glass and electrodes, with the end results being unsatisfactory.
The procedure herein will take 4 to 5 minutes when processing coated tubing and in the end will yield superior results. In the case of red tubes pumping times can be reduced considerably by filling tubes ‘hot, often at 150°C and even 175°C, provided the appropriate vacuum levels are obtained before filling and the filling pressure is adjusted accordingly.
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TUBE PROCESSING QUICK REFERENCE CHART
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FOR AVERAGE SIZE UNITS
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(15mm, ~8 feet long)
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BOMBARDING
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GLASS TEMPERATURE |
PRESSURE |
CURRENT |
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UP TO 150°C |
2 – 4 mm |
200 mA |
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150 - 200°C |
2 – 4 mm |
300 mA |
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200 - 250°C |
2 – 4 mm |
400 mA |
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250 - 300°C |
1 1/2 – 2 mm |
800 mA or maximum allowed for
electrodes used (10-15 x rating)
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PUMPING
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GLASS COOL DOWN |
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VACUUM OBTAINED |
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TEMPERATURE |
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IN MICRONS |
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GOOD BETTER |
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200°C |
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- - - < 10 |
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175°C |
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< 10 < 5 |
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150°C |
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< 5 < 1 |
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75°C |
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< 1 < 0.1 |
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RECOMMENDED FILLING PRESSURES
(AT 70-80°C GLASS TEMPERATURE)
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TUBE DIAMETER |
PRESSURE (Torr/mm Hg.) |
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8 mm |
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17 mm |
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9mm |
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15 mm |
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10 mm |
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13 mm |
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11 mm |
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12 mm |
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12 mm |
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11 mm |
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13 mm |
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10 mm |
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15 mm |
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9 mm |
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18 mm |
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8 mm |
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20 mm |
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7.5 mm |
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22 mm |
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7mm |
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25 mm |
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6 mm
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HP, 4 mm LP |
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(CLICK BELOW PICTURE TO ENLARGE)
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TUBE PROCESSING QUICK REFERENCE CHART
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FOR AVERAGE SIZE UNITS
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(15mm, ~8 feet long)
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BOMBARDING
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GLASS TEMPERATURE |
PRESSURE |
CURRENT |
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UP TO 150°C |
2 – 4 mm |
200 mA |
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150 - 200°C |
2 – 4 mm |
300 mA |
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200 - 250°C |
2 – 4 mm |
400 mA |
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250 - 300°C |
1 1/2 – 2 mm |
800 mA or maximum allowed for
electrodes used (10-15 x rating)
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PUMPING
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GLASS COOL DOWN |
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VACUUM OBTAINED |
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TEMPERATURE |
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IN MICRONS |
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GOOD BETTER |
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200°C |
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- - - < 10 |
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175°C |
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< 10 < 5 |
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150°C |
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< 5 < 1 |
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75°C |
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< 1 < 0.1 |
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RECOMMENDED FILLING PRESSURES
(AT 70-80°C GLASS TEMPERATURE)
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TUBE DIAMETER |
PRESSURE (Torr/mm Hg.) |
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8 mm |
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17 mm |
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9 mm |
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15 mm |
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10 mm |
........................... |
13 mm |
|
|
|
|
|
11 mm |
........................... |
12 mm |
|
|
|
|
|
12 mm |
........................... |
11 mm |
|
|
|
|
|
13 mm |
........................... |
10 mm |
|
|
|
|
|
15 mm |
........................... |
9 mm |
|
|
|
|
|
18 mm |
........................... |
8 mm |
|
|
|
|
|
20 mm |
........................... |
7.5 mm |
|
|
|
|
|
22 mm |
........................... |
7 mm |
|
|
|
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|
25 mm |
........................... |
6 mm
|
HP, 4 mm LP |
A WORD ABOUT GAUGES & METERS
There are four fundamental instruments necessary for proper luminous tube processing using the internal bombarding method. Each has its own specific purpose and function and one is equally as important as the other. Without the full compliment of these instruments to inform the processing technician of all aspects of the processing procedure, the process will be little more than guesswork as to the quality of the finished product.
