A new humidity generator has been constructed at the National Institute of Standards and Technology. Once fully operational, the NIST Hybrid Humidity Generator (HHG) will replace the Two-Pressure (2-P) Humidity Generator Mark II as the NIST primary humidity generation standard for frost/dew points from 70 C to +25 C using calibration gas-flow rates up to 150 standard liters per minute. The HHG will extend the NIST humidity generation range up to 85 C, and is expected to outperform the 2-P Generator in terms of accuracy. The HHG combines the two-pressure and divided-flow humidity generation techniques (hence the name hybrid ). The centerpiece of the HHG is a heat exchanger/saturator that is immersed in a temperature-controlled bath stable to within 1 mK. A precisely regulated pre-saturation process minimizes sensible and latent heat loading on the final saturator. For dew/frost point temperatures above 15 C, the two-pressure principle is employed. For frost points at or below 15 C, the water-vapor/air mixture is produced by mixing metered streams of moist air produced by the two-pressure principle with purified, dry air; here, the HHG saturates the wet air stream at a temperature close to the water triple point, reducing the uncertainty of the water vapor pressure.
Proceedings of NCSLI 2008 Workshop and Symposium
August 3-7, 2008
humidity, generator, standards, saturator, calibration, hygrometer, water vapor