Any body with at least one measurable property that changes as temperature changes can be used as a thermometer. Such a feature is called a thermometric property. The particular substance that exhibits variations in the thermometric property is known as a thermometric element.
Figure 1: Liquid-in-glass thermometer
A liquid-in-glass thermometer is one of the common devices for temperature measurement is shown in in Fig. 1, which consists of a capillary glass tube connected to a bulb filled with a liquid such as alcohol and sealed at the other side of the glass tube. The space above the liquid is occupied by the vapor of the liquid or an inert gas. As temperature increases, the liquid expands its volume, and there is a rise in the capillary level. The length ‘L’ of the liquid in the capillary depends on the temperature. Accordingly, the liquid is the thermometric substance, and ‘L’ is the thermometric property. Although this type of thermometer is used for ordinary temperature measurements, it is not suitable for applications where high accuracy temperature measurement is required. Various other types of thermometers have been devised to give accurate temperature measurements.
Thermocouples are used as sensors are based on the principle that when two dissimilar metals are joined, an electromotive force (emf) that is primarily a function of temperature will exist in a circuit. In individual thermocouples, one thermocouple wire is platinum of a specified purity, and the other is an alloy of platinum and rhodium. Thermocouples also utilize copper and constantan (an alloy of copper and nickel), iron and constantan, as well as several other pairs of materials. Electrical-resistance sensors are another important class of temperature measurement devices. These sensors are based on the fact that the electrical resistance of various materials changes in a predictable manner with temperature. The materials used for this purpose are usually conductors (such as platinum, nickel, or copper) or semiconductors. Devices using conductors are known as resistance temperature detectors, and semiconductor types are called thermistors. A variety of instruments measures temperature by sensing radiation.
They are known by terms such as radiation thermometers and optical pyrometers. This type of thermometer differs from those previously considered in that it does not come in contact with a body whose temperature is to be determined, an advantage when dealing with moving objects or bodies at extremely high temperatures. All of these temperature sensors can be used together with automatic data acquisition.