These four instruments are:
1. A gas fill pressure gauge (also used to monitor tube pressure during bombarding).
2. A high vacuum gauge to determine ultimate vacuum and system integrity.
3. An A.C. milliamperes meter (mA. meter) to measure bombarder current.
4. A temperature gauge to measure glass temperature.
Some equipment suppliers offer certain types of these instruments to the industry that are neither a good choice nor practical for luminous tube processing when using the high voltage internal bombarding method. They can even hinder the processing technician rather than help them. However, it is important to point out that although SVP sells what we feel is the best choice of instruments for our purposes and promotes them on this website, we are not the only company they are available from*. This is not an endorsement for something only we sell, with the exception of the SVP Bombarding Temperature Gauge. Similar products that will work well for our purposes are available from other manufacturers. Therefore, the following information is presented to inform interested readers rather than persuade them and is done so in an effort to improve the overall quality of neon throughout our industry. Following is a short discourse for each instrument that gives a brief summary of what the instrument is used for and covers key points to consider when selecting a specific instrument.
* The SVP Bombarding Temperature Gauge is the only instrument of it’s kind currently on the market and is specifically made for high voltage bombarding purposes.
GAS FILL PRESSURE GAUGE (back-fill gauge, filling gauge)
VACUUM GAUGE (micron gauge, ultimate vacuum gauge, high vacuum gauge)
BOMBARDING MILLIAMPERES METER (milliammeter, mA meter, current meter)
BOMBARDING TEMPERATURE GAUGE (heat gauge, pyrometer)
GAS FILL PRESSURE GAUGE:
This instrument is used to measure how much rare gas is put in the unit (the backfill pressure) after processing is complete. Because these instruments are absolute pressure gauges they are also commonly used to monitor tube pressure during the bombarding procedure.
There are several instruments currently on the market used for measuring the amount of gas fill pressure; Digital instruments with a simulated analog readout, older types of digital instruments with a numerical readout, an analog capsule dial gauge (typically referred to as a “Torr gauge”), oil manometers (typically referred to as a U-gauge or butyl gauge), as well as other less common instruments. The most popular and user friendly is the capsule dial gauge (Torr gauge). However, if the technician is concerned with accuracy and repeatability from unit to unit, this is not the best choice for several reasons.
Digital and dial gauges generally have an accuracy shift of about ± 2% of full scale. For the popular 0-40 Torr gauge it is in fact ± 2% of full scale. This is ± 0.8 mm (2% of 40). This means that the actual fill pressure, regardless of what the gauge is indicating, may vary by as much as 1.6 mm from one unit to the next (from -0.8 to +0.8 of the reading). When filling a 15 mm tube to the desired pressure of 9 mm, the actual fill pressure can be off by as much as 18% from one unit to the next. For a digital gauge the value of the accuracy shift depends on the range of the instrument, but typically they are 0-760 Torr (mm) full scale. When considering a possible accuracy shift of ± 2% of full scale for a 0-760 range the results are ridiculously poor at best and should be completely unacceptable to a technician who is concerned with producing quality, trouble free neon. This much deviation from the correct fill pressure can adversely affect a number of things; Overall tube operation, transformer load and the service life of each.
These already problematic gauges reveal other shortcomings as well. Both exhibit virtual leaks*. The 0-40 Torr gauge (or equivalent; several renditions are available) is of particular concern, so much so that the systems ultimate vacuum may never be reached even after days of continuous pumping. Depending on the particular method used to connect the gauge to the manifold, digital gauges can show similar problems. Even if the ultimate vacuum is reached due to the pumps capacity and ability to do so, closing the main vacuum stopcock will immediately defeat that achievement. Further, the Torr gauge has a very delicate movement. Even small debris such as phosphor powder will adversely affect the movement if allowed to infiltrate it – something that is easily accomplished through the normal course of processing. Larger particulates such as glass fragments that may inadvertently get into the movement due to a mishap during processing can damage the movement beyond repair. Once the movement is contaminated and compromised it does not move freely or accurately. Common observations are that of the needle sticking, not going to 0 when a hard vacuum is in the manifold, greater accuracy shifts than those previously mentioned, or all of these combined.
