torr to inHG question

[roverfixer]

When I go to an online conversion site ( http://www.onlineconversion.com/pressure.htm ), and enter 40 torr, the answer comes back as 1.574 inches of Mercury.

How does this translate to my roughing guage that reads 29 in HG?
Thnx

[Steve Hansen]

The conversion utility gives you absolute pressure, i.e. referenced from perfect vacuum. Dial gauges measure from the current ambient pressure which can be anywhere from about 600 Torr to 800 Torr depending on where you are (sea level elevation) and the current weather (barometric pressure). If we assume an average sea level pressure of 760 Torr (which rounds to 30 inHg) then you have to subtract 1.574 from 30 (28.4 inHg). 28.4 should be your reading on the dial gauge.