Appendix g: current vs. dewpoint – COSA Xentaur Xentaur HDT User Manual
Page 35
HDT User’s Manual
page 28
Appendix G: Current vs. Dewpoint
The current being drawn by the HDT, varies with the dew-
point being measured by the HDT. To use the current to cal-
culate 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 HDT 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.
A HART communicator is required to check and set the
Analog Output low and high ranges; the factory default set-
tings 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, ordering a HDT with
ppmV or LBS or G/M3 units, will cause the analog output
to be linearly proportional to those units (approximately
logarithmically proportional to dewpoint), refer to the graph
that follows. Naturally selecting °C or °F will cause the ana-
log 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
–
=