Pirani filaments.

[chrismb]

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

[johnf]

My guess
Cost!!

[chrismb]

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?

[Doug Coulter]

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....