CO2 Laser: DIY Vacuum gauge

CO2 Laser: DIY Vacuum gauge

I've decided to build a low-pressure pirani gauge to detect the leak in the tube. This site describe how to make a pirani gauge from scratch. I've use common flashlight lamp bulb even if they get low filament resistance. Actually my gauge work but is very sensitive the length of the wiring between the light bulb and the circuit get a significant resistance and change the value of the measure.

I've made own circuit on LTSpice, you can download it here:

The circuit

Tension by the equivalent resistance

This circuit maintain the resistance and so the temperature of the bulb constant. The voltage need to keep the filament at the same temperature is link to the energy take away by the air molecules and so the pressure. At atmospheric pressure the air takes away a lot of energy by convection but under medium vacuum air is much thinner and the convection much less effective, so you need less current to maintain it to the same temperature.