Ubya
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Building a DIY Pirani Gauge from a lightbulb
DISCLAIMER
If you copy exactly what I did, you'll be able to read a numeric value that without calibration is nearly useless. I'm ok with nearly useless because
i'm only interested in seeing this value going up or down and by how much, the exact numbers are not important to me. If you want to correlate this
"random" values to an actual pressure, you'll need to calibrate it with an actual working gauge (that I don't have at the moment).
Objectives of this project
I've been experimenting with plasma discharges for a few years already, and until now I've always totally eyeballed the vacuum level in my systems by
just looking at the color and opacity of plasma. A method that worked fine to distinguish if my vacuum chamber or my pump were total garbage, but
totally unusable to my untrained eyes to distinguish very minor leaks.
After upgrading to a 2 stages rotary vane vacuum pump, and an all metal KF apparatus, I even got toally rid of any plasma, and was left with
fluorescence cause by high energy electrons, but I still needed a way to know if the pressure was going up, down or staying still.
I needed a device capable of distinguishing changes in pressure in the range of 10-1Pa, the lowest pressure achievable by my rotary vane
vacuum pump, with junk laying around.
This project doesn't require any particular skills or tools, and all the steps can be done in about 1 hour.
The Pirani Vacuum Gauge
A pirani gauge is essentially a heated filament exposed to the vacuum you want to measure. By feeding a small current to the filament, it will heat
up. the amount of heating depends on the supplied current and the pressure of the environment. At higher pressures there are more molecules to carry
away the heat from the filament due to convection. At lower pressures there aren't many molecules, and the temperature of the filament increases.
When the filament heats up, its resistance also increases, so we can correlate the increase in resistance with a lowering of pressure.
This resistance change can be very small, not easy to detect with a simple multimeter, so we use a circuit called a Wheatstone Bridge.
When R1/R3 is equal to R2/Rx, Vout is 0V.
Any changes in Rx will cause a potential difference to be detected.
My DIY Pirani Vacuum Gauge
Some people wound their own filament from very thin tungsten wire, I thought of doing that too, but I already had a tungsten filament encased in a
vacuum tight vessel, just a regular incandescent lightbulb (even though they are pretty uncommon now).
I just had to make a hole in a lightbulb without cracking the glass, easier said than done!
I tried different kinds of lightbulbs, but the best ones are these little miniature lighbulbs that can be had for super cheap.
I bought the 4mm 24V model, and I'll say later why
After gently sanding the tip with some not so fine sandpaper, I epoxied it in a hole I drilled in a KF25 end cap.
I'm using KF parts because my vacuum system is based on those, but if you are using glass or random copper pipes and jam jars, you can glue this tiny
bulb wherever you like, just make sure it is a vacuum tight seal.
Some epoxies don't behave optimally under vacuum. JB-weld is apparently quite good for vacuum work, but I don't have that in my country, so I had to
try a few out before I found this that degasses the least.
Now the electrical part of the project.
This are the components I used for the Wheatstone Bridge
- R1 100 ohm 1W
- R3 100 ohm 1W
- R2 220 ohm 1W + 100 ohm potentiometer
- Rx 4mm 24V incandescent lightbulb
R1 and R3 can probably be normal 1/4W resistors, I went with 1W versions only to minimize the resistance change caused by joule
heating.
I'm supplying the wheatstone bridge with 5V, so one may ask, why did I buy a 24V rated lightbulb if I'm feeding it with 5V though a resistor of at
least 220 ohms.
Tungsten won't melt at 2000°C, but it will happily burn in a blink of an eye if exposed to oxygen.
A bigger temperature delta will give us a bigger resistance delta, and a bigger range of voltages between atmospheric pressure and "vacuum", but it
will also shorten the life of our gauge, or instantly kill it as soon as we apply power to it.
The 220 Ohm resistor was picked to make the filament barely glow when under high vacuum, and be only a couple hundred degrees when at atmospheric
pressure.
The 100 ohm potentiometer is there for us to set the 0V point at the vacuum lever we desire.
With this configuration the cold filament has a resistance of 60-70 ohm (varies from bulb to bulb) and reaches a max of 320 ohm when heated in a
vacuum.
The voltage output goes from -1.2V to 0V.
If in your setup you start getting positive values, it is ok, the pressure is still going down, as i said, these values are arbitrary, their direction
is more important than their absolute value.
Still, you can adjust the potentiometer to make your lowest pressure 0V.
If one is bothered by the negative voltage, swapping the leads of the multimeter solves the "issue" XD
Here is a quick video of the gauge in action
Attachment: DIY Pirani Gauge pumping down.mp4 (4.2MB)
This file has been downloaded 12 times
Pirani gauges are intrinsically not very precise, and my rendition of one hasn't for sure the best performance, but it allows me to distinguish
changes in pressures of a few Pascals or less
[Edited on 28-6-2024 by Ubya]
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Rainwater
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Quote: Originally posted by Ubya  |
When the filament heats up, its resistance also lowers, so we can correlate the increase in resistance with a lowering of pressure
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Raises
But spot on design.
The light bulb filament is the only thing stopping you from calculating the exact pressure.
