scope:

This International Standard describes hygrometers used for the determination of the water dew point of natural gases by detecting water vapour condensation occuring on a cooled surface or by checking the stability of the condensation on this surface. The water dew point of processed natural gases in transmission lines normally lies between - 25 °C and + 5 ° C, which corresponds to water concentrations of 50 to 200 ppm (V/V), according to the pressure of the gas. The hygrometers considered in this International Standard may be used for determining water vapour pressure, without requiring calibration, in a system operating under total pressures greater than or equal to atmospheric pressure. The relationship between water vapour partial pressure and the observed dew point confers on the method the quality of absolute measurement. If the test atmospheres contains gases which condense at a temperature in the region of, or above, that of the water dew point, it is very difficult to detect the condensed water vapour.