Operating instructions

8800 Series Trace Moisture Analyzer Instruction Manual Appendices

Appendix J: Analog Output vs. Dewpoint

The current output of the 8800 Series Trace Moisture Analyzer, varies with the measured dew-

point. 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 supplied by 8800 Series Trace Moisture Analyzer 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 -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 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 -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 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 5ppmV which is -65.5°C dewpoint, and the high range will be set to

150ppmV which is -38.5°C dewpoint. Now the current loop output will be 4.55 to 14.48 mA in

the range of 10 to 100 ppmV, the ~10mA variation is more than sufficient for a good measurement

by the user’s equipment. One may carry out similar calculations for LBS or G/M3 and choose the

appropriate settings. In general, if the dewpoint is monitored in °C or °F, there is no need to

change the factory default -100°C to +20°C settings, because the 4-20mA provides sufficient res-

olution to measure the output better than the specified accuracy of the sensor.

IZ–()HL–()×

----------------------------------------- L+=

12 4–()20 100–()–()×

------------------------------------------------------------100–()+40–=