W Wyatt Miller

High Level Humidity Generator for Nitrogen-Water Mixtures

Abstract: A high level humidity generator has been developed at the National Institute of Standards and Technology (NIST). This device provides the capability for research, testing and calibration at high levels of water vapor content in such gases as hydrogen, nitrogen, and ambient air for applications such as hydrogen-powered fuel cells, standard reference materials and food processing. The generator produces a gas stream of nitrogen-water vapor mixture having mixing ratios from 0.16 g to 1.47 g of water vapor per gram of dry nitrogen (g H2O/g dry N2). This is equivalent to dew point temperatures of 60 °C to 90 °C at standard atmospheric pressure and to water vapor pressures of 19 948 Pa to 70 182 Pa. The generated gas can be fed to a test chamber with independent temperature control from 25 °C to 200 °C and pressure control from 0.1 MPa to 1.5 MPa, for the calibration of hygrometers. Inter-comparisons with a NIST reference standard dew point hygrometer gave results that in all cases agreed to within 0.014 g H2O/g dry N2.

Trace-Humidity Calibration and Testing Services at the National Institute of Standards and Technology

Abstract: The National Institute of Standards and Technology (NIST) provides low-humidity calibration and testing services for hygrometers and humidity generators over a humidity range between 3 nanomole per mole (nmol/mol, parts per billion, or ppb) of water vapor in nitrogen and 3 000 micromole per mole (μmol/mol, parts per million, or ppm). Hygrometer calibrations are typically performed by subjecting the hygrometer to various H2O/N2 mixtures produced by the NIST Low Frost-Point Generator (LFPG), and comparing the water vapor concentration reported by the device under test to that of the LFPG. Humidity generator calibrations are typically performed by adjusting both the LFPG and the humidity generator under test to produce nominally the same concentration of H2O in N2, then measuring the difference in water vapor concentration through sequential measurements taken with a sensitive hygrometer. In the case of permeation-tube humidity generators, NIST offers either complete generator calibration, or calibration of water permeation-tubes. In addition to calibration services, NIST provides individually-customized testing services in which the LFPG routinely provides low-uncertainty H2O/N2 mixtures for prototype testing, development of new measurement technologies, and for scientific experiments. In this paper we discuss the trace-humidity calibration and testing services available at NIST. The LFPG, and its performance are presented. Examples are provided of how the calibration and testing of hygrometers and humidity generators are performed. We supply details of how to obtain NIST calibration and testing services.

Design and Performance of the New NIST Hybrid Humidity Generator

Abstract: 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. The design of the HHG is described, as well as the estimated uncertainty of the dew/frost-point and mole fraction of moist air it generates. The uncertainty estimate is based on a series of performance tests performed on the HHG. Finally, the humidity generated by the HHG is compared to the humidity generated by the other NIST humidity-generation standards.