How To Calibrate a WBGT Wet Bulb Globe Temperature Heat Index Meter


A Heat Index WBGT Meter, also known as a Wet Bulb Globe Thermometer, is calibrated for measuring the relative humidity (RH) at the factory and is ready to use upon receipt. There are three temperature readings measured by the Heat Index WBGT Meter. The Dry Bulb measures the air temperature but this does not indicate how the heat affects the human body. The Globe Thermometer measures the effect of radiant heat, wind, or solar load. The Wet Bulb simulates the effect of relative humidity and the body's ability for evaporation cooling. Generally it is the humidity that creates the most danger of heat stress. The Wet Bulb temperature is measured by a Dry Bulb thermometer that sits inside a wet cotton sleeve. Over time the end user may want to recalibrate the Heat Index WBGT meter to be certain that the Wet Bulb temperature reading is accurate. A user should recalibrate the Heat Index Wet Bulb Globe Temperature Meter by considering how often the Heat Index Meter is used or how clean/ dirty it may be.

Calibration salt mixes are available at many wholesale places. The choice of calibration salt mix depends on the percentage of humidity that the user wants the Wet Bulb to simulate and may vary among geographical regions or season. The ambient condition should be set at 23+/-2°C and stable humidity.

How it works (low reference):

  1. Remove the 2 screws holding the black globe in place.
  2. Carefully plug the sensor probe into the 33% calibration salt bottle.
  3. Depending on the model of Heat Index Meter, hold down the power, setting mode, and display mode buttons simultaneously so it starts the RH calibration.
  4. Hold buttons for 2 seconds until blinking “3X.X%” displays on the screen.
  5. Meter will finish calibrating in about 60 minutes (1 hour) or when the value stops blinking.

Hot it works (High Reference)

  1. Complete the steps above for the 33% calibration salt bottle first.
  2. Plug the sensor probe into the 75% calibration salt bottle.
  3. Press the power button for 2 seconds to start until blinking “7X.X%” displays on the screen.”
  4. Meter will finish calibrating in about 60 minutes (1 hour) or when the value stops blinking.

When the meter is finished calibrating, very carefully replace the black globe by fitting the sensor pin into the connector. Position the black globe by aligning the screw holes on the sheath and the meter to help place the black globe without damaging the sensor.

The meter will return to measuring mode once the calibration is complete. Be sure to check the RH value difference. The meter display and calibration standard should be within the +/-0.3%.