# hygrometer accuracy? **URL:** \ **Category:** Factual Questions\ **Created:** [May 24, 2004, 4:38pm UTC](https://boards.straightdope.com/t/hygrometer-accuracy/246821 "2004-05-24T16:38:18Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![bsane](https://avatars.discourse-cdn.com/v4/letter/b/8e8cbc/32.png) [@bsane](https://boards.straightdope.com/u/bsane)\ **Post date:** [May 24, 2004, 4:38pm UTC](https://boards.straightdope.com/t/hygrometer-accuracy/246821/1 "2004-05-24T16:38:18Z")
Does anybody know whether or not the cheap digital hygrometers are reasonable indicators of relative humidity? Also, how is humidity measured by one of those units? ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [May 24, 2004, 5:44pm UTC](https://boards.straightdope.com/t/hygrometer-accuracy/246821/2 "2004-05-24T17:44:47Z")
I’d guess your typical el-cheapo RH readout has an absolute 1-year tolerance of around ± 4 percentage points over most of the range. Of course, this is highly dependent on initial adjustment, calibration, quality of components & materials, QC, usage, etc. The only way to know for sure is to perform a calibration at known values of RH and temperature. I believe most of them work using a capacitive sensor. The capacitor’s dielectric has a dielectric constant that is a function of RH. The capacitor is part of an oscillator or RC time constant circuit. You’ll also notice there’s a thermistor or solid-state temperature sensor next to the capacitive RH sensor. This is to temperature-compensate the RH sensor. The temperature sensor is also used to measure the air temperature. ---
**Author:** ![bsane](https://avatars.discourse-cdn.com/v4/letter/b/8e8cbc/32.png) [@bsane](https://boards.straightdope.com/u/bsane)\ **Post date:** [May 24, 2004, 8:46pm UTC](https://boards.straightdope.com/t/hygrometer-accuracy/246821/3 "2004-05-24T20:46:50Z")
Sounds like it would be more than adequate. As long as the thing is reporting some semblance of actual RH and not some some calculations based solely on temperature.