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Abstract and Manuscript Management System - Abstract Detail |
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| Speaker: | Christopher Meyer |
| Title: | Design and Performance of the New NIST Hybrid Humidity Generator |
| Topic Group: | Humidity |
| E-mail: | cmeyer@nist.gov |
| Co-Authors: | W W Miller, D C Ripple, and G E Scace |
| Abstract: |
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. We describe here the design of the HHG as well as the results of a series of performance and validation tests on it. The performance tests include measurements of temperature non-uniformities inside and immediately outside the final saturator under various conditions (e.g., variations in bath temperature, gas-flow rate, and dew-point differences between the pre-saturator and final saturator). Other performance tests involve measurements of pressure stability in the final saturator under various pressures and gas-flow rates. Finally, performance tests have been conducted to determine the self-consistency of the two-pressure method as well as the dilution method. The validation tests include comparisons of the humidity generated by the HHG to that generated by the other NIST humidity-generation standards. This paper is intended for persons interested in hygrometer calibration and national humidity standards. Conference attendees will benefit by becoming aware of this new NIST humidity generator and learning about its capabilities. |
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