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September 2005
Special Calibration
Function
Location
Adjusting the
Pressure
VH 30
Constant
Hardware
Adjustment
of the
VH 38
Current Loop
at 4 mA
Hardware
Adjustment
of the
VH 39
Current Loop
at 20 mA
4-6
Table 4-3: Special Calibration Functions
The process pressure constant is entered in bar (absolute), and is used
to calculate ppm
. The moisture unit ppm
v
pressure to the total process pressure and is, therefore, independent of
the process pressure. The reason is that when compressing a gas
(process pressure) all partial pressures increase by the same factor
(Dalton's Law).
The gold/aluminum oxide sensor is selective to water vapor pressure
monitoring a higher vapor pressure when the total pressure (process
pressure) increases. The formula utilized by the analyzer refers to the
total pressure of 1 bar. An elevated pressure of the process has to be
corrected by programming the actual process pressure in bars to the
matrix field VH 30.
The system should be designed to maintain a constant pressure, for
instance, by using a pressure regulator in a bypass system.
Default: 1 bar (absolute)
By connecting an ammeter in the loop, the correct current (4 mA) can
be adjusted by increasing or decreasing the displayed digits.
Note: If the matrix input is locked (VH89), the calibration values are
displayed but the current output is unaffected. To enable adjustments,
VH89 has to be unlocked by entering "50" into this field.
Selection of this field assists during calibration, generating a nominal
20 mA signal, but the actual value must be 21.92 mA, an overrange to a
dewpoint of 22°C. By connecting an ammeter in the loop, the correct
current (21.92 mA) can be adjusted by increasing or decreasing the
displayed digits.
Note: If the matrix input is locked (VH89), the calibration values are
displayed but the current output is unaffected. To enable adjustments,
VH89 has to be unlocked by entering "50" into this field.
Description
is the ratio of water vapor
v
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