Although archaic by some opinions, the U-gauge oil manometer is by far the most accurate, repeatable and dependable instrument for our purposes. In comparison to other instruments the U-gauge is as accurate as how well you can see the scale**. Filling units to ± 0.1 Torr (mm) is entirely possible and repeatable from one unit to the next. There is no accuracy shift from one unit to the next; what you see is what you get. Also unlike other instruments, the only recalibration it ever needs is to periodically be cleaned and refilled with new oil of the appropriate type. The neon technician can do this in-house, thereby eliminating the need to return it to the factory for recalibration, as is the case with other fill gauges. There are no virtual leaks, delicate movements or circuit boards to be damaged.
* Virtual leaks are sources of gases (air, water vapor, neon & argon gas from back-filling, etc.) that are partially trapped but are released at a slow rate into the system. For example, un-vented screw holes or other threads exposed to the interior of the vacuum system will release gases into the system over a long period of time and make it impossible to reach and maintain ultimate vacuum until the partially trapped gases are completely exhausted. This can take days and even weeks. If the virtual leak is then again exposed to atmospheric pressure, or even back-fill pressures, it will be replenished and like starting over to remove the gases. Therefore, it is important to eliminate sources of virtual leaks from the system.
** The scale must be correctly calibrated for the particular fluid used and the gauge constructed correctly for the corresponding scale. Using butyl phthalate oil with a scale calibrated for silicone oil, or vice-versa, will give incorrect pressure readings. Similarly, a scale calibrated for a standard U-gauge cannot be used with the new advanced, compact SVP U-Gauge Fluid Manometer as there is a multiplied pressure differential between the two columns. Therefore, the scale for the new SVP Manometer is specific to this instrument when used with the oil supplied and no other gauge or oil.
VACUUM GAUGE:
A high vacuum gauge is used to measure the ultimate vacuum obtained within the manifold following the processing procedure and prior to backfilling the tube with inert gas. This instrument ensures that the tube was adequately evacuated. A high vacuum gauge is also valuable in determining the integrity of the vacuum system, as well as troubleshooting problems if they arise. Once a technician has become familiar with this instrument and how it reacts to various conditions and situations it can also be invaluable in avoiding problems.
A vacuum gauge capable of measuring down to at least 1 micron should be used and this level of vacuum should be strived for. A vacuum of 1 micron is where molecular flow begins to take place and where the high vacuum region begins. Analog vacuum gauges, rather than digital vacuum gauges, are best suited for our purposes for various reasons. The high voltage field produced by the bombarder is generally not favorable toward digital instrumentation (both vacuum gauges and temperature gauges). In addition to the influence of high voltage, the inexpensive (regardless of what they are sold for) digital vacuum gauges that are typically offered to the U.S. neon industry are not true high vacuum gauges, even though they are referred to as such by the suppliers offering them. Some sell these instruments with a gauge tube (sensor) that is designed for a range of measurement of 0-1,000 microns (the yellow coded Hastings DV-6M gauge tube) in an attempt to improve the accuracy of the unit. However, the gauge itself is actually designed for use with a 0-20,000 micron gauge tube. 20,000 microns = 20 Torr (mm), so this gauge is actually a 0-20 mm pressure gauge, not a high vacuum gauge intended to measure in the low micron range. A vacuum gauge designed to measure millimeters of pressure rather than pressures in the low micron range cannot accurately measure a few microns of pressure, much less measure 1 micron or below, regardless of what gauge tube is supplied with it and regardless of what the supplier claims. Further, repeatability of a gauge such as this in the low micron range is poor at best. This can lead the processing technician to think there is a problem with the vacuum system when there is not, or worse yet, think the vacuum is better than it actually is.
