Selecting probe mounting location and method - HMM170

HMM170 Quick Guide

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User guide

Relative humidity measurement is sensitive to temperature differences between the probe and the measuring environment. Even a small temperature difference may cause a large error. For example, if the temperature is +20 °C (68 °F) and the relative humidity 100 %RH, a difference of ±1 °C (±1.8 °F) between the environment and the probe causes an error of ±6 %RH. For this reason, temperature differences are an important consideration when selecting the probe mounting location and method.

The probe location should represent well the environmental or process conditions, and it should be as clean as possible. The probe head can be installed completely into the measurement environment, or through a wall with only the filter (and the sensors underneath it) extending to the measurement environment.

  • If the temperature of the measured environment differs greatly from ambient temperature, it is best to insert the whole probe and preferably plenty of cable inside the environment. This prevents measurement inaccuracy caused by heat conduction along the cable. The duct installation kit (Vaisala item code 210697) is a good accessory for this purpose as it allows the probe to be placed at a sufficient distance from the outer walls of the chamber or duct.
  • If there is no temperature difference between the two sides of the installation wall, installing the probe through the wall is a good option.

Other considerations:

  • Mount the probe head horizontally to prevent any water condensing on the probe head from running to the sensors.
  • Let the cable hang loosely to prevent condensed water from running along the cable to the probe head.
  • If you use probe heating, do not attach the probe directly to metal structures.
Figure 1. HMM170 probe installation through a wall