Calibration of hygrometers with the hybrid humidity generator

We describe here the calibration of customer hygrometers using the NIST Hybrid Humidity Generator (HHG). This generator produces humidified air with water amount fractions (water mole fractions) ranging from 1.0 10?7 to 0.57 using calibration gas flows up to 150 standard liters per minute. At ambient pressure these amount fractions correspond to dew/frost points ranging from 90 C to +85 C. The HHG may be used to calibrate instruments measuring water amount fraction, such as cavity ring-down hygrometers. It may also be used to calibrate chilled-mirror hygrometers measuring dew/frost-point temperature when an additional pressure measurement is made at the inlet of the hygrometer. Finally, the HHG may be used to provide humidified air for the calibration of relative humidity sensors in a temperature-controlled test chamber; for this, an additional temperature measurement is made in the chamber. These sensors may be calibrated over the range 2 % to 98 % for temperatures between ?34 C and 85 C. The HHG combines the two-pressure and divided-flow humidity generation methods (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 that are above 10.3 C, the two-pressure method is employed. For frost points at or below 10.3 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 and its test chamber. We also describe a series of performance and validation tests on the HHG and its test chamber 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 amount fraction, the budget yields a total expanded relative uncertainty of less than 0.2 % for amount fractions above 2 10?5. For relative humidity in the test chamber, the budget yields a total expanded relative uncertainty of less than 0.3 %.