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NIST's new primary standard humidity generator, the Hybrid Humidity Generator (HHG), replaces the NIST 2-Pressure Humidity Generator, Mark 2 (2-P). The HHG produces humidified air of known moisture content over the humidity range previously served by the 2-P (from -70 ºC frost-point to +25 ºC dew-point), with lower uncertainty. The HHG also extends NIST's humidity generation capability to a new maximum dew-point temperature +85 ºC (water vapor mole fraction of 57 %), enabling NIST to provide high-humidity calibration services.


Heat exchanger and saturator of the Hybrid Humidity Generator

The HHG will provide improved calibration services for various scientific and industrial applications. Uncertainty in dew/frost-point temperatures is reduced by as much as 4 times. The expanded range of the HHG addresses emerging industrial needs, such as the development and performance characterization of humidity sensors used in fuel cells.


Provide improved humidity calibration services; reduce the uncertainty of NIST humidity calibrations; and reduce the cost of NIST humidity calibrations.

Technical Approach:

The NIST HHG incorporates the two-pressure and divided-flow principles into a single, modular humidity generator design. The generator operates purely as a two-pressure generator for dew/frost-point temperatures greater than or equal to −15 °C. When operating as a two-pressure generator, various humidity test points are produced by flowing a stream of CO2-free air over isothermal water under conditions of constant temperature and pressure within the saturator of the humidity generator. By ensuring that the inlet gas stream has reached thermodynamic equilibrium with the water, the mole fraction of water vapor in the gas phase can be calculated from known thermodynamic properties, and is proportional to the vapor pressure of the water and the enhancement factor. This latter quantity is close to unity and accounts for departures from ideal solution behavior as well as non-ideal gas effects. The saturator of the HHG has an exceptionally high flow capacity of 150 standard liters per minute, allowing calibration of a wide range of specialized hygrometers.

Once saturated with water vapor, the gas mixture is expanded to a lower pressure. The water vapor partial pressure associated with the gas expansion is proportional to the ratio of total gas pressure at saturation to total gas pressure after expansion. Since the new water vapor pressure corresponds to a new dew-point temperature, a variety of dew-point temperatures may be generated for a given saturator temperature. In practice, the lower pressure value is kept constant at a value most favorable for hygrometers being tested, and the pressure within the saturator is varied. We use feedback control of a butterfly valve to actively stabilize the operating pressure.

In order to generate frost-point temperatures below –15 °C, the HHG is reconfigured as a divided-flow generator, in which CO2-free air, saturated by the HHG saturator, is mixed with purified ultra-dry air. Commercial laminar-flow elements provide accurate measurements of the molar flow rates of the metered streams of moist air produced by the HHG's two-pressure generator, and the purified diluent, respectively. Use of a high-performance air purifier for the diluent gas and careful attention to plumbing design and construction reduces the background water level in the diluent gas to less than 1 part in 108.

Dilution system of the Hybrid Humidity Generator

Major Accomplishments

  • In 2008, completed construction, testing, and validation of the HHG
  • In 2009, began performing calibrations of dew-point hygrometers
Created August 28, 2015, Updated June 23, 2017