Psychrometersaresimpleandcomparativelylowincost. Theysuffernoirreversibledamageat100 percent RH, as do the sensors of electric hygrometers. Unfortunately, complete wet-bulb depression readingsofpsychrometersbecomedifficultwhenrelativehumiditydropsbelow20percentorwhen thetemperatureisbelowthefreezingpoint.Forremotemonitoring,itisdifficult tokeepsufficient waterinthewaterreservoir.Therefore,psychrometersaresometimesusedtocheckthetemperature and relative humidity in the air conditioned space manually.
Mechanicalhygrometersdirectlyindicatetherelativehumidityofthemoistair. Theyarealso simpleandrelativelyinexpensive. Theirmaindrawbacksaretheirlackofprecisionoveranexten- sive period and their lack of accuracy at extreme high and low relative humidities. Electronic hygrometers,especiallythepolymerfilmresistanceandthecapacitancetypes,arecommonlyused for remote monitoring and for controls in many air conditioning systems. Both the electronic and mechanical hygrometers need regular calibration. Initial calibrations are usually performed either withprecisionhumiditygeneratorsusingtwo-pressure,two-temperature,anddivided-flowsystems orwithsecondarystandardsduringmanufacturing(referto ASHRAEStandard41.6-1982,Stan- dardMethodforMeasurementofMoist AirProperties).Regularcalibrationscanbedonewitha precision aspiration psychrometer or with chilled mirror dew-point devices.
Aircontaminationhassignificant influence ontheperformanceofthesensorofelectronicand mechanical hygrometers.This is one of the reasons why they need regular calibration.