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. |
