Humidity standards for low level water vapor sensing and measurements

Abstract

An intercomparison between the two most widely used types of low level humidity standards constituting a permeation-tube standard and an evaporation–diffusion standard is presented. Using a precision chilled-mirror hygrometer as a transfer standard, frost-point temperatures of nitrogen–water-vapor mixtures produced by the standard generators are measured in the water vapor concentration range of 5–70 nmol mol −1. Measured differences between the standard generators are found to be within the standards’ expanded uncertainties for moisture content of greater than≈15 nmol mol −1. Below 15 nmol mol −1, the differences are greater than the combined uncertainties of the standard generators.