Development of a new primary humidity measurement standard

Abstract

A new primary humidity generator was developed at the National Standards Authority of Ireland (NSAI). The principles of operation and overall design of the generator are discussed. The flow regime within the saturators was analysed using computational fluid dynamics (CFD) to determine if sufficient mixing takes place within the final saturator. This was later investigated by experiment and it was found that sufficient mixing takes place for flow rates up to 3 l min−1 at standard flow conditions of 101 325 Pa and 293.15 K. The uncertainty due to non-ideal saturation at 90 °C dp is discussed. This uncertainty was analysed with respect to the effect that the flow speed and latent heating in the saturators has on the saturation efficiency, separately. The uncertainty associated with saturation efficiency at 90 °C dew point was estimated from the temperature gradients along the saturation path arising due to latent heating effects. The standard uncertainty due to non-ideal saturation was found to be ±3 mK dp.