When considering a vacuum gauge, one that will measure pressures between 1 micron and 1,000 microns is suitable for general neon tube production. However, one that can measure 0.1 micron should be used for critical neon work and cold cathode lamp production. Generally speaking, the broader the measuring range that the instrument covers the less accurate it will be in the low micron range, i.e., a vacuum gauge that has a scale of 0-1,000 microns will not measure a vacuum of 1 micron as accurately as a gauge that has a scale of 0-100 microns. A comparison of these two analog vacuum gauge ranges can be seen Here. For reference, a vacuum chart with pressure conversions is available Here.
CAUTION!
Whatever vacuum gauge is used, whether it is line voltage operated or battery operated, a stopcock between the vacuum gauge tube (sensor) and main manifold body must be used to isolate the gauge tube from the main manifold. This protects the gauge and gauge tube from possible damage due to bombarder high voltage, spark tester frequencies and voltage, as well as the influx of contaminants, which are a normal result of the bombarding process and venting the manifold to atmosphere. The optional SVP Vacuum Gauge Stopcock and how the gauge tube is connected to the stopcock can be seen Here.
BOMBARDING MILLIAMPERES METER:
A bombarding milliamperes meter (mA meter) monitors the amount of current generated by the bombarder through the tube being processed. The use of this instrument is necessary to ensure that the right amount of current is being applied at the appropriate times. This eliminates any guesswork and provides the processing technician with the information necessary to avoid the problems normally associated with tube processing if this instrument is not used. Damage to the phosphor coating inside the tube, electrode sputtering and structural damage to the glass tube will result if the bombarding current is not monitored with an appropriate milliamperes meter.
When considering an A.C. milliamperes meter to measure bombarding current, one that has a range of 0-1,000 mA. is suitable for general neon work as the maximum current applied will typically be less than 1,000 mA. For larger diameter, 25mm Cold Cathode work a 0-2,000 mA. meter should be used due to the higher currents required to process the larger electrodes. Bombarding current in this instance will easily exceed 1,000 mA.
A True RMS Iron-Vane meter should be used rather than an inexpensive rectified meter. A rectified A.C. milliamperes meter, which gives average values rather than actual values, is not accurate enough or suitable when used in close proximity to the high voltage field produced by the bombarding transformer. Further, certain types of bombarder choke controls, if not closely matched to the bombarder, distort and/or chop the sine wave and create excessive signal noise. The more distorted the waveform is and the more signal noise there is the more inaccurate the reading will be on a rectified meter. This accuracy shift can be as much as 20% from the actual value. For example, a reading of 600 mA on a rectified meter may actually be anywhere from 480 mA to 720 mA. By comparison, a good quality true RMS Iron-Vane meter is unaffected by the applied voltage, wave distortion or signal noise and will be within ± 2% of the actual value. Each type of meter is easily identified visually. A comparison of the two different meter types and how to identify each can be seen Here.
BOMBARDING TEMPERATURE GAUGE:
A Bombarding Temperature Gauge* monitors the glass temperature during the processing procedure. A minimum glass temperature is necessary to ensure that the maximum amount of contaminants and impurities are released from the internal surface of the tubing, which if allowed to remain, will affect the life, efficiency and overall quality of the finished unit. Too high of a glass temperature is also undesirable. Excessive glass temperature will damage phosphor coatings, cause deformation of the glass structure and induce stress points into the glasswork. This instrument, if properly designed for this specific purpose, eliminates speculation as to the actual glass temperature and provides the technician with reliable information. In the case of the SVP Bombarding Temperature Gauge, the instrument is also used to correlate glass cool-down temperatures with vacuum levels obtained as indicated by the vacuum gauge.
Because of the high voltage field, consideration must be given when choosing a bombarding temperature gauge. Inexpensive infrared pyrometers and inexpensive digital gauges (regardless of what they are sold for) as well as thermal mediums such as temperature crayons** are not suitable for various reasons and are found to be inaccurate in close proximity to the bombarder high voltage field. However, good quality analog type pyrometers are historically the best choice as they are typically unaffected by the high voltage field.
When used in close proximity to high voltage an infrared pyrometer requires special circuitry. If the lens, or signal pick-up or head, is to be placed close to the tube being bombarded a special lens and focal point must also be used for the gauge to function properly. Such instruments are available, but the design requirements and construction criteria add considerable cost to the finished product. Because of the cost factor, infrared temperature gauges suitable for bombarding are rarely used. However, they are available.
