Thermocouple gauges are great vacuum gauges for measuring pressures from
atmospheric on down to 10-4 Torr. (.0001 mm) There are
some problems or issues that can be associated with these gauges. First off, we
are dealing, as with most gauges, with a 2 component system. On one hand
we have the controller head or meter head and on the other hand we have the gauge tube or
transducer. In order for the system to operate correctly we have to
have confidence in both components.
The gauge controller should be zeroed out. In the case of
atmospheric pressure this generally can be done with just the controller
itself. There sometimes is an adjustment screw or pot that can be
adjusted. Review of the manual accompanying your gauge should
provide the location for this. In the case of the low pressure or
high vacuum reading the gauge should be tested or "set".
This can be accomplished by using a reference gauge tube which has a known
vacuum pressure in it, or using a vacuum system capable of generating
pressures better than 10-4 Torr. In the second case the
gauge can then be set at it's maximum vacuum pressure number with some
degree of confidence. In the first the gauge is set to the rated
reference tube pressure. These reference tubes should be submitted
for recalibration occasionally because they have a tendency to loose
pressure over time.
These systems change over time. If the tube resides in a high
vacuum situation it can become contaminated by the gases evolved in the
system. Many times it is oil vapor, but can be just about
anything. There are two "noble" wires in the transducer head that
measure the potential drop from one wire to the other. The two wires present that measure the potential become
coated with impurities or get dirty and when dirty the tend to read high toward atmosphere. Supposedly,
it is possible to "clean" these tubes with carburetor cleaner or
acetone although our efforts to
date have consistently met with failure. Usually the only thing that
can be accomplished is to replace the tube. The best
method is to use the transducer for your pressure measurement and when not
in use, to isolate the transducer through use of a valve or
stopcock. We also have gauge guards which is a small
filter between the gauge tube and the system. Some users swear by
Other problems that can be present to varying degrees. A common one is in neon
tube production to "fry" the gauge tube when bombarding.
If high voltage is to be supplied into the system being evacuated then the
use of a battery operated gauge will eliminate this problem area. The
gauge then does not offer a path to ground. Another solution is to
again isolate the gauge tube during the process with a valve of some kind.