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Air conditioner compressor pressure?

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Arcstarter

Wed Dec 02 2009, 10:54PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Hi guys,

I recently bought an air conditioner compressor, to suck air down from glass globes, or jars, to play around with plasma. I will be using a high frequency high voltage source like a Tesla coil for excitation, and i will likely find some argon to charge it with, and i am getting a bit of helium from my brother and i am buying some valves and fittings...

The question is not if it will work, i know it will. The question is about how hard of a vacuum it can pull. I have asked my dad, who works at a refinery, and my brother that is attending college for instrumentation (both need to be very trained with reading pressure and calibrating meters) and neither have a clue.

I have seen people that got 20" Hg or harder with a fridge compressor, and an air conditioner pulls a harder vacuum, apparently near full vacuum, though perfect is unachievable. Let's say that mine can pull 27" (Sam Barros' compressor from a fridge did 27", and A/C unit's pumps can apparently pull harder vacuums). How many microns would that be? I know nothing of the pressure of vacuums, i know that about 10 microns is around what is found in a vacuum tube, and TV's are around 300 microns or something, which i never considered them to have a very hard vacuum. A large CRT when broken is not as loud as a small tube

.

As a side note, some guy used one of the compressors to fill up a tank, and it was high enough pressure to break the needle on a 200psi gauge. I know that vacuum is miles from measuring high pressure, but just to get an idea...

Also, how exactly should i keep it oiled? Every few minutes of use, before i put the globe on, just a few drops of low viscosity general use oil in the intake?

Thank you guys for any answers! I will be anxiously waiting the arrival of my new compressor, which i will get Christmas

.

Matthew.

Turkey9

Wed Dec 02 2009, 11:43PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I'm pretty sure TV's are at a much lower pressure than that. And they have safegaurds built in to make implosion not as dangerous so i bet that's what makes them more quiet. I'm sure that they need at least as good of a vacuum as tubes if not much much better. Anyway 27" Hg is about 680,000 microns. Vacuum units are very confusing, but google pressure converter and there are a bunch of great unit converters. In terms of playing with plasma, your type of pump will be fine. But in terms of fun physics experiments, you at least need a rotary vane vacuum pump. Those can easily reach 100 microns, and are readily found on eBay.

When using a pump for what it is meant for (like compressing a gas), you can get high pressures. But the type of pump that works great at compression doesn't work nearly as well at vacuum. You can slowly build pressure when compressing using a weak pump and get a pretty high pressure, but not the same when using a weak pump for vacuum. So how well a pump can compress won't necessarily give a good idea of what vacuum it will pull.

I heard somewhere that you shouldn't oil those pumps... They run for years in a a/c unit and never have to be oiled. If there is some oil in it already, that should be sufficient.

rp181

Thu Dec 03 2009, 12:34AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

turkey:
they do need to be oiled. in operation, its a closed loop, so not oil is lost over time. When you disconnect it and use it, oil is lost. You need to pour some oil in the inlet every once in awhile to keep it healthy.

plazmatron

Thu Dec 03 2009, 01:08AM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

An Air conditioning compressor should get down to about 20 microns or so (IF it is a rotary vane pump).
You MUST however use the correct oil for them (vacuum oil, not ordinary mineral oils).

The vacuum in a CRT, is a high vacuum, way way less than a micron. To pump down that far, you would need either a Diffusion pump, or a Turbomolecular pump.

Les

Turkey9

Thu Dec 03 2009, 01:30AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Sorry about that, oil it!!!

Do they use rotary vane pumps in the actual a/c systems? I thought Arcstarter was just going to use a compressor in reverse. I know they use rotary vane pumps to recharge a/c units, and if that's what you're using, it's great! I've had one of those for a few years and done a lot of fun plasma things with it.

klugesmith

Thu Dec 03 2009, 03:19AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Arcstarter: Here's a good beginning tutorial about vacuum levels & units of measurement:

I think the question among us is whether you are getting a refrigerant compressor made to be part of a fridge/AC system, or a vacuum pump used to service air conditioners & refrigeration equipment.

A small, quiet refrigerant compressor may pull a vacuum of "27 inches of mercury". That leaves an absolute pressure of about 1/10 atmosphere = 76 torr = 3" Hg. Low enough for some interesting luminous discharges with high voltage & high frequency, esp. in inert gases. You may be able to get somewhat lower pressure if two are cascaded -- I think the limit is that compressed gas has to push open a check valve.

gives practical hints about lubrication and cooling.

Rotary-vane vacuum pumps used to service refrigeration equipment usually have a minimum pressure rating between 0.01 and 0.1 torr (10 and 100 microns).

When you get down to 1% of normal pressure (7.6 torr), the automobile-service vacuum gage reads 29.6", a value that will depend on your altitude and weather. Time for completely different kinds of vacuum gages, which measure absolute pressure. (I've been reading about these in preparation for potting a HV transformer in oil, first time for me).