The digital temperature gauges most commonly offered to the neon industry at the present time are also affected by the bombarder high voltage electromagnetic field. The printed circuit board and related components that comprise these instruments were simply not designed for this. The result is an instrument that can display various readings at any given temperature depending on different factors including the load on the bombarder, which changes the characteristics of the emitted high voltage field. General observations reported have also been temperature readouts that go “haywire” once the gauge passes a certain temperature: i.e. The numbers begin to rapidly bounce around both upscale and downscale, or the numbers, which are comprised of LCD’s, “scramble” and are illegible. The manufacturer of these gauges is aware of these inherent problems. Their solution is to wrap the circuit board in aluminum foil in an attempt to shield it from the high voltage field.
Regardless of which instrument and/or method are used to measure glass temperature, it should have the ability to easily compare glass cool-down temperatures with vacuum levels obtained during the evacuation stage. This is an important consideration in determining how well the unit was evacuated and therefore the quality of the finished product. For example, at a glass cool-down temperature of 175°C (the temperature at which vaporized contaminants will begin to re-condense inside the tube) the vacuum must be better than 5µ, preferably better. Obviously any type of thermal medium cannot do this. Only an instrument that provides temperature readout can.
SVP Neon Equipment is the only equipment manufacturer (or materials supplier) who specifies evacuation speeds vs. glass cool-down temperatures. Refer to our Recommended Bombarding Procedure, also listed on the TECHNICAL page for more detailed information.
SVP Neon Equipment is proud to be the only manufacturer and supplier of a Bombarding Temperature Gauge that is truly made specifically for high voltage bombarding purposes. With a large 4½” easy to read analog meter face especially designed to aid the processing technician with at-a-glance information, both during the heating stage as well as the cool-down evacuation stage, it is the most user-friendly and accurate Bombarding Temperature Gauge on the market.
* Paper, regardless of which kind, should not be considered as an alternative or substitution. A very convincing argument against its use is to place a ½” wide strip of paper on a 75 watt light bulb for 10 minutes. Try several kinds of paper for comparison. The light bulb never changes temperature and the temperature is far less than bombarding temperature, but the paper will progressively get darker the longer it stays on the bulb. Different papers will char at different time intervals, but they will all turn brown in much the same manner as they do when using them to “measure” glass temperature. The amount of moisture in the paper and ambient atmospheric conditions will also affect the time required to char the paper. Attempting to use paper to measure glass temperature during bombarding, where a variety of conditions can exist, is an old unreliable method of doing so.
** Thermal mediums such as temperature crayons are compounds that rely on a series of chemical reactions to achieve the desired result. Their accuracy depends on the absence of anything other than heat that may influence the reaction, thereby changing the result. The influence of the high voltage field is why temperature crayons are not accurate for bombarding purposes because of the very nature of the medium. As the crayon is being heated it is going through chemical changes. First it liquefies, then begins to change colors, during which time it solidifies again in a different form. These changes are chemical reactions taking place. According to both the Department of Chemistry and Chemical Engineering Department at the University of South Carolina, whenever a voltage is introduced into a chemical reaction such as this, it changes the end result of the reaction. In the instance of high voltage, the more voltage that is present the more the end result of the chemical reaction will be changed. In our case the high voltage is more than sufficient to cause the crayon to change colors before it is supposed to. In other words, the crayon may say “300°C” on the label, but it will actually change color at a much lower temperature than this. To reflect on this situation, a supplier of temperature crayons to the neon industry has changed which temperature-range crayon they offer at least 3 or 4 times since they first started promoting them. The replacements have progressively been of a higher temperature rating; 260°C, 280°C, etc. With the latest offering the crayon labels have even been removed to hide what temperature rating the crayon is supposed to be. It is our opinion that the temperature designation of the crayon was changed to compensate for the affects mentioned above in an effort to get the crayon to change colors at an actual glass temperature closer to what it should be, rather than what the crayon is specified to change color at. However, the effort still seems to fall short of the target temperature. SVP has tested different renditions of these crayons over the years during tube processing. The older “green” crayon that was once used was marked as “300°C”. During high voltage processing this crayon would turn to almost black at ~180°C and stop “smoking” at an actual glass temperature of ~225°C. The new tan/brown crayon with no label will turn to the required “chestnut” color at ~175°C and stopped smoking at ~200°C – far short of the required glass temperature. However, when a large amount of the tan crayon was applied it increased the color change temperature to ~200°C and the “stopped smoking” temperature to ~250°C – hardly a reliable, repeatable, consistent method for measuring glass temperature during high voltage bombarding.