I would suggest nicrome wire. It is easier to get and more chemically resistant across your desired temperature range. More importantly, you can
measure its length accurately. You can not do that with a light bulb filament.(its a spring, made from a spring)
Measuring the length, resistance, and weight you can characterize the heater. Them measure the thermal dissipation and compare it to a reference
chart.
stolen from google
As you can see the area useful for radiant thermal emissions is.... more than difficult... to calculate. A stright peace of wire would be much more
manageable
"You can't do that" - challenge accepted
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Ubya
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Thanks Reinwater for spotting my mistke. Of course I meant increases but I managed to screw up while typing (I had to retype the entire post becase
trying to load the video managed to reset the page.....)
Yeah for sure, a more involved design allows a much better control on the various variables, i went with a lightbulb because it is something that
anyone can buy and expect similar performances.
I also tried to make metal to glass vacuum tight seals in the past, and it wasn't as easy as I expected.
There are also ways to improve my design, but I went with the simple stupid one, because it is stupid simple to make.
I will try other designs now 
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Jenks
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This is a nice experiment and I will remember it. I wonder if silicone might work better than epoxy to bond the light bulb to a glass or metal tube.
I've been obsessed lately with the potential of silicone cement.
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Ubya
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| Quote: Originally posted by Jenks | | This is a nice experiment and I will remember it. I wonder if silicone might work better than epoxy to bond the light bulb to a glass or metal tube.
I've been obsessed lately with the potential of silicone cement. |
I haven't tried any silicone based adhesives in this case, but I've tried using silicone caulking in past vacuum experiments.
It is veeery permeable. I thought I could DIY my own gaskets with silicone, but it was like having a leak
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Rainwater
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At that pressure, stay away from epoxy, silicone, rubber, steel, zinc, lead, tin, most copper and a long list of other stuff .
Temperature permitting, carving wax also called Faraday Wax will be the cheapest to acquire and easiest to use.
If your wax feels oily, its no good. You want it rock hard at 22c.
"You can't do that" - challenge accepted
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Twospoons
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Actually nichrome is a terrible choice for a temperature sensing element, as it has a Tcr an order of magnitude less than tungsten (0.0004 vs
tungsten's 0.0045). You'd do better with nickel (0.006) or iron (0.005). Or maybe platinum (0.00392) just because it will be more inert, and is
available in very thin gauges.
Another way to drive the Pirani gauge is to create a feed back circuit that keeps the gauge wire resistance (and temperature) constant by varying the
bridge drive voltage. This keeps the radiated losses constant so only the conduction loss affect the bridge. Bridge voltage is then related to
pressure (probably not linear though - these things seldom are). Easy to do with a single opamp.
The only question I have with DIY Pirani gauges is how to calibrate it?
[Edited on 29-6-2024 by Twospoons]
Helicopter: "helico" -> spiral, "pter" -> with wings
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Ubya
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Exact reason why I added a disclaimer at the top of the post 
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bnull
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Glue a small piece of carborundum with epoxy putty to the head of a 2 mm Ø steel nail and attach the nail to a portable screwdriver. The carborundum
can come from a sharpening stone or from a thick metal cutting disc.
Edit: By the way, nice idea.
[Edited on 29-6-2024 by bnull]
Quod scripsi, scripsi.
B. N. Ull
P.S.: Did you know that we have a Library?
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Ubya
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| Quote: Originally posted by bnull |
Glue a small piece of carborundum with epoxy putty to the head of a 2 mm Ø steel nail and attach the nail to a portable screwdriver. The carborundum
can come from a sharpening stone or from a thick metal cutting disc.
Edit: By the way, nice idea.
[Edited on 29-6-2024 by bnull] |
I have glass drill bits and diamond coated dremel bits (different shapes).
it mostly depends on the type of bulb how easy is to crack it.
someone also suggested me to try the drilling/sanding under water, but I fear it would damage the delicate filament
probably the best way would be to use the flat side of a diamon coated disc
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bnull
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What about waterproof sandpaper for metal with a little bit of vaseline or mineral oil as lubricant? I never had issues with a 150 grain sandpaper at
low speed (<1000 RPM), and the surface gets rough enough to permit a good adhesion.
Quod scripsi, scripsi.
B. N. Ull
P.S.: Did you know that we have a Library?
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Ubya
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| Quote: Originally posted by bnull | | What about waterproof sandpaper for metal with a little bit of vaseline or mineral oil as lubricant? I never had issues with a 150 grain sandpaper at
low speed (<1000 RPM), and the surface gets rough enough to permit a good adhesion. |
I ordered more lightbulbs to make other things. I might use the excuse to try different cutting methods.
I didn't want to use oil in this case because it would have been impossible to remove fully without damaging the filament (maybe soaking for multiple
times in clean solvent) or spoiling the epoxy seal or vacuum
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bnull
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On the other hand, you can use the sandpaper dry. Just blow it and the bulb now and then to cool and remove glass dust. It takes a little longer to
drill but, again, I never had issues.
Quod scripsi, scripsi.
B. N. Ull
P.S.: Did you know that we have a Library?
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