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XDT User’s Manual
18
parts per million by volume computed at the sensor pressure (more about pressure later). The LBS
and G/M
3
are density measurements, pounds of water per million standard cubic feet and grams
of water per standard cubic meters, respectively, both in Natural Gas, they are computed accord-
ing to data derived by IGT Research Bulletin 8, taking into account sensor pressure.
Note that the analog output is linear to the selected engineering units, therefore be mindful that
while scrolling through various units the analog output may change even though the measured
dewpoint is stable.
The PSI legend flashes at the bottom of the display, when there is pressure correction in the com-
putation of the displayed values. A short press of the ‘pressure correct’ button toggles the unit in
and out of pressure correct mode. When there is no pressure correction applied, the PSI legend
does not appear.
Sensor Pressure is used in the context that this is the pressure inside the sample cell when per-
forming the measurement, i.e. it is the operating pressure of the sensor.
Gas Pressure is used in the context that this is the pressure at which the dewpoint is to be calcu-
lated.
Pressure Correction is used in the context that the values displayed signify the moisture content at
some pressure (we refer to this as the ‘Gas Pressure’) different from the pressure at the sensor.
Note that PPM, LBS and G/M
3
readings are by definition unaffected by pressure correction
because only the pressure at the sensor affects their value. While °C and °F are affected by pres-
sure correction by reporting what the dewpoint would be at the Gas Pressure when the dewpoint
is what is measured at the pressure at the sensor. However, this also implies that whether pressure
correction is applied or not the PPM, LBS and G/M
3
readings are affected by the setting of the
sensor pressure.
A long press of the Pressure Correct button, while in the pressure correct mode (flashing PSI leg-
end), changes the unit to the View/Set Sensor Pressure sub-state. The display has the ‘SET’ and
‘PSI’ legends on, and alternately shows SEN and the currently set value for the sensor pressure.
The up and down buttons allow the user to modify the sensor pressure, while a short press of the
pressure correct button toggles the Sensor Pressure setting between whatever value is on the dis-
play and 14.7 psi ---- this is a quick way to go back to atmospheric settings. A long press of the
pressure correct button changes the unit back to the Viewing Dewpoint Mode. Pressing the
‘Mode’ button changes the unit to the View/Set Gas Pressure sub-state. The display has the ‘SET’
and ‘PSI’ legends on, and alternately shows GAS and the currently set value for the gas pressure.
The up, down and pressure correct buttons operate in the same manner as in the Sensor Pressure
sub-state. Pressing the ‘Mode’ button changes the unit back to View/Set Sensor Pressure sub-
state, and so forth.
Notes: 1. Instruments are shipped from the factory in the locked mode and must be unlocked
before this procedure can take place (see section 4.3 #7 to unlock).
2. The factory default settings are: 14.7psi for both sensor and gas pressure and pressure
correction disabled.
3. When Pressure correction is disabled all dewpoints are computed by assuming that
both Sensor and Gas Pressures are 14.7psi.
4. Pressure correction will effect the analog output but not the alarm relay trip-points.
The analog output will track the pressure corrected dewpoint, the relays will trip on the
uncorrected dewpoints.