Validation of a dew-point generator for pressures up to 6 MPa using nitrogen and air

Abstract

A new primary humidity standard was developed at VSL that, in addition to ordinary operation with air and nitrogen at atmospheric pressure, can be operated with other carrier gases such as natural gas at pressures up to 6 MPa and SF 6 at pressures up to 1 MPa. The temperature range of the standard is from −80 °C to +20 °C. In this paper, we report the validation of the new primary dew-point generator in the temperature range −41 °C to +5 °C and the pressure range 0.1 MPa to 6 MPa using nitrogen and air. For the validation the flow through the dew-point generator was varied up to 10 l min −1 (at 23 °C and 1013 hPa) and the dew point of the gas entering the generator was varied up to 15 °C above the dew point exiting the generator. The validation results showed that the new generator, over the tested temperature and pressure range, can be used with a standard uncertainty of 0.02 °C frost/dew point. The measurements used for the validation at −41 °C and −20 °C with nitrogen and at +5 °C with air were also used to calculate the enhancement factor at pressures up to 6 MPa. For +5 °C the differences between the measured and literature values were compatible with the respective uncertainties. For −41 °C and −20 °C they were compatible only up to 3 MPa. At 6 MPa a discrepancy was observed.

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