A dirty vacuum guy

This is a beginning. Of what, I don't know yet. I intend to write about the world of industrial vacuum. From time to time, as subjects or applications arise I will post some notes. I don't know how I'm going to do it without giving away too much detail (industrial spies, you know), which could affect my work, but I'll try. It will be a learning process, for sure.

I have spent 40 years in the field, working for a number of companies that produce a wide variety of types of vacuum pumps. The one thing that they all had in common was that the pumps they produced were used in the rough vacuum field. This covered vacuum levels from atmospheric to 29.9 in.HgV (0.5 torr). We would occasionally combine them with roots-type blowers to pump down to about 20 microns.

In my experience, folks who work in the high vacuum field tend to look at us with scorn, since we are in the "dirty" vacuum range. But, I would contend that they would have difficulty with many applications in our range. Some things get more complicated in our world; take pump-down.

Pump-down equations use average volumetric flow (acfm) to determine time to evacuate a closed chamber. this is OK when a pump that can pull down to a deep vacuum (low absolute pressure) is used, since the acfm curve is flat, thus the average acfm is, well, average. What I mean is that it stays constant, whereas a rough vacuum pump may have a curve that is, well, a curve. Being a curve, the average is a variable. In order to calculate the average it is necessary to perform the calculations in steps, then total all the averages. The points between the two pressures must become narrower as you approach the pump's ultimate pressure, since pump-down is logarithmic. It should be noted that the higher the pump's ultimate absolute pressure, the more pronounced the curve.

For the high-vacuum guys, the equation is simple. They perform it one time. For us dirty guys, we must perform it many times. A spreadsheet makes this process quite easy, however. It is also possible to create a list of correction factors, based on the pump's ultimate pressure. This can get complicated, however, since there are many possible ultimate pressures.

Anyway, that's the first post. I welcome your comments--if you don't mind getting "dirty".