Appendix g: current vs. dewpoint – COSA Xentaur Xentaur LPDT User Manual
Page 47
LPDT User’s Manual
page 40
Appendix G: Current vs. Dewpoint
The current being pulled by the LPDT, varies with the dew-
point being measured by the LPDT. To use the current to
calculate the value of the dewpoint measurement, one must
know the settings of the low and high ends of the analog
output range, then:
where:
I = current drawn by LPDT loop in mA.
H = value of High end of Analog Output range
converted to selected engineering units
L = value of Low end of Analog Output range
converted to selected engineering units
D = dewpoint measured by instrument in selected
engineering units.
Consult section 3.4.5.7&8 and/or appendix B, to check and
set the Analog Output low and high ranges; the factory
default settings are -100°C and +20°C respectively.
For example a unit with factory default settings, drawing
12mA is computed to be measuring a dewpoint of -40°C:
Note that the computation is such that the current is linear to
the selected engineering units. Hence, selecting ppmV or
LBS or G/M3 units, will cause the analog output to be lin-
early proportional to those units (approximately logarithmi-
cally proportional to dewpoint), refer to the graph that
follows. Naturally selecting °C or °F will cause the analog
output to be linearly proportional to dewpoint.
When monitoring in ppmV or LBS or G/M3, the analog
output low & high ranges may have to be adjusted to pro-
D
I
4
–
(
)
H
L
–
(
)
×
16
----------------------------------------
L
+
=
12
4
–
(
)
20
100
–
(
)
–
(
)
Ч
16
------------------------------------------------------------
100
–
(
)
+
40
–
=