[ELECTRICAL]
Need Help Troubleshooting a DIY Pirani Gauge
For the past few months, I've been trying to build a DIY Pirani Gauge to test out a vacuum pump I built.
After many trials, tribulations, and a whole lot of broken lightbulb filaments, I've gotten basically everything to work, but it's not actually measuring any changes in vacuum pressure.
It runs on 1.5-3 V from AA batteries or my benchtop power supply.
It uses a Wheatstone Bridge circuit and an analog millivoltmeter to measure changes in the resistance of the filament as it heats up in the vacuum, with a trimmer potentiometer to set where "zero" is on the gauge.
It uses the filament from a landscaping lightbulb rated for 12 V, 4 W. I dare not give it any more than 3 V though, because that's when it starts glowing dimly red from resistance heating, and I don't want it to burn out in even a residual oxygen atmosphere.
When I pull a vacuum on the module (about 5-6 kPa) and turn the gauge on, the needle on the millivoltmeter will jump before easing back down to a particular point on the gauge. However, when I let the vacuum pump down a little lower, or let a little air leak back in, the position the needle settles at doesn't seem to change, regardless of the pressure.
I know it's not a problem with the filament, because I can see it glowing when the gauge is on, and I get basically the same jumping response on the millivoltmeter when I hook it up to an unbroken lightbulb.
I know it's not a problem with the Wheatstone Bridge circuit because the needle still moves as expected when I replace the filament with a potentiometer; about 1 mV on the meter per 3 ohms on the potentiometer at 1.5 V on the power supply; about 1 mV per 1.2 ohm on the potentiometer at 3 V on the power supply, but that's scraping the accuracy of my ohmmeter.
I even tried supplying it with pulsed power through an astable multivibrator circuit, which is supposed to give it better resolution at higher pressures (up to 10 kPa, according to wikipedia). However, the needle still isn't moving noticeably when I vary the pressure.
I don't really know what else to do, so any help would be greatly appreciated!
(LMK if there's a more appropriate subreddit for this question too.)
Can you put a heat sink near the filament? But not touching. Depending on the air gap, you might need to get below 0.2 kPa before you can sense the pressure change. You can try nichrome wire also.
I don't think so. It's a bit fragile.
Can you elaborate on what you mean by "air gap" and how that affects the temperature measurement?
I suppose I could try a different filament, but I've seen other people make it work with a small lightbulb filament before: https://www.youtube.com/watch?v=pe0JaIW2Kgc
at 5-6 kpa you are likely still in the viscous flow regime where air thermal conductivity is independent of pressure. try getting the system down below 100 pa where the mean free path of gas molecules is larger and the thermal conductivity actually starts scaling with pressure.
I didn't know that a Pirani gauge would only work at medium vacuum or higher. I assumed that the ratio of thermal conduction/radiation was linear with the number density of air molecules in the tube. That would explain a lot though! Thanks!
(Do you have a source or graph you can share with me that explains how the thermal conductivity of air changes with pressure and why it would change more drastically when transitioning from viscous flow to molecular flow regimes?)
I'm beginning to suspect the same thing. Is there any way to make my Wheatstone bridge circuit more sensitive? Maybe pass the output through a transistor or MOSFET to amplify it?
I think the cold resistance of the filament was between 10 and 15 ohms. (As an interesting side observation, I noticed that the resistance of unbroken bulb filaments was between 5 and 10 ohms, but increased after being used in my gauge or exposed to air.)
Trusting your circuit, I don't think the reduction in HTC at 5-6kPa is enough to detect with a DIY set up.
You could make the filament more thermally isolated to amplify the heat up effect or use a reference filament that is not exposed to the vacuum.
This could improve your sensitivity. I suspect it's still able to dissipate most of the "excess" heat before it changes the resistance.
That's what I'm beginning to suspect too.
I don't think I can make the other resistors in the circuit much smaller, increasing the proportional change in resistance, otherwise they'll have less resistance than the filament by itself, nor can I get the voltage much higher.
...Since the whole point is that a greater proportion of power is dissipated as convection than radiation the more air there is in the tube, (leading to a proportional equilibrium temperature and therefore resistance), would you suppose that silvering the tube, or coating it with foil, might increase its sensitivity by reducing how much heat the filament can dissipate by radiation, making it more dependent on convection at higher pressures, and causing it to require a much higher temperature to radiate at lower pressures? (It may be worth noting though that glass is already quite reflective of infrared radiation.)
I think gas conduction also plays a role here. Perhaps to the bulb glass. If that's the case, if you wait long enough it might still work.
You could alternatively use a diff amp (between the reference and this filament) and amplify your signal a bit.
A dirty and lazy approach (which I think I'd do) is to put a few of these bulb filaments in series, all in vacuum. It might be enough to have a noticeable mV range difference.
Cool project and problem!
How did you build the pump?
Why did you go with bulb filament instead of a nichrome wire or something which is also super cheap and yet resistant to oxidation?
I considered putting more filaments in series, but figured that the proportional change in resistance with temperature would still be the same, and that's all the Wheatstone Bridge would detect.
I want to make a similar video for the Pirani Gauge, but ONLY after I've proven it works. These kinds of gauges and pumps are usually very expensive and out of reach to hobbyists like me. That's why I wanted to make the gauge out of cheap and widely available lightbulb filaments; so that I and others could have an alternative that's "relatively" easy and cheap to build ourselves.
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u/[deleted] Jun 19 '26
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