From the support site of the software I'm using:
Gas pressure will always be measured as a percentage. We do not support other units, as conversion between them is error-prone. Using percentage rather than other units is also in-line with our existing strategy for Giesen, Loring and Coffeetool.
This is a list of specifications that a gas pressure meter must adhere to, in order to get the best results in conjunction with Phidgets boards.
Pressure meter with an output range from 0-5V (preferred) or 4-20mA.
The output of the pressure meter must be calibratable. e.g. if the meter measures from 0-4 bar and the pressure from the roaster is 0-2 bar we must be able to set the input maximum on the pressure meter on 2 bar. Note: If this is not possible, it means that the RI will show incorrect values if the max/min value can't be reached)
The pressure output signal is expected to be between 0 and 100%. In the above example 0 bar is 0% and that 2 bar is 100%.
If the pressure meter outputs Volts (0-5V), one can connect it directly to a Phidgets 1018 or Phidget 1019 via a 3-pin 30XX sensor cable (XX means that there are different length available).
If the pressure meter outputs Milli-Ampere (4-20mA), one needs a Phidgets 1018 or Phidget 1019 as well as a Phidget 1132 (there is also an acrylic enclosure available).
There are active and passive 4-20mA loops. Which defines the physical location of the ground. Please refer to the the Phidgets 4-20mA Interface Primer which contains wiring diagrams.