2 replies to this topic

[stu]

Dear All,
Please explain how much Vaccum(max) can we create in a system.
Thanks in advance.
stu.

[Art Montemayor]

Stu:

I don't know what level of student you are, but you seem to be the lazy type. All you have to do is use the Google Search Engine and you get:

http://www.newton.dep.anl.gov/askasci/gen01/gen01040.htm

as the perfect answer to your question.

Let your fingers do the walking. Do some of your own research in order to learn how to seek out answers fast and efficiently.

[DRS]

I would like to say something at the cost of being a devil's advocate.

[rant] For a student, an answer as simple as absolute zero pressure is good enough. A starting engineer, however, should first know that creating vacuum is oxymoron. Many of the vacuum pump manufacturers talk about gauge vacuum and specify pump capacities at sea level (but they don't mention it). They never bother to pass on information regarding various correction factors required for specific applications. The only extensive technical write up I could get my hands on was from SIHI and it took me about 10 years. It was, unfortunately, pulled off from their website now.

As we don't use absolute gauges most of the times, it is important for engineers to know why the gauges show different readings at different places though the system is same. It is easy to understand that atmospheric pressures differ at different altitudes, but when it comes to understanding vacuum gauge reading, many people get confused. The maximum attainable vacuum, theoretically, is nothing but the local atmospheric pressure, but on negative side (when speaking in terms of gauge pressure). Then we have many types of vacuum devices and their drawbacks. There are couple of issues that need to be addressed in the link provided by Mr Montemayor. The first comment that the ambient air pressure difference is insignificant is superficial in an engineer's point of view. Secondly, pumps that acheive 10^-9 torr are not that common.[/rant]

My intention is not to point out mistakes in Mr Montemayor's references. Like other members of this forum and other forums, I held him in high esteem all the time and see him in awe most of the times. I am sure that the response was a desperate attempt to correct a lazy student.

If I mean any offence in my post, my sincere apologies. It is due to lack of my knowledge on english but not intentional.

PS: I have attached an excel spreadsheet that calculates atmospheric pressure at different altitudes. I used all the formulae I came across so far and the results are graphically compared in the third sheet. I use the fifth formula for all practical purposes related to HVAC systems and is from US Department of Mining.

Attached Files

•  AmbientPressure.xls   25KB   47 downloads