thermometer(redirected from external thermometer)
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An instrument that measures temperature. Although this broad definition includes all temperature-measuring devices, they are not all called thermometers. Other names have been generally adopted. For a discussion of two such devices See Pyrometer, Thermocouple. See also T emperature measurement
This thermometer consists of a liquid-filled glass bulb and a connecting partially filled capillary tube. When the temperature of the thermometer increases, the differential expansion between the glass and the liquid causes the liquid to rise in the capillary. A variety of liquids, such as mercury, alcohol, toluene, and pentane, and a number of different glasses are used in thermometer construction, so that various designs cover diverse ranges between about -300°F and +1200°F (-184°C and +649°C).
In this thermometer the differential expansion of thin dissimilar metals, bonded together into a narrow strip and coiled into the shape of a helix or spiral, is used to actuate a pointer. In some designs the pointer is replaced with low-voltage contacts to control, through relays, operations which depend upon temperature, such as furnace controls.
This type of thermometer has a bourdon tube connected by a capillary tube to a hollow bulb. When the system is designed for and filled with a gas (usually nitrogen or helium) the pressure in the system substantially follows the gas law, and a temperature indication is obtained from the bourdon tube. The temperature-pressure-motion relationship is nearly linear. Atmospheric pressure effects are minimized by filling the system to a high pressure. When the system is designed for and filled with a liquid, the volume change of the liquid actuates the bourdon tube.
Vapor-pressure thermal system
This filled-system thermometer utilizes the vapor pressure of certain stable liquids to measure temperature. The useful portion of any liquid-vapor pressure curve is between approximately 15 psia (100 kilopascals absolute) and the critical pressure, that is, the vapor pressure at the critical temperature, which is the highest temperature for a particular liquid-vapor system. A nonlinear relationship exists between the temperature and the vapor pressure, so the motion of the bourdon tube is greater at the upper end of the vapor-pressure curve. Therefore, these thermal systems are normally used near the upper end of their range, and an accuracy of 1% or better can be expected.
In this type of thermometer the change in resistance of conductors or semiconductors with temperature change is used to measure temperature. Usually, the temperature-sensitive resistance element is incorporated in a bridge network which has a reasonably constant power supply. Although a deflection circuit is occasionally used, almost all instruments of this class use a null-balance system, in which the resistance change is balanced and measured by adjusting at least one other resistance in the bridge. Metals commonly used as the sensitive element in resistance thermometers are platinum, nickel, and copper.
This device is made of a solid semiconductor with a high temperature coefficient of resistance. The thermistor has a high resistance, in comparison with metallic resistors, and is used as one element in a resistance bridge. Since thermistors are more sensitive to temperature changes than metallic resistors, accurate readings of small changes are possible. See Thermistor
a device for measuring temperature through contact with the medium being studied. The uses of thermometers are extremely varied. There are household thermometers (room thermometers, thermometers for air and water, and clinical thermometers), industrial thermometers, and precision thermometers for experimental and meteorological work. The operation of thermometers is based on such physical properties as the thermal expansion of liquids, gases, and solids and on the temperature dependence of electrical resistance, thermal electromotive force, the magnetic susceptibility of a paramagnet, and the pressure of gases or saturated vapor.
The most common types of thermometers are liquid-filled, filled-system, resistance, and thermoelectric thermometers (see). In addition, condensation, gas, acoustic, and magnetic thermometers are used to measure low temperatures. There are also thermometers for special purposes—for example, meteorological thermometers, hypsometers, and deep-sea thermometers.
Bimetallic thermometers are sometimes used. Their operation is based on the difference in thermal expansion of the substances comprising the strips of their sensitive elements. There are also quartz thermometers, using the temperature dependence of the resonance frequency of a piezoelectric quartz crystal, and capacitance thermometers, using the dependence of the dielectric constant of ferroelectrics on temperature.
D. I. SHAREVSKAIA