Thermometer, Liquid-Filled

a temperature-measurement device whose operation is based on the thermal expansion of a liquid. Liquid-filled thermometers provide a direct reading of the temperature.

Liquid-filled thermometers are commonly used in technology and in the laboratory for measuring temperatures from –200° to 750°C. They consist of a transparent glass (rarely quartz) bulb fused to a stem of the same material. The scale, calibrated in degrees Celsius, is applied directly to the thick-walled stem, as in an etched-stem thermometer, or to a strip rigidly attached to the thermometer, as in a graduated-scale thermometer. In thermometers of the “einschluss” type, both the stem and the attached scale are enclosed in a glass or quartz sheath. The liquid fills the entire bulb and part of the bore in the stem. Liquid-filled thermometers may contain various substances, depending on the temperature range. The most commonly used are pentane (–200° to 20°C), ethyl alcohol (–80° to 70°C), kerosine (–20° to 300°C), and mercury (–35° to 750°C).

Mercury is usually used in liquid-filled thermometers, since it remains liquid in the temperature range from –38° to 356°C at normal pressure and up to 750°C with a small increase in pressure, which is attained by filling the bore with nitrogen. In addition, mercury is easily purified and does not wet the glass, and its vapor creates only a low pressure in the bore. Liquid-filled thermometers are made of specific types of glass and are subjected to special thermal treatment, called aging, which eliminates shifting of the zero point on the scale caused by repeated heating and cooling of the thermometer (correction for shifting of the zero point must be made in exact measurements). Liquid-filled thermometers have scales with divisions from 10° to 0.01°C. The precision of a liquid-filled thermometer is determined by the magnitude of a scale division.

Liquid-filled thermometers with an abbreviated scale are used to attain the required accuracy and convenience; the most accurate types have the point for 0°C on the scale, regardless of the temperature range used. The measurement accuracy depends on the depth of immersion of the thermometer in the medium to be measured. A liquid-filled thermometer should be immersed up to a reference scale division or a specially applied line on the scale (partial-immersion thermometer). If this is not possible, a correction is made to the column of mercury that depends on the temperature being measured and the temperature and height of the column. The main disadvantages of liquid-filled thermometers are slow response time and sometimes inconvenient dimensions.

Meteorological thermometers, Beckmann thermometers, and clinical thermometers are specially designed liquid-filled thermometers. Clinical mercury thermometers have an abbreviated scale (34°–42°C) and scale divisions of 0.1 °C. They operate on the principle of a maximum-registration thermometer, in which the mercury column in the bore remains at the maximum level upon heating and does not descend until the thermometer is shaken.