Temperature measurement/Catalog of methods in biophysics

Temperature is a physical quantity that describes the state of an organism as a biological system. A change in normal temperature is a significant and easily identifiable symptom of a large group of possible disorders. However, it turns out that relatively easy local measurement of the dynamics of temperature changes can be used as a method to examine the properties of individual components of a biological system, which are completely unrelated to temperature.

Temperature as a typical state quantity cannot be measured directly, but known physical phenomena are used, which demonstrate the dependence of other physical quantities on temperature. Temperature measurement methods can be divided according to different criteria. We can tell by the method of contact with the organism depending on whether or not the sensor must be in direct contact with the surface of the measured object.
 * invasive methods, where the own sensor is introduced into the body,
 * and non-invasive methods, which we further divide into
 * touch,
 * and contactless,

Thermometers based on the longitudinal expansion of solids and the volumetric expansion of liquids
This type of thermometer is still the most widespread, in the health sector, although it is gradually being replaced by thermometers with electronic evaluation, which use other principles. The most famous is the medical mercury thermometer, which exists in two modifications. The most common of them is the so-called maximum thermometer, which records the highest temperature reached on the scale, the second type is called „speedometer“ measures the instantaneous temperature in a significantly shorter time to stabilize compared to a maximum thermometer. The temperature stabilization time for the maximum thermometer is several minutes. Both types of thermometers consist of a mercury reservoir and a capillary equipped with a scale, into which the mercury column is forced due to thermal expansion. In addition, with the maximum thermometer, the capillary is narrowed right at the exit from the reservoir, which means that the mercury does not spontaneously return to the reservoir after cooling and remains at the maximum temperature reached. Before further use, the mercury in the reservoir must be shaken off. The greater the volume of the reservoir and the smaller the radius of the capillary, the greater the sensitivity of the mercury thermometer. Medical mercury thermometers measure with an accuracy of tenths of a degree, special laboratory thermometers with an accuracy of up to two hundredths of a degree.

Bimetallic thermometers are also based on thermal longitudinal expansion, which use the deformation of the tape created by joining two metals with different coefficients of longitudinal expansion. The temperature deformation of the tape can be mechanically converted to a scale, but it is more common to use these types of thermometers as temperature switches or protections for various devices.

Metal resistance thermometers
A metal resistance thermometer is based on the change in electrical resistance of metals as a function of temperature. The advantage of these thermometers is linearity in a wide range of temperatures and a simple evaluation device. Theplatinum thermometer is most often used, which is suitable for the temperature range from 100 °C to 440 °C and measures with an accuracy of thousandths of a degree. This type of thermometer is used mainly in technical practice.

Thermocouple thermometry
A thermocouple is an electrical elementthat is inherently suitable for measuring temperature. The theoretical principle of the thermocouple was first described in 1822 by Seebeck. He discovered that in a closed electric circuit formed by the connection of two conductors made of different metals, an electric current flows in situations where these connections have different temperatures. We generally call these conductors thermocouples. When disconnecting this circuit, we can measure the electromotive voltage, causing this current, which is sometimes called the relative Seebeck voltage according to the author. From a thermodynamic point of view, a thermocouple is a device that converts thermal energy into electrical energy, therefore the magnitude of the thermovoltage is determined by the temperature difference between the junctions. Therefore, if we want to use a selected pair of thermocouples for temperature measurement, one of the junctions must be located at a known temperature, which is called the reference, and the junction is referred to as the reference point (cold end of the thermocouple). The second thermocouple connection is then called and used as a measuring one. The ideal reference temperature is 0 °C, because in that case the voltage directly corresponds to the temperature of the measuring point. However, it is technically easier to maintain the temperature of the reference point at a higher temperature (e.g. the temperature of the laboratory) and it is even better to continuously measure the temperature of the reference point, e.g. with the already mentioned platinum thermometer. In this case, we obtain the actual temperature of the measuring junction by adding the voltage equivalent of the temperature of the reference point to the measured thermal voltage. In general, we are talking about compensation of the temperature of the reference point.

Thermistor thermometry
Thermistor thermometry is based on the dependence of the electrical resistance of a semiconductor on temperature, where the density of free electrons increases with temperature, which results in a decrease in resistance. The use of thermistors for temperature measurement offers a number of advantages, if a completely miniature sensor size is not required. Advantages include a higher signal level, accuracy up to 10-3 K, relatively simple evaluation part of the device. Thermistor sensors are mostly designed for invasive measurements, e.g. in the form of needles, while the thermistor is fixed in the tip of the needle. The accuracy of the thermistor measuring apparatus is mostly not used, as the real accuracy with which it makes sense to read the temperature in a generally very heterogeneous biological system is approximately 0.1 °C.

Thermometry based on the use of optical sensors
In invasive methods of temperature measurement, the use of optoelectronics represents the most technically demanding approach. The device consists of a light source of a suitable wavelength, a detection device, which analyzes the incident light and usually converts changes in spectral composition into electrical changes, which are processed in the usual way. The light guide consists of a thin insulated glass fiber, terminated by its own temperature detector. The most common types of fiber thermometers use, for example, the dependence of the length of the afterglow of a phosphorescent substance on temperature. Optical sensors have a fundamental advantage in radiofrequency or microwave heating in hyperthermia. Since the mentioned sensors and the supply do not interfere with the electromagnetic field, there is no need to take into account their location relative to the field, it is possible to work in continuous irradiation mode, and the risk of registering artificial temperatures decreases at the same time. In addition, optoelectronic sensors are particularly suitable for temperature measurement in deeper tissues,

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