chapter  75
Manometric Thermometers
Pages 14

Manometric thermometers are deŸned in this handbook as those thermometers that make use of the pressure of a gaseous medium as the physical quantity to obtain temperature. Very seldom are they available from commercial sources; for example, the temperature control of a home freezer is o£en of this kind. Consequently, instead of simply buying one, every user must build his or her own when this type thermometer is needed. œey can be a quite useful choice since, in the era of electronic devices and sensors, it is still possible to make a totally nonelectronic thermometer, though of modest precision, which in addition keeps its calibration indeŸnitely, as long as the quantity of substance sealed in it remains unchanged. œe range of temperatures that can be covered by this kind of thermometer depends on the principle and on the substance used. When the thermodynamic equilibrium between the condensed phase of a substance (either liquid or solid) and its vapor is used, one has a “vapor-pressure thermometer,” and the temperature range spanned by each substance is generally narrow. In addition, only substances that are gaseous at room temperature (i.e., condensed at temperatures lower than 0 °C) are normally used, conŸning the working range to below room temperature; however, some substances that are liquid at room temperature and have a high vapor pressure (i.e., which easily evaporate) have been used, but do not result in a sizable extension of the working range much above room temperature. A special case of vapor pressure being used at high temperature is the device called a “heat pipe,” which is not used as a thermometer, but instead as an accurate thermostat [1]; using sodium, the working range is pushed up to ∼1100 °C. When a pure substance is used only in its gaseous state, one has a “gas thermometer,” whose temperature range can be very wide, especially for moderate accuracy, depending mainly on the manometer; on the other hand, its fabrication is somewhat more complex and its use normally much less straightforward.