The NPL standard humidity generator: an analysis of uncertainty by validation of individual component performance

Abstract

The NPL standard humidity generator-the reference for traceable humidity calibrations in the United Kingdom-has been validated by assessment of the performance of individual components of the generator system. The investigation has explored the effects of saturator efficiency, temperature conditioning of the gas, desorption, leaks, pressure variation and other factors that contribute to the uncertainty in dew-points generated by this instrument. The results of these experiments have led to some improvements in the design of the standard. Repeatability of overall performance has been monitored over a period of two years. The improvements in design, together with better characterization of the performance of the instrument, have enabled a smaller uncertainty in generated dew-point to be assigned. This uncertainty is now +or-0.05 degrees C dew-point in the middle of the operating range, rising to +or-0.08 degrees C and +or-0.15 degrees C dew-point at the upper and lower extremes of the range: +80 degrees C and -75 degrees C dew-point respectively.

Metrological challenges for measurements of key climatological observables. Part 4: atmospheric relative humidity

“…In other generators the input gas may be humidified directly to a non-saturated level - using partial saturation, diffusion or blending approaches - that is, definitively characterised in terms of mass, volume or amount of component substances, and that enables the mole fraction to be calculated (Hasegawa and Little, 1977; Stevens and Bell, 1992; Heinonen, 2002, Lovell-Smith, 2009; Mackrodt, 2012; Meyer et al, 2012). …”

An effective laboratory air humidity generator for drying research

“…These methods produce an accurate humidity level gas. However, most of them are used for calibration purposes, which are complex and occupy a large amount of space (Stevens & Bell, 1992). Moreover, the quantity of humid gas generated is quite small although some commercial versions have relatively large capacities of up to 300 ml/min.…”

A Novel Method and an Equipment for Generating the Standard Moisture in Gas Flowing through a Pipe

“…To calibrate and to validate the moisture sensors, the accurate generation of moisture at certain predetermined values is crucial. Different methods have been proposed and implemented including the diffusion tube method [ 9 ], NPL method [ 10 ] and a method using permeation tubes [ 11 ].…”