Operation 3-7
January 2006
Measuring Vapor Pressure
With a Pressure Sensor
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)
Measuring Dew Point With
or Without a Pressure
Sensor
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)
P1 = 500 psia
P2 = 14.7 psia
As measured by the GEI chilled mirror dew point sensor:
Tdew2 = –40°C @ P2
e2 = 0.1283 mbar
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:
Tdew1 = –4.022°C
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