Design and Performance of the New NIST Hybrid Humidity Generator
Christopher W. Meyer, W Wyatt Miller, Dean C. Ripple, Gregory E. Scace
A new humidity generator has been constructed at the National Institute of Standards and Technology and is now fully operational. The NIST Hybrid Humidity Generator (HHG) has replaced 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 flows up to 150 standard liters per minute. The HHG extends the NIST humidity generation range up to 85 C, and outperforms 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 divided-flow method is used. For this method, 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. To our knowledge, this is the first primary generator that incorporates the divided-flow technique. We describe here the design of the HHG as well as the estimated uncertainty of the dew/frostpoint and mole fraction of moist air generated by it. The uncertainty estimate is based on a series of performance tests performed on the HHG. Finally, we include comparisons of the humidity generated by the HHG to that generated by the other NIST humidity-generation standards.