Pirani filaments.

How to get to vacuum, what the classes are, and what is needed for what job.

Pirani filaments.

Postby chrismb » Mon Jan 24, 2011 1:58 pm

Out of interest, does anyone know what the pro's and con's of gold plated tungsten versus platinum/rhodium filaments are?
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Re: Pirani filaments.

Postby johnf » Mon Jan 24, 2011 2:39 pm

My guess
Cost!!
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Re: Pirani filaments.

Postby chrismb » Mon Jan 24, 2011 3:11 pm

Well, that was the obvious thing. I've just acquired an Edwards active gauge APG with platinum/rhodium filament. On Edwards website, this one is up for $495 and the other at $471. But does that extra $20 buy you longer life, or better chemical resistance, better accuracy between gas types, all/other?
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Re: Pirani filaments.

Postby Doug Coulter » Mon Jan 24, 2011 4:13 pm

I use small wattage "grain of wheat" type bulbs, and they last a long time and cost under a dollar. The trick there is I don't heat them up much. One that is 27 ohms cold I heat to balance a bridge at 47 ohms, well below where the tungsten-air or tungsten-water reaction kicks in. They last a very long time, or until the next inrush accident if you don't baffle them off in a a few inches of bent copper tubing.

I started with panel lamps from computers (yes, the old ones I used to fix for a living had incandescent lamps on the panels, they were quite pretty). Now I use an equivalent 14v lamp (again, 27-28 ohms cold) and don't heat it to quite the double resistance point. A high power opamp suffices to do that with no buffering needed.

Circuit diagram and explanations here.

I should add that these do fail in some tens of hours when powered up in air -- so I turn them off when I'm not pumping. Interestingly they get more sensitive right up to failure, as the tungsten gets thinner....
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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