There are various types of instruments used for measuring temperature.
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Thermometers do not measure temperature directly, instead, they measure certain properties of substances that are dependent on temperature.
There are three basic types of thermometers.
One type is based on the thermal expansion of substances, that is, the fact that an increase in temperature results in an increase in the volume of the material. From the amount of volume change we can calculate the change in temperature.
The second type is based on the temperature dependence of the electrical properties of the materials. These are called electric thermometers.
The operation of non-contact thermometers is based on the electromagnetic radiation emitted by warm bodies, the composition of which depends on the temperature. With this method, it is possible to measure temperature even from a distance.
As the temperature increases, the particles making up the substance move faster and as a result, they occupy a larger space. In other words, the volume of the substance will increase. The only exception is water, which reacts the opposite way between 0 and 4 degrees Celsius (32 and 39.2 degrees Fahrenheit), but it is true for all other liquids, gases, and solids.
Thermometers based on the principle of thermal expansion take advantage of this phenomenon.
In the case of liquid thermometers, a liquid must be chosen that does not freeze or boil when measuring within wide temperature ranges. So it is mostly alcohol or mercury that is selected for this purpose. However, the use of mercury is not allowed in many countries because if the thermometer breaks, the released mercury vapor may pose health risks.
Gas thermometers are used primarily to measure low temperatures because of the low freezing point of gases.
Metal thermometers are less suitable to perform accurate measurement, but since thermal expansion generates strong tension in the solid material, they can be used for operating switches.
Such thermometers are bimetallic thermometers, made by welding two different metal strips together. When the temperature changes, the welded metal strips, which have different thermal expansion properties, bend and cause electrical switches to turn on and off. These type of thermometers were used to operate staircase timer switches, the temperature control of electric clothes irons, and the thermostats controlling the temperature of the engine coolant in cars.
Electric thermometers are also suitable to perform accurate measurements, and since they are small, they only slightly affect the temperature of the material being measured.
There are three different types of electric thermometers: resistance thermometers, thermistors and thermocouples.
Resistance thermometers are based on the temperature-dependence of the resistance of metals: it usually increases with the increase of temperature. Since the resistance can be measured very accurately, the temperature can also be determined quite precisely.
Thermistors differ from metal resistance thermometers in that they are made of a semiconducting material. This is why they are much more sensitive to temperature.
Thermocouples are made by welding or twisting two different metal wires together. Where the surfaces of the two metals touch, voltage is produced because the energy levels of valence electrons in dissimilar metals are different, so some of the higher energy level electrons move to the other metal until the energy is balanced. The voltage thus generated is temperature dependent, so by measuring the voltage, the temperature can be determined too. Thermocouples provide measurement over a wider range of temperatures than thermistors.
Pyrometers and infrared thermometers can measure temperature from a distance.
These thermometers can be used to measure the temperature of objects that are difficult to reach or are too hot for other types of thermometers (such as molten metals). They are also useful when looking for sources of heat loss in buildings.
The base of non-contact temperature measurement is that objects emit electromagnetic radiation, the intensity and composition of which is temperature-dependent. The largest part of this radiation falls into the invisible infrared segment of the electromagnetic spectrum, but incandescent objects hotter than 600 °C (1,112 °F) also emit visible light.
Non-contact thermometers consist of an optical system, a detector and a signal processing unit. They are often equipped with a laser pointer to show exactly the part of the object's surface being measured.
Some non-contact thermometers measure the intensity of incident radiation from which we can infer the temperature of the object if we know its distance from the thermometer. The object being measured has to fill the field of view of the measuring instrument's lens, otherwise it shows a lower temperature.
Other non-contact thermometers do not measure the intensity of the radiation but the wavelength composition which is characteristic only of temperature, while the intensity also depends on the size of the surface and the distance from the measuring instrument. The warmer the object, the lower the wavelength of the most intense radiation. Since the color of the object distorts the emitted radiation, and other factors may also interfere, for example, dusty air, this measurement method is not always accurate.