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HMP155ATemperature and Relative Humidity Probe

When installed in close proximity to the ocean or other bodies of salt water, a

coating of salt (mostly NaCl) may build up on the radiation shield, sensor, filter

and even the chip. NaCl has an affinity for water. The humidity over a

saturated NaCl solution is 75%. A buildup of salt on the filter or chip will

delay or destroy the response to atmospheric humidity.

The filter can be rinsed gently in distilled water. If necessary, the chip can be

removed and rinsed as well. Do not scratch the chip while cleaning.

Long term exposure of the HUMICAP



relative humidity sensor to certain

chemicals and gases may affect the characteristics of the sensor and shorten its

life. TABLE 8-1 lists the maximum ambient concentrations, of some

chemicals, that the HUMICAP



can be exposed to.

TABLE 8-1. Chemical Tolerances of HMP155A

Chemical Concentration (PPM)

Organic solvents 1000 to 10,000

Aggressive chemicals

(e.g., SO

, H

HCl, Cl

, etc.)

1 to 10

Weak Acids 100 to 1000

Bases 10,000 to 100,000

Recalibrate the HMP155A annually. Refer to the Assistance page at the

beginning of this document for the procedure for returning the HMP155A to

the factory to get recalibrated.

9. Troubleshooting

Symptom: –9999, NAN, –80 °C, or 0 % relative humidity

1. Check that the sensor is wired to the correct input channels as specified by

the measurement instructions.

2. Verify the range code is correct for the datalogger type.

3. Verify the red power wire is correctly wired to the 12V, switched 12V, or

SW12V power switch. The terminal the wire is connected to will depend

on the datalogger program.

Connect the red wire to a 12V terminal to constantly power the sensor for

troubleshooting purposes. With the red wire connected to12V, a

voltmeter can be used to check the output voltage for temperature and

relative humidity on the yellow and blue wires respectively (temperature

°C = mV * 0.14 – 80.0; relative humidity % = mV * 0.1).

Symptom: Incorrect temperature or relative humidity

1. Verify the multiplier and offset parameters are correct for the desired units

(TABLE 7-3 and TABLE 7-4).

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