One additional note on the Pirani. The Pirani's filament operates at constant temperature: usually 120 deg. C. The shell of the tube has a temp comp element either attached or wound around it. Needless to say, as the temp of the tube gets to 100 degrees or more, the tube will cease to work properly simply because there's no temp differential from filament to shell. That does not mean it will get destroyed.
At some temp the bridge components in the base of the tube will be affected, probably about the time the connector starts to melt. That should be somewhere around 150 degrees or more. Fixes are simple if your chamber runs real hot. A small muffin fan could be employed in an extreme case.
On the problems with plasmas and Pirani tubes, I've seen a number of tubes that got fried when neon sign makers had Pirani tubes connected close to their neon tubes while going through the bombardment cycle. The plasma gets to the filament and then the rather highish voltage & current zaps the electronics. This can be gotten around with a long/thin connecting tube or, better, a close mesh metal screen in the sensor's connecting tube.
Steve
Created on Thursday, April 05, 2001 6:41 PM EDT by Steve Hansen