A WATER HYPSOMETER UTILIZING HIGH-PRECISION THERMOCOUPLES

A boiling-point barometer-commonly called hypsometer-has been develope d for use on meteorological radiosondes. In this hypsometer, water is heated electrically, and its boiling temperature is measured with a th ermocouple. Once the boiling temperature is known, pressure is determi ned via the water vapor saturation pressure curve. The pressure range required is 1050-10 hPa, that is, slightly more than two orders of mag nitude. 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. Thi s formidable requirement calls for very accurate calibration procedure s that are novel in thermocouple thermometry. However, once the thermo couple is calibrated, individual hypsometers utilizing thermocouples m ade of the same batch of material do not require calibration. For comp uting pressure from boiling temperature, the Goff-Gratch reference fun ction is suggested; if approximations cannot be avoided, they must be specially selected. When using another liquid (e.g., fluorochlorohydro carbons) 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 l eads to a practically constant relative pressure error dp/p over the e ntire pressure range. Consequently, heights computed for the hypsomete r sondes are more accurate than those obtained from sondes employing o ther pressure sensors (e.g., aneroids), as was confirmed in an interco mparison. The hypsometer is operationally used in the SRS radiosonde b y the Swiss Meteorological Institute; so far nearly 3500 successful fl ights have been made.