hygrometer accuracy?

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?

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.

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.