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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

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