Xentaur XDT User Manual
Also see for XDT: User manual
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Contents
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XDT User’s Manual Appendices50Appendix J: Analog Output vs. DewpointThe 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 endsof the analog output range, then: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 unitsL = value of Low end of Analog Output range, converted to selected engineering unitsD = 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 highranges; the factory default settings are -100°C and +20°C respectively.For example a unit with factory default settings, supplying 12mA is computed to be measuring adewpoint 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 linearly propor-tional to those units (approximately logarithmically proportional to dewpoint), refer to the graphthat follows. Naturally selecting °C or °F will cause the analog output to be linearly proportionalto dewpoint.When monitoring in ppmV or LBS or G/M3, the analog output low & high ranges may have to beadjusted 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 -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 (consult with the graph on the following page). In most instancesthis would be an unacceptable output for proper monitoring of the measurement. In this examplethe user should adjust the analog output low & high ranges such that the output range is bettersuited to the measurement of 10 to 100 ppmV. It may be useful to select the low and high rangesto be 5 and 150 ppmV respectively, thus out of range conditions will be detected properly. Thenthe low range will be set to 5ppmV which is -65.5°C dewpoint, and the high range will be set to150ppmV which is -38.5°C dewpoint. Now the current loop output will be 4.55 to 14.48 mA inthe range of 10 to 100 ppmV, the ~10mA variation is more than sufficient for a good measurementby the user’s equipment. One may carry out similar calculations for LBS or G/M3 and choose theappropriate settings.While making these computations it may be useful to obtain a copy of Xen-taur’s dewpoint calculator, this is a Microsoft Windows™ program which simplifies the processof converting dewpoint measurement units. It is available at www.xentaur.com. If you are not cer-tain how to carry out such calculations, send by e-mail to xentaur@xentaur.com or by fax to (631)345-5349, your system specifics, and some one will get back to you with appropriate analog out-put settings.In general, if the dewpoint is monitored in °C or °F, there is no need to change the factory defaultD I Z–( ) H L–( )×S----------------------------------------- L+=12 4–( ) 20 100–( )–( )×16------------------------------------------------------------ 100–( )+ 40–= |
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