January 2006
3-4 Operation
Process Pressure The Optica provides several ways of measuring or specifying sample
gas pressure (needed for pressurized humidity measurements). You
can:
• actively measure the pressure at the dew point sensor, or
• manually enter the pressure, if it is known and stable, or
• “sample-off” a high-pressure gas sample for measurement at a
lower pressure (usually atmospheric) and let the Optica calculate
dew point at the process pressure.
For example: a sensor with a maximum pressure rating of 300psi
cannot be subjected to a process pressure of 500 psi. A sampling
system can be arranged to allow the measurement to be made at
atmospheric pressure. The Optica can then calculate and display
the dew point at the process pressure (see Scenario 2 on page 3-6).
Actively Measuring
Process Pressure
Use when the process pressure is within the specifications of the dew
point sensor and can be measured directly. In the Pressure Input
menu, set the Pressure Input to V or I as appropriate for the pressure
sensor. Set the Process Pressure Status to Disabled.
Note: The pressure measured by the pressure sensor will be used to
compute the vapor pressure.
Manually Entering
Pressure
Use when the process pressure is a known and fixed value, and will
not be actively measured. In the Pressure Input menu, set the Pressure
Input to Use Default. Enter the pressure into the Default field. Set the
Process Pressure Status to Disabled.
Note: The default pressure entered will be used to compute the vapor
pressure.
Measuring at a Different
Pressure
Use when: the humidity must be measured at a pressure that is lower
or higher than the process pressure, but the reported value must
represent the humidity at the process pressure. Scenario 2 below gives
examples of programming the Optica for this case.
Note: The process pressure is entered manually and the humidity
sensor pressure may be measured or manually entered.
Examples of the use of the Process Pressure menu are shown
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