Vacuum pump

[Alberto]

Hello.

I want to buy a Vaccum pump to make experiments with plasma, make a CRT and that kind of things. I think it´s probably that the final vaccum is sometimes a "lie" like the lumens in the flashlights. Of course good brands like Yellow jacket are reliable.

With a budget about 300$ I can get a Yellow jacket YJ II that gets 20 microns vaccum. The question is, with a 2 stages vaccum pump is it possible to get vaccums bigger that 15-20 microns or not really? To know if it´s worth to expends a little bit more and gett a greatter vaccum.

Or maybe 5 microns is not really a big difference here and I can archieve that with a getter

Thank you

[Twospoons]

I don't think I've seen a mechanical pump spec to below 25u.  But thats plenty low enough for plasma experiments.  You can get low enough for that with just a water aspirator, especially if your water is cold.
Below that you  need a diffusion pump (+cryo trap?) or turbomolecular pump as well. That would get you into CRT territory.

[klugesmith]

I agree that 25 micron pressure is reasonable with a two-stage rotary vacuum pump. Low enough to extinguish most easy plasmas. Low enough for CRT play, as seen at sparkbangbuzz.com.  Set aside some money for a vacuum gage.

[Alberto]

Thank you for your answers!

Any advice about the vaccum gage? I have no idea about this gages.

[alan sailer]

The cheapest I know of are the standard dial type. They will not tell you what your ultimate vacuum is but
can let you know if your system has a leak.

The next step up is a thermocouple sensor and readout. They can get down to 10-3 torr.

After that the gauges get crazier and more expensive. Convectron is the best known. Capacitance manometer
(Baratron) is another brand that measures absolute pressure, very useful if you are dealing with different gasses at higher
pressures(plasma tubes).

Ebay is your best friend here as new vacuum gauges can be very expensive.

[davekni]

Any advice about the vaccum gage? I have no idea about this gages.

The next step up is a thermocouple sensor and readout. They can get down to 10-3 torr.

Not sure what thermocouple sensors you are thinking about.  The ones I'm familiar with do not start working until under 10-3 Torr, good from there down to 1 micron or so (0.001 Torr).  I have such a meter by "Yellow Jacket".  It claims to work to 760 Torr, but doesn't really function until under 10 Torr.  Rated down to 1 micron.  My  single-stage vacuum pump can't get low enough to test that range.

Before buying the above commercial one, I made a simple sensor.  Took a small incandescent bulb (one from a series-connected Christmas light string), broke off the tip, and inserted into rubber vacuum hose.  Used tungsten temperature coefficient for sensing and same element for heating.  Simplest circuit is to apply power through a resistor.  Low enough power to avoid burning out filament, but high enough to raise filament resistance by 3-4x.  Measure filament or resistor voltage as vacuum increases.  Filament resistance will start increasing (temperature increasing) as mean-free-path of air molecules approaches the distance from filament to glass housing.  Above that pressure, thermal conductivity of air is roughly constant.  Below that pressure, conductivity drops as pressure decreases.  With ~2mm from filament to housing, temperature increases significantly when mean free path is that length, around 50 microns pressure for 2mm.  Certainly not a precision tool, but provides a rough idea of pressure getting reasonably low.

[alan sailer]

I was talking about the ordinary TC gauge. I used one on the fusor I built a few years back because all my fancy gauges were going nuts from all the electrical noise. The operating pressure of the unit was about 20 microns. The gauge was really all I needed since I had already verified that I was getting vacuum with a Convectron gauge.

It gave readings up to atmosphere but since the meter was an old old galvometer with a logarithmic looking display telling higher pressures was pretty inaccurate. But you could use it to verify a bad leak.

Is there a better intermediate priced vacuum gauge between a Bourdon gauge and a Convectron that I am unaware of?

[Twospoons]

http://www.cientificosaficionados.com/tbo/medidores%20vacio/medidor%20de%20vacio.htm
http://www.belljar.net/articles.html

[davekni]

The next step up is a thermocouple sensor and readout. They can get down to 10-3 torr.

I was talking about the ordinary TC gauge. I used one on the fusor I built a few years back because all my fancy gauges were going nuts from all the electrical noise. The operating pressure of the unit was about 20 microns. The gauge was really all I needed since I had already verified that I was getting vacuum with a Convectron gauge.

Alan:  Did you intend to say 10-3 microns rather than 10-3 Torr?  That would make more sense.

[alan sailer]

I have attached a picture of the gauge I was using for the fusor. According to the gauge the readings are in millitorr.
Unless I am going crazy (always a possibility) a millitor is 0.001 torr. And according to the internet 1 micron is equal
to 1 millitorr.



In any case I was just trying to suggest to the original poster that a TC gauge might be useful. I could be totally wrong.

[davekni]

I have attached a picture of the gauge I was using for the fusor. According to the gauge the readings are in millitorr.
Unless I am going crazy (always a possibility) a millitor is 0.001 torr. And according to the internet 1 micron is equal
to 1 millitorr.



In any case I was just trying to suggest to the original poster that a TC gauge might be useful. I could be totally wrong.

Agree.  Millitorr is the same as micron.  Such a TC gauge would be perfect.  I was just confused by the post earlier saying 10-3 torr, rather than 10-3 millitorr or 10-3 microns.

[Alberto]

Thank you for all your help.

So 2 things. Probablye a vacuum pump from VEVOR of 23 micros are scam right?

