Campbell Scientific HMP155A Temperature and Relative Humidity Probe - Manual (Page 22)

HMP155ATemperature and Relative Humidity Probe
datalogger control between measurements. For lead lengths greater than 6.1 m
(20 ft.) or when the probe will be continuously powered, use a differential
analog measurement. For a discussion on errors caused by long lead lengths
see Section 7.4.
HMP155A sensors purchased directly from Vaisala with serial
numbers < E4430001 require approximately 5 seconds warm up
time.
The HMP155A output scale is 0 to 1000 millivolts for the temperature range of
–80° to +60°C –112 to +140°F) and for the relative humidity range of 0 to
100%. Multipliers and offsets for converting voltage to temperature and
relative humidity are listed in TABLE 7-4 and TABLE 7-5 respectively.
TABLE 7-4. Parameters for Temperature
Units Multiplier
(degrees mV –1 )
Offset
(degrees)
Celsius 0.14 –80
Fahrenheit 0.252 –112
TABLE 7-5. Parameters for Relative Humidity
Units Multiplier
(% mV –1 )
Offset
(%)
Percent 0.1 0
Fraction 0.001 0
7.4 Long Lead Lengths
This section describes the error associated with measuring the HMP155A with
a single-ended measurement if the probe has a long cable. To avoid these
problems, Campbell Scientific recommends measuring the HMP155A using a
differential analog measurement (Instruction 2) when long lead lengths are
required. Generic datalogger connections for measuring the HMP155A using a
differential measurement are given in TABLE 7-2.
Understanding the details in this section is not required for the general
operation of the HMP155A with Campbell Scientific’s dataloggers.
The signal reference (white) and the power ground (black) are in common
inside the HMP155A. When the HMP155A temperature and relative humidity
are measured using a single-ended analog measurement, both the signal
reference and the power ground are connected to ground at the datalogger. The
signal reference and the power ground both serve as the return path for 12 V.
There will be a voltage drop along those leads because the wire itself has
resistance. The HMP155A draws approximately 4 mA when it is powered.
The wire used in the HMP155A (pn 9721) has resistance of 27.7 Ω/1000 feet.
Since the signal reference and the power ground are both connected to ground
at the datalogger, the effective resistance of those wires together is half of 27.7
NOTE
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