XDT User’s Manual Appendices
Appendix J: Analog Output vs. Dewpoint
The current output of the XDT, varies with the measured dewpoint. To use the current to calcu- late the value of the dewpoint measurement, one must know the settings of the low and high ends of the analog output range, then:
(I – Z) ⋅ (H – L)
D =
S
where: I = current supplied by XDT analog output in mA.
Z = value of zero output of current loop in mA: for 4/20 = 4, for 0/24 = 0.
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.
S = span in mA of current loop output: for 4/20 = 16, for 0/24 = 24.
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
For example a unit with factory default settings, supplying 12mA is computed to be measuring a dewpoint of
(12 – 4) ⋅ (20 –
16
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 linearly propor- tional to those units (approximately logarithmically 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 provide a useful output. Consider an example where the area of interest to be moni- tored is 10 to 100 ppmV, and the analog output is set up with the factory defaults of
In general, if the dewpoint is monitored in °C or °F, there is no need to change the factory default
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