Invented in 1724 by Daniel Gabriel Fahrenheit, the mercury thermometer is now largely replaced by other technologies. Although there are many different types of thermometers, capable of measuring certain physical characteristics, the five most common are:liquid expansion devices, bimetallic, thermal resistance or thermistor, thermocouples and infrared radiation devices. /strong>
Their operation is quite simple to understand. A reservoir is linked to an extremely fine capillary. The known expansion of the liquid under the effect of the heat applied to the tank propels it into the graduated capillary:the fineness of this capillary as well as the thermal expansion coefficient of the liquid are essential to measure a strong displacement with a small variation in volume. Thus, the fluid travels the further the hotter it is.
The use of a liquid rather than a gas makes it possible to reduce the atmospheric pressure bias. Then, for economic and health reasons, colored alcohol gradually replaced mercury. However, if you come across a silver liquid thermometer, it could also be galinstan, an alloy of gallium, indium and tin which has mechanical characteristics close to mercury.
They generally have a needle display placed on a dial. This needle or arrow which gives the temperature on the graduated dial, is connected to a circular spring in the center of the probe. This spring is made of two different metals that expand differently, but predictably when exposed to heat. The expansion of this spring with heat pushes the needle onto the dial.
Bimetal thermometers are very inexpensive but nevertheless, it usually takes several minutes to reach the target temperature. Not to mention that their metal coil (up to several centimeters long) must be fully immersed in the material to be measured to get an accurate reading.
The fundamental principle is the measurement of the effects of heat on electronic currents. Indeed, the internal resistance of this type of component is strictly a function of the temperature applied to the probe. On an RTD, resistance increases with heat in a very linear fashion, which is handy for accurately measuring even high temperature, up to 800 degrees Celsius. But this is a rather expensive technique.
Conversely, the thermistor sees its intrinsic resistivity drop as a function of temperature. In fact, the two advantages here are a lower manufacturing cost and a fast measurement time, at the expense of the reduced temperature range of less than 300 degrees Celsius.
Thermocouples operate on the rule that when two dissimilar metals are connected over a different temperature range, the junction or solder generates an electric current. This current changes, in a predictable way, with variations in temperature. Common thermocouples weld together nickel and chromium (called Type K), copper and constantan (Type T), or iron and constantan (Type J).
The probe of this type of thermometer uses this weld directly as a heat sensor. The thermocouple remains a good compromise between RTD thermal resistance and thermistor:limited manufacturing cost, reduced measurement time, to the detriment of a slight drop in precision compared to RTDs. It is often acceptable for most specific and even industrial applications.
More recent, wireless thermometers use the natural electromagnetic radiation of bodies and objects. This allows temperature measurement from a distance and without contact with the fluid or solid in question. The most easily measured and economically accessible radiation is in the infrared. Thus, a relationship between this quantity and the temperature to be measured is known and calibrated.
Then, a man-machine interface can restore an almost instantaneous reading of its value. The applications are notable in the field of health, where microbial contamination is common, as well as in hostile industrial environments, where dangerous radiation or high temperatures make any direct measurement difficult.
