Appendix G: Current Vs. Dewpoint - Teledyne 8800T Instruction Manual

8800T User's Manual

Appendix G: Current vs. Dewpoint

The current being pulled by the 8800T, varies with the dew-
point being measured by the 8800T. 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 drawn by 8800T loop in mA.
H = value of High end of Analog Output range
L = value of Low end of Analog Output range
D = dewpoint measured by instrument in selected
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:
12 4
–
-------------- --------------------------------------------- -
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 lin-
early proportional to those units (approximately logarithmi-
cally 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 pro-
I 4
–
= --------------------------------------- -
D
converted to selected engineering units
converted to selected engineering units
engineering units.
20
–
16
–
H L
16
100
–
100
+ –
page 40
+
L
40
= –