GE Optica Operator's Manual page 51

Measuring Vapor Pressure
With a Pressure Sensor
Measuring Dew Point With
or Without a Pressure
Sensor
Operation
To measure vapor pressure with a 4-20 mA, 0-30 psia pressure sensor,
connect the equipment as shown above and make the following
entries in the Pressure Input menu section.
•
Input: Iin 4-20
•
Units: psia
•
Upper: 30.00
•
Lower: 0.00
In addition, to accommodate the pressure expansion, the following
entries are required in the Process section:
•
Status: enabled
•
Pressure: 500 (specify the process pressure with the same units
as the default pressure specified above)
In Scenario 2, the dew point measurement requires both the sensor
pressure and the process pressure to be known. Sensor pressure can
be entered as a default value, or measured, as above, and the process
pressure must be entered in the Process Pressure field.
Example (using standard atmospheric conditions at 25°C)
As measured by the GEI chilled mirror dew point sensor:
Tdew2 = –40°C @ P2
Per Dalton's Law of Partial Pressure:
e1 = (P2/P1) × e2 = (500/14.7) × 0.1283 = 4.36 mbar
Using standard vapor pressure equations, the actual pressurized dew
point is calculated by the Optica to be:
P1 = 500 psia
P2 = 14.7 psia
e2 = 0.1283 mbar
Tdew1 = –4.022°C
January 2006
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