Q. Your pressure transmitters which are available in compound ranges are calibrated in inches Hg over the vacuum portion of their range, and in psig over the pressure portion. How do I scale my BAS input using two engineering units?

A. Vacuum is usually measured in inches of mercury (''Hg), while positive pressure is measured in pounds per square inch gauge (psig). To interpret the output of your pressure transmitter, the entire range must be measured in the same units, usually psig. 30'' Hg vacuum is equivalent to 14.73 psig. To determine pressure from the 4-20 mA output of a compound pressure transmitter,
you may use the formula:

Pressure (psig) = [(mA-4)/16 x Range] + Offset

Where Range = Total pressure range of transmitter in psig
Offset = -14.73

Example: A PTX1-15 (30'' Hg to 30 psig) is outputting 16 mA. What is the pressure?

Pressure (psig) = [(16-4)/16 x [30 psig - (-14.73)]] + (-14.73 psig) = 18.8 psig

What is the pressure at 6 mA output?
Pressure (psig) = [(6-4)/16 [30 psig - (-14.73)]]+ (-14.73 psig) = -9.14 psig

This may be easily converted into '' Hg vacuum: (-9.14/-14.73 ) 30'' Hg = 18.6'' Hg vacuum