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The NIST Hybrid Humidity Generator (HHG) provides calibration services for a variety of humidity–measuring instruments. It is operated by the Thermodynamic Metrology Group of the Sensor Science Division, which is part of the Physical Measurement Laboratory. Calibrations are performed by subjecting the instrument under test to air with an accurately known moisture content produced by the generator. The calibration system includes a test chamber (a chamber into which the humid air is directed and where the temperature and pressure are sufficiently constant and uniform), which is used for calibrating relative humidity sensors. The calibrations may be performed using humidity definitions of dew/frost point, relative humidity, water amount fraction (water mole fraction), or water mass ratio. Relative humidity calibrations can be performed on both relative humidity sensors and chilled-mirror hygrometers that have an external temperature probe. Relative humidity sensors are calibrated by placing them in the HHG’s test chamber with humidified air flowing through it. Relative humidity calibrations for chilled-mirror hygrometers are performed by having the hygrometer measure the dew point of the air generated by the HHG while comparing the temperature measured by the hygrometer’s external temperature probe with that of a reference thermometer. The instruments and ranges of calibration include but are not restricted to:

  1. Chilled-Mirror Hygrometers, calibrated at customer-specified points over the dew/frost range of –90 °C to +85 °C, or alternatively over the relative-humidity range 2 % to 98 % for temperatures between −34 °C and 85 °C.
  2. Temperature and Relative Humidity Sensors (Thermohygrometers), calibrated over the relative humidity range 2 % to 98 % for temperatures between −34 °C and 85 °C.
  3. Cavity Ring-Down Hygrometers, calibrated at customer-specified points over an amount fraction range of 1×10–7 to the upper limit of the hygrometer.

Uncertainties of calibrations with the NIST HHG are provided below:


Humidity Parameter


Expanded Uncertainty

Dew/frost point temperature TDP, TFP (°C)

–90 °C < TFP < –85 °C

0.12 °C

–85 °C < TFP < –55 °C                

0.07 °C

–55 °C < TFP < 0 °C

0.04 °C

0 °C < TDP < 85°C

0.04 °C

Relative humidity, RH, at test chamber temperatures of 2 °C to 85°C

0.5% < RH < 98%

0.5% of value

Water mole fraction x

1.0×10-7  < x < 2.0×10-7

2.0 % of value

2.0×10-7  < x < 1.0×10-6

0.8 % of value

1.0×10-6  < x < 0.57

0.2 % of value


How to Arrange a Calibration

  1. Contact the staff member in charge of the calibration service for your artifact with any technical or calibration cost questions.
  2. Calibrations are performed on a first come first serve basis.
  3. Ship your artifact with a copy of the purchase order. Use a sturdy box in which we can return the calibrated instrument.
  4. The purchase order may be sent to contact by e-mail, fax, or mail. Please include the following information:
  • service ID for the calibration
  • model and serial number of your artifact
  • customer contact name, address, phone, and e-mail
  • return shipping method (e.g. shipping account number)

For contact information, shipping address, or cost of calibration, see Humidity Measurements.

Publication Resources

Created May 5, 2009, Updated November 28, 2022