I have an empty beer bottle and a vacuum capable of pulling 26 inches of vacuum, which I believe is about 2 psi of absolute pressure. Is the beer bottle strong enough to withstand that? How about a champagne bottle?
Yeah, no problem with the strength of either. Vacuum tubes were evacuated down to a few microns or less, with walls perhaps 1/5 the thickness of a beer bottle. There could be a flaw that would allow implosion, but a beer bottle is small enough that the velocity of any shards that missed the middle would be pretty low. You might have issues getting it clean enough inside and a good enough seal at the mouth to not leak down fairly quickly, so you will probably have to keep pumping on it.
But you can’t equate in-Hg gauge vacuum to absolute pressure without knowing the ambient pressure which is NOT barometric pressure unless you are at sea level. Derate the expected pump rating by 1"Hg for every 1000’ elevation.
The range of vacuum you mention is very soft…basically only enough for clamping applications, bleeding brakes, testing auto accessories. And not deep enough for AC evacuation, making water boil, nor any cool scientific stuff that happens when you get down in the micron range.
You just answered another question that I have had for about a year. I have used this pump for evacuating A/C’s in cars and when I lived in NY I could get about 28 inches (according to the gauge). Now I live about 3000 feet up in CA and my gauge will only go down to about 26 inches. I no longer use it at work for that reason. I did not know altitude would affect it.
Now that the OP has been answered, can anyone help me get a whiskey bottle to hold a feather duster? A gin bottle to do windows? I’d like to get the entire bar to help clean up the place.
Thanks.
I could be wrong here, but I don’t think the altitude is affecting the operation of the pump at all. It’s simply that when you’re higher up, the ambient atmospheric pressure is lower, so the difference in pressure between the “vacuum” and the atmosphere is less than it would be at sea level. That doesn’t mean that the actual hardness of the vacuum inside the evacuated vessel is any less. Or is there another factor at work here?
To be clear, all of these measures of pressure in length units (inches or microns) are referring to the pressure of a mercury column, right? It seems odd to me that those units would still be used once you got down to microns, since you’re obviously not going to have a “column” of mercury that low.
I can’t help you with that, but I can get a surprising amount of freshly cut thyme into a beer bottle, like in that Jim Croce song.
:smack: …and I’ve been stuffing it into a coconut all this time!
What, like an hourglass?
An inexpensive vacuum gauge compares the pressure inside the evacuated vessle to atmospheric pressure and displays the difference. Thus it will measure a perfect vacuum as ~29.6"Hg in Miami, and ~24"Hg in Denver. Note: The Barometric pressure reported for Denver will always be a value that has been adjusted by a Kollsman table offset to sea level equivalence, Thus Barometric pressure, and ambient pressure are only the same for sea level locations. This is done so that weather forcasters don’t have to account for the altitude of each reporting station.
This article (not a PDF) explains it pretty well:
http://www.belljar.net/basics.htm
In short:
There is no gauge that works well from atmosphere down to zero pressure. Several types are used, which have overlapping ranges to provide coverage.
At mild vacuum, the pressure is measured directly either by manometers, or mechanical gauges with diaphrams or bourden tubes. These are typically referenced to atmosphere, NOT zero pressure. These are known as gauge readings (vs absolute readings) and are properly written with a “g” appended to the units. (23"g for example.) Even absent the “g” When the units are "Hg, or "H2O you can be pretty sure that the measurement is intended to mean “depression below atmospheric” NOT “pressure above zero”. (Except when you are talking about the atmospheric pressure itself!) When the units are microns this almost always means microns of Hg above zero. I don’t know what units are used for weak vacuum in metrified locations. You can usually assume “inches” means inchesHg, if it is water column reading it will either say H2O or WC…but some trades get sloppy this way, and air conditioning techs that work with both the refrigeration and air handling sides can end up confused.
Such measurement is sufficient for many purposes like measuring engine intake pressures, barometrics, even altitude, but not quite good enough to insure good evacuation of Air Conditioning systems. Normal changes in barometric pressure, not to mention altitude mean that there is no reliable way to tell if a gauge reading indicates a dry system, or one that still has a lot of water in it. Lacking a better gauge, the procedure is to pump for a long time, then test and see if a sealed system will hold the gauge reading…This is usually good enough, but there is know way to tell a leaky system from a wet system. A better equipped refrigeration tech will have a thermally based “micron gauge” and will only need to pump as long as required to dry the system. A tech with a micron gauge probably is getting paid by the job instead of by the hour.
At lower pressures, the units still represent the height of a perfect vacuum referenced mercury column. No, you can’t measure an actual mecury column with micron precision, so even though those are the units, the measurements are done other ways.
One way is to take a large volume sample at the pressure to be measured, and compress this to a much smaller volume (higher pressure) where you can make a reasonable measurement. This is how a McCleod gauge works.
Another way is by measuring the insulating properties of a vacuum. These are known as Thermocouple or Pirrani gauges.
Finally, at very low pressure levels, you can measure the electrical properties of the remaing gas, and these are known as ionization gauges, of which hot cathode, and cold cathode are the two basic types, with some variations within those two types.
All of these lower pressure techniques are subject to errors due to gas composition, condensation, etc. There is quite a bit of knowledge involved in the technique of high vacuum.
If this vacuum stuff interests you, you could do much worse than Strong’s “Procedures in Experimental Physics” as a starting point. Alibris often has used copies in good condition at wildly variable pricing.
I understand, but it’s lime in the coconut, and thyme in a bottle.
Ah, one thing I couldn’t figure out how to say well in my last post:
A huge amount of confusion is caused by measuring “vacuum” rather than pressure. Vacuum is not a thing like pressure, it is the lack of pressure, and pressure is only positive, never negative…but lots of trades continue to use vacuum (negative) pressure measurements, because gauges that use the atmosphere as a reference are cheaper than gauges that use a deep vacuum chamber (anaeroid) as a reference.
So on one scale a perfect vacuum is about -29.6"HG and on the other it is zero. Sort of like Centigrade vs. Kelvin…the numbers go in opposite directions below zero C.
To be fair to my little vacuum, it does not come with a gauge. I rigged a vacuum gauge to an A/C schrader valve connector to monitor the vacuum in the system and to check for leaks. I believe my vacuum is rated to 75 microns but my gauge never read much below 28 even at sea level. I guess the accuracy of my gauge might be to blame too as Kevbo mentioned since it was originally meant for engine intake vacuum.
Millimeters mercury is equivalent to Torr, which is the unit all of my vacuum gauges down to 10^-4 Torr have used. I’ve certainly seen mass specs that were pulling 10^-7 Torr. Granted, it is not an SI unit, but it is extremely common in measuring vacuum well below where any actual column of mercury would be used.