Neon signs are filled to a pressure of a few torr, but first are evacuated to much less than 0.1 torr (100 microns) to get the air out. Not to mention the high temperature high voltage bombardment step.

The pressure in practical CRT's (other than "fine beam" tubes for the lecture hall) is less than 1e-6 torr (0.001 micron). But some amateurs have made working CRTs at pressures of hundreds of microns:

Arcstarter

Thu Dec 03 2009, 07:50PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

plazmatron wrote ...

An Air conditioning compressor should get down to about 20 microns or so (IF it is a rotary vane pump).
You MUST however use the correct oil for them (vacuum oil, not ordinary mineral oils).

The vacuum in a CRT, is a high vacuum, way way less than a micron. To pump down that far, you would need either a Diffusion pump, or a Turbomolecular pump.

Les

I have never heard of a rotary vane pump in a air conditioner or fridge. I have always seen piston pumps, and i am pretty sure that is what this is.

I guess this was a bad idea :/. 680,000 microns can not be right, though that is what i calculated. I mean, that is no kind of vacuum :P.

My point about how high pressure they can pump is that internally it is strong.

I have seen pumps on eBay that can get down to 10 microns, which would be nice, but if these could pump down to 20 microns like plazmatron suggests, that'd be fine. I know for a fact these work, though. There is even a video on youtube. This guy got a 'line' of plasma in a jar, probably 6 inches long, with the jar not being charge, and with some 20KHZ and maybe 20Kv with an ac LOPT. If i charge it after evacuating it, and it is in the (probably) 5-30MHz numerous kv field, it should easily be excited. Right?

Thanks for all of the answers. I did the research, but i could not understand it, because 680,000 microns is not low pressure :P.

klugesmith

Thu Dec 03 2009, 11:00PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Arcstarter wrote ...
Thanks for all of the answers. I did the research, but i could not understand it, because 680,000 microns is not low pressure :P.

Do you get it now, the difference between gage and absolute pressure?
When automobile vacuum gage reads 27" that means -27" Hg (-680000 microns) with respect to atmosphere. The corresponding absolute pressure is +3" Hg (80 torr or 80000 microns).
I think terms like Torr or micron are used almost exclusively for absolute pressure.

27" vacuum (80 torr absolute pressure) is nothing to sniff at. Probably lower than the argon pressure in clear light bulbs that can exhibit interesting HV plasma effects. Try it and show us some pictures. Don't forget, it can implode a pickle jar almost as violently as a hard vacuum.

Fabio

Fri Dec 04 2009, 12:42PM

Registered Member #122 Joined: Fri Feb 10 2006, 12:55PM
Location: Milano Italy
Posts: 148

i found an old fridge compressor capable of about 7000 microns of vacuum (7 millimeters or 0.27 inch), not to bad for low vacuum experiment but obviously surely not enough for x-rays, valves, particle accelerators, IEC fusors, LASERs and similar cool experiments

even a high vacuum double stage pump can have problems in reaching his final pressure if the vacuum contaniner is large and/or the pipeworks are long and thin

the pump shown in these photo is able to reach 30 microns of ultimate vacuum with the gauge directly attached on his fitting in about 50 minutes (and about 130 microns immediately after the oil warmup, about 8 minutes in case of a cold start or 15 seconds if the pump was switched off recently)

if i connect some container to this pump the time required to reach his final pressure become outrageous large, for example, to evacuate this maltese cross tube (about 3/4 of a liter) to a final pressure of 42 microns i took 4 days (and i never reached the final pressure of the pump!)

there are many problems regarding vacuum.

first of all, the pumping speed of any vacuum pump will lowering when the pressure inside of the vessel is lowering, at his final pressure the pumping speed is ZERO

second, the fluid dinamic conductance of a pipework is proportional to his diameter inversely proportional to his lenght, and to the difference of pressure between his terminals,
since the pressure differential is almost zero the conductance of a standard pipe is almost zero, and this is the reason because in the ultra high vacuum system only large and short flanged pipework are used

third, when the pressure go extremely down, even the tiniest leak or the smallest outgassing can be a large problem, a simple fingerprint inside on a ultra high vacuum chamber may require more than a week for outgassing, also, if you tighten a screw in a blind hole you leave some air between the hole and the screw, this air will slowly leak inside the chamber and again, weeks may required to degass it!
of course, i made no mention at a real leak!

playing with low vacuum is fun, but plaiyng with high and ultra high vacuum could be painful

ciao!
Fabio

jpsmith123

Fri Dec 04 2009, 01:55PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

If you want to learn about vacuum practice, here is a useful resource:

This place is good too:

If you're going to be experimenting with vacuum, I recommend buying one of the relatively cheap rotary vane pumps of the type used for *servicing* air conditioners...you can get a 2 stage, 3 CFM pump on ebay for less than $200.

I have one of these, which, surprisingly, pumps right down to "zero" on my Varian TC gauge (although it wouldn't go below about 250 microns until I replaced the inlet fitting with one that didn't leak).

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