TC gauge sensitivity?

[johnp]

The vacuum system in our set-up has a Granville-Philips 260 gauge controller that runs a pair of GP 260-009 TC gauges and an ion gauge. Neither of the TC gauges reads below .1 torr even when the ion gauge is telling us 1E-4 torr. From 760 t down to .1 t they seem to work fine, but they won't go any lower. This is an old system; do TC gauges 'go bad'? Could the controller be flaky? Need your advice.

[MarkS]

TC gauges don't really go bad per say. They're simply two wires joined thermally, one is heated and the other one is used to measure resistance, so if the wires haven't broken I'd say that the gauge head probably works just fine. The controller is probably OK. You might need to adjust the zero point on the controller if it allows for that.

[johnp]

Mark, thanks for the tip. A few minutes digging the manual out & tweaking the controller with a screwdriver did the trick.

[DaveC]

John -

As you probably now have discovered, the "Zero" pressure reading of a TC thermocouple type gauge, is set arbitrarily at a low pressure, usually that achieved with the high vacuum pump operating. Typical :"zero"value is at or below 1 mTorr.
Similarly the "Atm" or full scale reading it gotten by setting the gauge readout at atm press.

It takes some serious attention to destruction to kill off a TC tube. Using the wrong tube for the readout system is one way that could do that. But generally these things are very hardy.

Dave Cooper