Calibration of Hygrometers with the Hybrid Humidity Generator
Christopher W. Meyer, Joseph T. Hodges, Peter H. Huang, W Wyatt Miller, Dean C. Ripple, Gregory E. Scace
We describe here the calibration of customer hygrometers using a new humidity generator that has been commissioned at the National Institute of Standards and Technology. The NIST Hybrid Humidity Generator (HHG) generates frost/dew points from 70 °C to +85 °C (mole fractions from 1×10−6 to 0.57) using calibration gas-flow rates up to 150 standard liters per minute. 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. 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 method with purified, dry air. In this special publication, we describe the design of the generator. We also describe a series of performance and validation tests on the HHG and use the results of these to construct an uncertainty budget for the generator when used in two-pressure mode and when used in the divided-flow mode. These tests include measurements of temperature gradients and pressure stability in the generator under various operating conditions, and comparison of the humidity generated by the HHG to that generated or measured by the other NIST humidity standards. For dew/frost point temperatures, the uncertainty budget yields a total expanded uncertainty (k = 2) of less than 0.025 °C for dew/frost point temperatures above 60 °C. For mole fraction, the budget yields a total expanded relative uncertainty of less than 0.2 % for mole fractions above 2 ×10−5.