And in Spain it is hard to find technical equipment. One like this one it would be good for that experiments right?

https://www.elitecheu.com/products/elitech-wireless-micron-gauge-1-19-000-microns-digital-vacuum-gauge-for-hvac-and-automotive-refrigerant-vacuum-meter-1-4-sae?_pos=2&_sid=de2604845&_ss=r&variant=43105279410435

I would prefer one like alan sailer posted, but I cant find, but the one from the link is in amazon for 100$

[davekni]

So 2 things. Probablye a vacuum pump from VEVOR of 23 micros are scam right?

Might be a bit optimistic for a single-stage pump, but not way off.  A two-stage pump is likely to be lower than 23 microns.  23 microns is about 3 Pascals in metric units.  VEVOR appears to have a typical range of vacuum pumps typically used for clearing refrigerant lines before filling.  I'd guess you could achieve 5 Pa with single-stage pumps and below 1 Pa with two-stage pumps.
Be aware that pumps are listed for "ultimate vacuum".  This is what can be achieved with no input flow and a long run time to get pump internals all degassed.  Expect this value if connected to a vacuum gauge and nothing else and run for 30 minutes (and start with clean oil).

https://www.elitecheu.com/products/elitech-wireless-micron-gauge-1-19-000-microns-digital-vacuum-gauge-for-hvac-and-automotive-refrigerant-vacuum-meter-1-4-sae?_pos=2&_sid=de2604845&_ss=r&variant=43105279410435

No specific information, but that looks like a good vacuum gauge.  Typical specifications for a thermocouple (or other related gas thermal conductivity) measurement method.

[Alberto]

So 2 things. Probablye a vacuum pump from VEVOR of 23 micros are scam right?

Might be a bit optimistic for a single-stage pump, but not way off.  A two-stage pump is likely to be lower than 23 microns.  23 microns is about 3 Pascals in metric units.  VEVOR appears to have a typical range of vacuum pumps typically used for clearing refrigerant lines before filling.  I'd guess you could achieve 5 Pa with single-stage pumps and below 1 Pa with two-stage pumps.
Be aware that pumps are listed for "ultimate vacuum".  This is what can be achieved with no input flow and a long run time to get pump internals all degassed.  Expect this value if connected to a vacuum gauge and nothing else and run for 30 minutes (and start with clean oil).

https://www.elitecheu.com/products/elitech-wireless-micron-gauge-1-19-000-microns-digital-vacuum-gauge-for-hvac-and-automotive-refrigerant-vacuum-meter-1-4-sae?_pos=2&_sid=de2604845&_ss=r&variant=43105279410435

No specific information, but that looks like a good vacuum gauge.  Typical specifications for a thermocouple (or other related gas thermal conductivity) measurement method.

Thanks for your answer, and sorry, I meade a mistake, I mean 2-3 microns.

And looking for the pump in the english web of bebor, they don´t even know how to make the conversion. They say that the pump get 0,3 pascales, 25 microns.

I´ll go for the yellow jacket.

And about the gauge, Imust look for  a termo couple type right?

[klugesmith]

Thermocouple or Pirani type gage, or a variant called Convectron. Dave may have mentioned a DIY Pirani gage.

Pirani has heated filament connected as one segment of a Wheatstone bridge.  The electric power to maintain constant R, so constant temperature, goes down as gas pressure is reduced.

 

[davekni]

Pirani has heated filament connected as one segment of a Wheatstone bridge.  The electric power to maintain constant R, so constant temperature, goes down as gas pressure is reduced.

Wasn't familiar with the label "pirani", but that is actually what I built.  Held filament at constant temperature and measured power.  Described a simpler version in my previous post here.

The common theme is that all these related gauges measure thermal conductivity of gas to deduce pressure.  Temperature of a heated element is measured/controlled and power to keep it hot is also measured.  Thermocouples are only one method of measuring temperature.

Although 1 micron resolution is common for such gauges, accuracy isn't likely better than 5 to 10 microns.  Not sure you will be able to tell if you are achieving 2 or 3 microns vs 7 or 8 microns.  I don't know much about gauges for lower vacuum levels.  I think we had an ionization gauge in the physics department along with a diffusion pump.

[Alberto]

Yes, I have built a pirani gauge, the problem is I need to calibrate it. I mean, I need a cacuum meter to know the values of the resistance at diferent presures

[klugesmith]

One can calibrate the electronic gauge using a McLeod gauge.
That's what Mr. Pirani did, as a step toward having fewer McLeod gauges to maintain in the factory where he worked.

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I have a few McLeod gauges, and enough clean mercury, but never enough bench space to do the exercise.

[Twospoons]

And if you don't have a McLeod gauge you'll find buying one is more expensive than buying a used commercial vacuum gauge, like a Baratron.
Best is if you have a standards lab nearby who can do the calibration for you. Probably similar money though. But at least you will be referenced to a known standard. One difficulty with Pirani gauges is the response curve varies dependent on the gas being measured, so ideally you would calibrate against multiple gases, or at least the ones you want to use.

It also occurred to me it would be possible to do a crude calibration based off the boiling points of various liquids - heavier compounds boiling at lower pressures. Liquids boil when the ambient pressure matches the vapor pressure of the liquid.

[klugesmith]

>>Liquids boil when the ambient pressure matches the vapor pressure of the liquid.

Good idea there. It's easy to find youtube videos showing water boiling at room temperature, when pressure gets below 15 or 20 torr.

I bet familiar boiling gets weird at sub-torr vapor pressures, for two reasons.
R1: Hydrostatic pressure is significant at very small depth below liquid surface.
R2: Surface tension significantly increases pressure inside vapor bubbles.

Has anyone here explored those issues?