I picked up a vac pump from the local university salvage today with a 15 millitorr 1.23CFM rating. I get the CFM but what does the millitorr rating mean?
Joe
It’s questions like this that interest me… you have a vaccum pump which will pull a maximum of 1.23 CFM and…?
Now, attacking this logically - a torr is a measurement of pressure, equivalent to 1 mm of mercury. And standard atmospheric pressure is 760mm mercury.
Okay, so - a millitorr would be 1/1000 of a torr, or 1/760,000 of an atmosphere of pressure.
15 millitorr has to be how low it can get the pressure. 'Cos if lowering the pressure in a vessel or container by 15 milltorr was all it could do, well, you can get the pressure down a hell of a lot more than that by sucking on it with a plastic straw. It wouldn’t be a vaccum pump with a rating like that, it’d be a fan…
So, your vac pump can at least theoreticly get the pressure down to 15/760,000 of an atmosphere or 0.00002 atm. This is equivalent to 0.00029 PSI or about 1.2 Pa or 0.2 kg/sq meter… that is a pretty impressive vaccum pump rating. It wouldn’t be pulling 1.23 cu ft/min at that pressure, but in reasonable vaccum bagging ranges it’d probably draw plenty.
If the rest of a vaccum bagging system would stand it, I think it’s squash your foam blank flat. I’d suggest that you couple this beastie with a vaccum switch as mentioned in some of the other vaccum bagging posts. I’d think this, coupled with some sort of inline collection system for resin and such , will handle any vaccum bagging work you intend to do.
Good score. I must see what my local college has gathering dust in the back rooms.
hope that’s of use
doc…
ummm, i stand corrected… thanks Glenn. sounds like a great pump…
peace,
Brennan
this took 1/2 second to find on Google by typing in “millitorr”:
The More Common Units of Pressure Measurement
Traditionally, the pressure in a system is stated in terms of the height of a column of mercury that may be supported by the pressure in the system. At one standard atmosphere the force is 1.03 kg/sq. cm (about 14.7 pounds per sq. inch). This pressure will support a mercury column 760 millimeter high (as in a barometer). One millimeter of mercury is the equivalent of 1 Torr. A thousandth of a millimeter is referred to as a micron of mercury or, in more current terminology, 1 milliTorr (mTorr). To be proper in the modern scientific world, the SI system of units is used. Here pressure is referred to in terms of newtons/sq. meter or Pascal ¶. To convert Torr to Pascal, divide by 0.0075.
Millitorr is the same measurement as Micron. His pump is a 15 micron pump or a 15 millitorr pump, same thing. It is a powerful pump by most standards, a oil rotary vane for sure. Micron and Millitorr go from 1 (29.91996"Hg) to 1000 (29.88"Hg). For those (lab workers) that are really into the higher vacuum levels this is considered a Medium Vacuum pump often just called a roughing pump.
When you go above 1 micron you use other measurement like TORR (mm Hg.) and you are getting into High Vacuum level, which is from 1x10(-3) to 1x10(-6) TORR (mm Hg.). Above 1x10(-6) Torr you are getting into the Very High Vacuum range. So there are Low, Medium, High, and Very High vacuum levels.
There is also the measurements of Pascal, Mbar, and Atmosphere. It can be confusing.
But Very High Vacuum is not something that is easily obtainable without very expensive pumping equipment. So when you hear someone say “I’ve got a pump that can pull 30"Hg of vacuum”…well they probably don’t.
The pdf chart I’m attaching does not go into the upper levels of High Vacuum, or the levels of Very High Vacuum at all.
Remember, (and please correct me if I am wrong) but atmospheric pressure represents the upper limit of the force that can be exerted by a vacuum setup. in other words, it’s not like you could pump such a vacuum that it would crush anything…and anything reasonably close to a high vacuum would be near the max as far as the PSI pressure exerted on the vac bag is concerned. The fine differences between 1/20 of an atmosphere and 1/100 of an atmosphere and 1/100,000 of an atmosphere might matter if you were building an electron tube, but not for vac bagging.
right?
Thanks everybody for your input it was very helpful. I really liked Docs’ image.
