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Checking Humidity Sensor Calibration

Many people don't know that the air above a saturated solution of a salt and water will reach an equilibrium %RH depending on the salt. I know I didn't until looking around for a way to verify my sensor readings. 

This article gives some more detail and describes the "conventional" method of using a salt solution in a glass jar to calibrate sensors. The "two point" calibrations is often used with sodium chloride (75% RH) for the high end and magnesium chloride (33%) for the low.

Because cheese aging humidity is in the 75 to 85% range typically, a single point calibration with table salt is usually adequate. Here is the procedure we use which is simple and relatively fast:

What you need:

  • 1 gal. zip-lock plastic bag
  • 1 1/2 tsp. table salt
  • 1 Tbs. tap water
  • paper towel
  • one or more humidity sensors

Procedure: Place the salt and water in the bag and mix until no more salt dissolves. Place the paper towel in the bottom of the bag and let it absorb the salt solution. Lay the bag on an incline or over the edge of a cutting board so the solution and wet paper towel are lower than the rest of the bag. Place the sensor probe(s) in the top part of the bag and seal it, leaving enough air in the bag to keep it from lying flat. Allow the humidity in the bag to come to equilibrium at least 12 hr. Once equilibrium has been reached the sensor should read 75% RH plus or minus it's accuracy (usually from 3 to 5% RH).

If the sensor reading is a few percent out of this range you can probably still use it, but make sure you account for the error in you readings. If error is more you should replace your sensor probe.

Note: Before tossing a sensor or meter you can try letting it set out of the cave for a couple of weeks. Sometimes sensors can get fouled in the high-humidity environment of the cave, but after "airing out" will revert to reading correctly.