|
U-GAUGE OIL MANOMETER*
OIL FILL & SCALE PLACEMENT INSTRUCTIONS
(*Pronounced ma-nom-eter, not man-o-meter)
AMOUNT OF OIL
There is no set amount of oil that has to be put into the U-gauge. But it needs to be enough so that if a neon unit is filled to 20mm pressure the oil in the U-gauge does not go past the bottom of the U-tube, which would cause the fill gas (or air) to bubble up the right side of the gauge. If this happens it does not cause any permanent damage, but will delay accurate readings until the oil drains down the tube and settles again.
ATTACH SCALE
Clip the scale onto the right side of the U-gauge board. Adjust the scale so the top of it is about even with the top of the 9mm tube on the right side of the gauge, where it flares out into the bottom part of the stopcock. This does not have to be an exact placement.
OIL FILL
Unscrew and remove the stopcock plug from the top of the U-gauge. Stand it up on the end of the black control knob so it doesn't lie on the table and get dirt on the O-rings. Using an eyedropper, extract some oil from the bottle. Carefully insert the tip of the eyedropper into the stopcock barrel and past the stopcock "seat" (where the tip O-ring seals). Gently squirt the oil into the tube. Repeat as necessary until the oil is one or two millimeters below the zero mark on the scale. Wait 10-15 minutes for the oil to drain down the tube and settle. Fill the gauge the rest of the way to the zero mark by adding a few drops of oil at a time. Slightly under filling or over filling is not a concern as the scale is easily adjusted. Once the oil is in, adjust the scale so the zero mark is exactly aligned with the top of the fluid level. Wipe any oil out of the stopcock barrel with an acetone-wetted piece of lint free paper towel (such as Bounty) wrapped around a fire polished piece of tubulation. Replace the stopcock plug but leave the stopcock open.
FIRST USE
SLOWLY open the main stopcock to apply vacuum to the manifold. The oil may foam up considerably at first. If this happens, immediately close the main stopcock until the oil stops foaming. If the main stopcock is opened too quickly the oil can foam up over the top of the left side of the gauge and down into the manifold. So go slowly! Open and close the main stopcock a little at a time until full vacuum is applied. Once the oil is done foaming and outgassing you can open and close the main and air vent stopcocks as quickly as you want without the oil foaming again.
DAILY USE
Follow the instructions in our RECOMMENDED BOMBARDING PROCEDURE for step-by-step instructions on how to use a U-gauge manometer to monitor bombarding pressure and measure fill pressure.
Neon Electrodes for Sign Lighting
Through our precise manufacturing techniques, overseen by engineers with a tremendous commitment to quality, Voltarc produces Masonlite MilleniumTM Neon Electrodes – the unrivaled choice for signage professionals worldwide.
 At Voltarc we use the best raw materials, including shells deep drawn from the finest-quality, pure soft iron, and nickel plated in a custom-built, dedicated plant. The result is first-rate electrodes without the risk of contamination.
Our lead wires are made of standard nickel wire for corrosion-resistance and flexibility, along extra length of copper-clad Dumet sealed into the pinch to enhance confidence in the metal-to-glass seal, and solid-nickel wire for high purity and low outgassing, attached to the shell for reliability via a welding method pioneered by Masonlite.
Masonlite Millennium™ Neon Electrodes - Bombarding and Pumping Procedure
The following procedure is for 15/50C and 15/50CT electrodes on 15mm or larger diameter coated Sign Tubing. Adjust the procedure for other electrode sizes, smaller diameter tubing, and tubes less than 18" long.
