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© Servomex Group Limited 2017
AquaXact Issue 1.0
Appendix B Current vs. dew point
The current being drawn by the AquaXact 1688 varies with the dew point being measured.
To use the current to calculate the value of the dew point measurement, one must know
the settings of the low and high ends of the analog output range, then:
where:
I = current drawn by AquaXact 1688 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 = dew point measured by instrument in selected engineering units
An AquaXact Digital Controller is required to check and set the analog output low and high
ranges as well as the engineering units and the process pressure. The factory default
settings are -100 °C and +20 °C respectively, dew point °C and ambient pressure. For
example a unit with factory default settings, drawing 12 mA is computed to be measuring a
dew point of -40 °C.
Note that the computation is such that the current is linear to the selected engineering
units. Hence, ordering an AquaXact 1688 with the units set to ppm will cause the analog
output to be linearly proportional to those units (approximately logarithmically proportional
to dew point), refer to the graph in Appendix C. Naturally selecting °C or °F will cause the
analog output to be linearly proportional to dew point.
Note: The analog output range can only be set by the factory or using the AquaXact
Digital Controller.
When monitoring in ppmV, the analog output low and high ranges may have to be
adjusted via the digital controller or as a factory supplied option to provide a useful output.
Consider an example where the area of interest to be monitored is 10 to 100 ppmV, and
the analog output is set up with the factory defaults of -100°C to +20°C (which is 0.014 to
23612 ppmV); then the current loop output will vary only from ~4.1 to ~4.2 mA in the area
of interest (see the graph in Appendix C). In most instances this would be an unacceptable
output for proper monitoring of the measurement. In this example the user should adjust
the analog output low & high ranges such that the output range is better suited to the
measurement of 10 to 100 ppmV. It may be useful to select the low and high ranges to be
5 and 150 ppmV respectively, thus out of range conditions will be detected properly. Then
the low range will be set to 5 ppmV which is -65.5 °C dew point, and the high range will be
(I – 4) x (H – L)
16
+ L
D =
(12 – 4) x (20 – (-100))
16
+ (-100) = -40 °C
