Vacuum unit conversions

[madmapler]

I'm confused. Often times I'll look at a pump and it will show vacuum in torrs so I get an online conversion chart and it still does'nt make sense to me. FI this pump I looked at recently says 14 cfm and .75 torr. (Its 1.5 hp.) According to my chart .75 torr is .0296 hg. Another chart says 1 torr=29.88hg(guage)or.03937hg (absolute). Can anyone help me make sense of this?:confused: Thanks.

[BreezyHill]

Sean, here is the table I have used in the past. It has several different ways of looking at measurements of vacuum in a table for so it is easy to correlate the different units of measurement. I suggest finding a unit you are familiar with and go from there. Interesting pump that is rating at such a high " of mercury rating.

Good Luck!

http://www.engineeringtoolbox.com/vacuum-converter-d_460.html

[madmapler]

Ben, The table you showed me happens to be the one of the tables that I am referring to. I dont understand what the difference is between absolute hgs and guage hgs. The other table I look at (listed below) says that .75 torr = the fractional (absolute) hg figure. I'm afraid I'm still lost.
http://users.nlamerica.com/kevin/Pressure.htm

[GeneralStark]

Ben, The table you showed me happens to be the one of the tables that I am referring to. I dont understand what the difference is between absolute hgs and guage hgs. The other table I look at (listed below) says that .75 torr = the fractional (absolute) hg figure. I'm afraid I'm still lost.
http://users.nlamerica.com/kevin/Pressure.htm

What exactly are you trying to figure out? For determining the capabilities of a vacuum pump you should be concerned about its CFM rating at whatever vacuum level (gauge) you hope to operate it at. Torr is a metric reading (mm of mercury) and Inches of HG is English standard.

Inches mercury absolute pressure is the absolute pressure which is gauge pressure + atmospheric pressure. For maple applications you are concerned about the vacuum pressure inside your tubing system, so use the "gauge pressure". Vacuum measurements are relative to atmospheric pressure, If you look at the chart Ben provided, look at the column on the left which is % vacuum. 0% vacuum is absolute atmospheric pressure or in other words no vacuum in your tubing system. In an ideal world, you are shooting for 100% vacuum in your tubing system (difficult to do) or as close as possible.

[madmapler]

In all the discussions I've read about hgs. I never determined that there were 2 different figures to consider. Thanks for all the info.