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maintain accurate readings the sensor calibration should be checked at least annually.
For sensors located in areas where there is severe dust and atmospheric pollutants, it
is recommended that the sensor be checked more often.
4.2 Field Checking Relative Humidity
If the sensor to be checked is located in a remote area, NovaLynx recommends taking
along a second RH sensor element to replace the original sensor element in case it is
out of the specific accuracy range. Due to the stability of the sensor design, it is not
necessary to perform an actual calibration adjustment to the sensor electronics.
Replacement of the sensing element is recommended every two years for best results.
If a second RH sensor is unavailable, the next easiest way to check the probe operation
in the field is to compare the relative humidity sensor output against an accurate
psychrometer. If the humidity readings are within ± 4% RH the sensor is good and does
not need to be replaced. If the reading error is greater, then the decision must be made
whether to replace the sensor or to use it for a longer period of time. Remember to
include the accuracy of the test instrument (psychrometer) in the decision process.
Most often the accuracy of the test instrument or psychrometer will be less than the
accuracy of the electronic sensor.
For best test results, the Assmann style psychrometer, NovaLynx Model 225-5230, is
recommended. The Assmann psychrometer uses a spring-driven fan and has precision
thermometers. An easier method is to check the sensor operation using a second
electronic sensor such as Model 225-HM34-C. The 225-HM34-C uses the same
sensing elements and allows quick and easy checking of the humidity and temperature.
Reading of the 225-HM34-C is direct using the built-in LCD display.
Should the RH probe appear to be out of calibration even after changing the sensing
element, contact NovaLynx for instructions. For best results, the RH sensor should be
tested in an accurate RH calibration chamber (such as the Model 220-HMK11) under
controlled conditions.
Any signal conditioning provided for use with the sensor has been adjusted at the
factory by simulating the sensor with a precision DC voltage source. Verify that the
signal conditioning or monitoring equipment is operating correctly and make any
necessary adjustments before testing the sensor. Retest the sensor with the signal
conditioning or monitoring equipment after making any adjustments to determine
whether or not the sensor is correctly calibrated.
NovaLynx offers the Model 220-HMK11 for checking the calibration of electronic RH
sensors by the user. The calibration chamber uses saturated salt solutions to check the
%RH at 75% and at 12%. The calibration chamber works best in stable temperature
conditions such as those found indoors or in laboratories, but can also be used to make
quick and accurate field tests of an electronic humidity sensor calibration since the
chamber is always at the indicated humidity. Sensor readings can be taken within 5
minutes. The chamber humidity reading is corrected according to the temperature
