A boiling-point barometer—commonly called hypsometer—has been developed for use on meteorological radiosondes. In this hypsometer, water is heated electrically, and its boiling temperature is measured with a thermocouple. Once the boiling temperature is known, pressure is determined via the water vapor saturation pressure curve.
The pressure range required is 1050–10 hPa, that is, slightly more than two orders of magnitude. In order to achieve an accuracy of 0.05% in pressure (0.5 hPa at 1000 hPa), boiling temperature must be measured to about 0.01 K. This formidable requirement calls for very accurate calibration procedures that are novel in thermocouple thermometry. However, once the thermocouple is calibrated, individual hypsometers utilizing thermocouples made of the same batch of material do not require calibration. For computing pressure from boiling temperature, the Goff-Gratch reference function is suggested; if approximations cannot be avoided, they must be specially selected. When using another liquid (e.g., fluorochlorohydrocarbons) in the hypsometer, the accuracy required for the temperature measurement would be reduced, however, water was chosen because it is environmentally harmless.
Apart from the fact that the hypsometer does not require individual calibration, its advantage over other pressure sensors is the fact that a given uncertainty in boiling temperature leads to a practically constant relative pressure error dp/p over the entire pressure range. Consequently, heights computed for the hypsometer sondes are more accurate than those obtained from sondes employing other pressure sensors (e.g., aneroids), as was confirmed in an intercomparison. The hypsometer is operationally used in the SRS radiosonde by the Swiss Meteorological Institute; so far nearly 3500 successful flights have been made.