Table 3: Comparison of Mercury dry and wet bulb psychrometers with alternative methods** Usage (AWS, Range** Accuracy Response time Linearity Stability Calibration Cost or manual) AWS, or manual Electrical fast response with digital 0 – 100%RH ±1% - ±5%RH Near linear ±1%RH/yr Easy to calibrate Low price Capacitance <50 ms display AWS or Electrical Few seconds, Poor but can be manual with 15 – 95%RH ±3% - ±10%RH ±3%RH/yr Easy to calibrate Low price Resistance 5 s without filter compensated digital display AWS or Dry and wet bulb High degree of manual with 0 – 100%RH ±2% - ±5%RH medium Good ±0.5 to 1%RH/yr Expensive PRT calibration digital display Dew point ±1.2%RH, or Manual with Better than 1.5°C hygrometer 5 – 100%RH ±0.15°C for dew slow to fast Linear Easy to calibrate Expensive digital display dew point drift/yr (Chilled-mirror) point Inexpensive way Non-linear but ±5%RH if treted to obtain a Hair hygrometer Manual 0 – 100%RH 5 minutes can be up to 10%RH/Yr hard to calibrate carefully week’s record for compensated hygrograph Mercury dry and wet bulb slow response, Medium to high psychrometers * Manual 0 - 100%RH ±0.2°C depending on Linear ±1%RH/yr Easy to calibrate cost (Sling or construction Assmann) * Mercury is banned due to Minamata Convention and to be phased out by 2020. ** The table contains information on typical values of instrument’s properties, based on the available literature. Cross reference must also be considered from WMO No. 8, Guide to Meteorological Instruments and Methods of Observation, under Annex 1D, Operational Measurement Uncertainty Requirements and Instrument Performance.
Table 3: Comparison of Mercury dry and wet bulb psychrometers with alternative methods** Usage (AWS, or manual) Range** Accuracy Response time Linearity Stability Calibration Cost Electrical Capacitance AWS, or manual with digital display 0 – 100%RH ±1% - ±5%RH fast response <50 ms Near linear ±1%RH/yr Easy to calibrate Low price Electrical Resistance AWS or manual with digital display 15 – 95%RH ±3% - ±10%RH Few seconds, 5 s without filter Poor but can be compensated ±3%RH/yr Easy to calibrate Low price Dry and wet bulb PRT AWS or manual with digital display 0 – 100%RH ±2% - ±5%RH medium Good ±0.5 to 1%RH/yr High degree of calibration Expensive Dew point hygrometer (Chilled-mirror) Manual with digital display 5 – 100%RH ±1.2%RH, or ±0.15°C for dew point slow to fast Linear Better than 1.5°C dew point drift/yr Easy to calibrate Expensive Hair hygrometer Manual 0 – 100%RH ±5%RH if treted carefully 5 minutes Non-linear but can be compensated up to 10%RH/Yr hard to calibrate Inexpensive way to obtain a week’s record for hygrograph Mercury dry and wet bulb psychrometers * (Sling or Assmann) Manual 0 - 100%RH ±0.2°C slow response, depending on construction Linear ±1%RH/yr Easy to calibrate Medium to high cost * Mercury is banned due to Minamata Convention and to be phased out by 2020. ** The table contains information on typical values of instrument’s properties, based on the available literature. Cross reference must also be considered from WMO No. 8, Guide to Meteorological Instruments and Methods of Observation, under Annex 1D, Operational Measurement Uncertainty Requirements and Instrument Performance.
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