Instruction manual

HMP155A Temperature and Relative Humidity Probe
7.3 Datalogger Programming
The temperature and relative humidity signals from the HMP155A can be
measured using a single-ended analog measurement or a differential analog
measurement.
Use a single-ended analog measurement when the HMP155A signal lead
length is less than 6.1 m (20 ft) or if the probe will be turned on and off under
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, Long Lead Lengths.
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 7-3 and TABLE 7-4 respectively.
TABLE 7-3. Parameters for Temperature
Units Multiplier
(degrees mV
–1
)
Offset
(degrees)
Celsius 0.14 –80
Fahrenheit 0.252 –112
TABLE 7-4. 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.
NOTE
13