7 replies to this topic

[Art Montemayor]

Okara:

This is a good student question because it is candid, honest, and straightforward. This is a good example of spotting something that doesn’t seem to make sense and asking the question – without being afraid of admitting ignorance or failure to understand. As it turns out, I believe the employment of a negative pressure is bad and misleading nomenclature used by either lazy or ignorant engineers. I believe whoever wrote that notation was trying to describe a partial vacuum and they used a negative notation thinking that the pressure scale is analogous to the temperature scales. It is not.

I am going to assume what you are dealing with are partial vacuum pressure values. If so, then there is only one accurate and simple way to talk about vacuum, in my opinion: in absolute pressure units. That way, there is no confusion created in dealing with the values and communicating with them. For example, do you mean (by -0.8 bar) 0.8 bar below atmospheric pressure (whatever that is, where you are at)? If you stated, instead, 0.2 barA (approximately) everyone would understand what you mean. If you know the value of the local atmospheric pressure, then you can quantify the exact value of the absolute vacuum pressure. Always employ absolute pressure values when dealing with vacuum. This leaves nothing to conjecture or guess.

The temperature scales are not analogous to the pressure scale because in the pressure scale(s) the value of “zero” gauge pressure depends on where you are located at. In other words, the base atmospheric pressure differs with location. This is not a factor in the temperature scales and that is why the negative values of the gauge temperatures are understandable and accurate to describe.

If you are working with vacuum and would like to have an Excel workbook that explains vacuum technology and its workings, send me your email and I’ll send you a copy of it. Vacuum readings and notation are so varied and misunderstood that I had to prepare this workbook for young graduate engineers that worked under me some years ago. This helped in training them to understand just what vacuum is and how it is measured and handled.

I hope this helps out.

[Ankur Shah]

umm.. why not just attach the file to the forum?

it would work better and fewer posts also!

Ankur