Preheat
-
Open vacuum valve and evacuate tube to approximately 2 to 3 Torr (2 to 3 mm Hg) pressure. Close vacuum valve.
-
Turn on bombarder and bring current to between 150 and 200 mA.
-
Continue bombarding until the tube reaches 275° to 300°F (135° to 150°C). Always release bombarder switch prior to opening of main stopcock. Then open vacuum valve. Evacuate tube(s) for 45 to 60 seconds, depending on tube length and configuration.
Step 2
-
 Close vacuum valve, refill tube(s) with 2 to 3 Torr
(2 to 3 microns) of dry air.
-
Turn on bombarder and raise current to 325 mA;
continue heating until tube reaches 375° to 400°F
(190° to 205°C). Note: for uncoated tubes, heat to
482°F (250°C). At this point reduce pressure to 1 Torr
or slightly less.
Step 3
-
Increase current to 800 mA.
-
Bring neon electrode shells to a bright, cherry-red color, (1,652° to 1,832°F (900° to 1,000°C)) while maintaining ½ to 1 Torr pressure. Once all shells are of a uniform color, release the bombarder switch and open the vacuum valve.
-
Evacuate the tube(s) to the lowest possible pressure - at least 3 to 5 millitorr (3-5 microns). Continue pumping until you can comfortably handle the tube(s) - about 122 °F (50°C).
-
Release accumulated moisture in the tabulation(s) at this time with the aid of a heat gun or hand torch. This prevents transfer of moisture to the finished tube(s) during flushing and/or backfilling of the inert gas(es).
-
Backfill the tube(s) to the designated pressure with the desired gas using a positive pressure gauge if possible.
Masonlite
Millennium™ Electrode Type |
Preheat Current
Pressure |
Step 2
Pressure |
Step 3
Pressure |
|
12/30C |
150 mA |
225 mA |
450 mA |
|
12/30CT |
|
|
|
|
13/30C |
2 Torr |
2 Torr |
1 Torr |
|
13/30CT |
|
|
|
|
12/25C |
75 mA |
125 mA |
300 mA |
|
12/25CT |
|
|
|
|
13/25C |
2 Torr |
2 Torr |
1 Torr |
|
13/25CT |
|
|
|
|
15/30C |
150 mA |
225 mA |
450 mA |
|
15/30CT |
2 Torr |
2 Torr |
1 Torr |
|
15/50C |
200 mA |
325 mA |
800 mA |
|
15/50CT |
2 Torr |
2 Torr |
1 To |
Masonlite MillenniumTM Electrodes
Lead Glass Electrodes: Mica Disc - Ceramic Collar - Premium Steel Shell
|
Part #1
|
Description2
|
Rating (mA)
|
Glass Length
|
|
20270
|
10/20C-2
|
20
|
2"
|
|
20271
|
10/20CT-2
|
20
|
2"
|
|
20304
|
12/25C-2
|
25
|
2"
|
|
20305
|
12/25CT-2
|
25
|
2"
|
|
20274
|
12/30C-2 1/2
|
30
|
2 1/2"
|
|
20275
|
12/30CT-2 1/2
|
30
|
2 1/2"
|
|
20276
|
13/25C-2
|
30
|
2"
|
|
20277
|
13/25CT-2
|
30
|
2"
|
|
20282
|
13/30C-2 1/2
|
30
|
2 1/2"
|
|
20283
|
13/30CT-2 1/2
|
30
|
2 1/2"
|
|
20285
|
15/30C-2 3/8
|
45
|
2 3/8"
|
|
20286
|
15/30CT-2 3/8
|
45
|
2 3/8"
|
|
20288
|
15/50C-2 3/4
|
80
|
2 3/4"
|
|
20289
|
15/50CT-2 3/4
|
80
|
2 3/4"
|
|
20298
|
18/120C-3
|
120
|
3"
|
|
20299
|
18/120CT-3
|
120
|
3"
|
|