“15 millitorr has to be how low it can get the pressure. 'Cos if lowering the pressure in a vessel or container by 15 milltorr was all it could do, well, you can get the pressure down a hell of a lot more than that by sucking on it with a plastic straw. It wouldn’t be a vaccum pump with a rating like that, it’d be a fan…”
Joe
Wayull… the physicist Richard Feynmann was kind of an intellectual hero of mine, and he described how he would work with an idea: by constructing a mental picture of it. And in my far less sophisticated way, I try to do the same thing…
Also, I cheat. In top of the desk is something called the Pocket Ref ( http://www.sequoiapublishing.com/pdt_pocketref3ed.htm ) and one of these little marvels, the HP11c that was given to me in school when the previous owner hated RPN ( http://www.hpmuseum.org/hp11c.htm ) so that I could do some quick and dirty figuring…
To start with, a torr. From the Pocket Ref, I find out that it’s the pressure equivalent of a millimeter of mercury in a mercury barometer. So-
760 torr = 760mm Hg = 1 ATM = about 14.7 PSI = about 34 feet if your barometer uses water.
A millitorr is 1/1000 of a torr ( thank Huey for a technical education )
So, in a pale attempt at playing Feynmann, I can start making a mental picture:
15/1000 of a torr is 15/760,000 of an atmosphere or the rough equivalent of 1/50,000 of an atmosphere - 1/50,000 of a 34 foot column of water. Calculator time, or do very, very rough figures 'cos I just discovered the batteries in the 11c are dead, less than 1/1000 of a foot.
Typical straw that you get with your jumbo sized Frozen Dacquiri is at least 10 ", so that is in a whole different order of magnitude from 15 millitorr. If it’s a vaccum pump, and it can only get the pressure down by 15 millitorr, it’s a pretty sad excuse for a vaccum pump. The rating has to be what said pump can get the pressure down to.
And that’s where the picture comes from.
Glenn now has me curious about 'oil rotary vane ’ pumps… and that’s what I get a kick out of, so many new things to find out.
“I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”
In Reply To
Glenn now has me curious about 'oil rotary vane ’ pumps…
Doc, if you don’t mind me giving you my take on oil rotary vanes.
Oil rotary vanes are fine if you either have a use for the higher vacuum level or if you build a system like that described in the joewoodworker.com site. Then you could use lower vacuum level to make an EPS board or use a higher vacuum level to make for instance fins or do other composite things.
Most oil rotary vanes are meant to run cooler at full vacuum. If let ran without pulling a vacuum for too long most will overheat or get hot to the point of causing excessive wear (unlike a vacuum cleaner which is just the opposite). Pumps like the Robinair, J/B, Ritchie would typically be considered throwaways for the most part, but if taken care of will last a good long time. They are used mostly in the air-conditioning maintenance/repair industry. New these pumps can be had for around $250 these days…can you say C h i n a…
Pumps like Welch, Leybold, Alcatel, Edwards, etc are typically lab type pumps that are rebuildable and much more durable, but cost very much new. Typical full rebuild kits (do it yourself) for smaller versions of these are around $250+. So if you buy off eBay consider this. They also for the most part come equipped with special vacuum fittings for example like the “NW25” or “NW40” which need centering rings and clamps, all of which can be pricey too. New these pumps alone start in the $1200 range.
Oil is also pricey for rotary vane vacuum pumps, and for the later ones I mentioned, as well as the others too, a gallon of vacuum pump oil is around $20 from vacuum pump sales/repair businesses, unless you have a connection with an oil distributor.
Also used lab pumps off eBay could have been used in conjunction with dangerous gases and chemicals. Wear gloves and don’t drink the oil.
Many thanks, Glenn- it’s a pleasure to hear from somebody who obviously works with stuff and knows it.
Had a quick look-see on the subject, as much to see how they work as anything else, stumbled across http://www.bestechind.com/VP_Vacuum_Pumps.htm and the diagram at the bottom intrigued me - put me in mind of the Wankel engines I was a bit more familiar with. Visualising the thing in operation, and you have a very interesting rotary engine variant.
The geometry of it is kinda cute too - the vane must move through the rotor shaft and the sum of the projecting lengths of the vane from the rotor must be a constant.
The vaccum oil must be interesting stuff too. No violatile parts to speak of?
Yep, like I said, it’s questions like this that interest me.
Thanks again, Glenn
doc…