Thermometer

A thermometer ( ancient Greek: θερμός thermos " hot " and μέτρον métron " measure, measure " ) is a measuring instrument for determining the temperature.

Many thermometers are based on the temperature dependence of the expansion of liquids, gases or solids whose expansion coefficient is known. Others use the relationship of the electrical conductivity with temperature. Pyrometers, however, to determine the temperature based on the heat radiation. However, other physical effects, which have a temperature dependency, can be used for the construction of a thermometer.

Each thermometer comprises a temperature sensor ( for example a substance in a suitable manner of thermal expansion shown) and a display device ( for example, the scale on the thermometer ).

Thermometers are calibrated on the basis of fixed temperature points, such as the triple or melting points of certain materials, or against a certified reference thermometer.

History

The development of the thermometer can be sure not to associate with the invention of a single person. Rather, numerous scientific knowledge were necessary, which led to our present concept of temperature and enabled the introduction of a temperature scale, and the technical implementation.

The relationship between the thermal expansion of air was already known in antiquity ( Philo of Byzantium, Heron of Alexandria ). For the thermoscope a glass container was immersed in water and rose or fell depending on the temperature of the water level. In the second century AD, Galen led eight " degrees of heat and cold ", which he defined with a mixture of ice and boiling water.

In a biography of Galileo Galilei Giambattista Nelli cited letters between Galileo, and his pupil Giovanni Francesco Sagredo from the years 1612 and 1615, in which from one instrument to measure heat is the question. The doctor Santorio Santorio from Padua, who was with Galileo in contact whose scientific findings used medical and used both Thermoskope for temperature measurement, as well as pendulum for measuring pulse. Santorio has two reference points ( snow and candle flame ) used to calibrate the Thermoskops. Also around this time the Dutchman Cornelius Drebbel and the Englishman Robert Fludd based on the experimental setup of Heron, but also on a manuscript from the 12th century.

All Thermoskope previously used did not use the thermal expansion of the liquid, but that of air. They were like basically a barometer and were therefore particularly affected by atmospheric pressure, such as 1643/44 was the latest known by Evangelista Torricelli. Ferdinando II de ' Medici, Grand Duke of Tuscany had, in 1654 produced the first thermometer that exploited the incidence of alcohol in a closed glass tube.

1724 Daniel Gabriel Fahrenheit struck before the temperature scale named after him, which defined the coldest point of a freezing mixture as 0 ° F, the melting point of water as 32 ​​° F and the human body temperature of 96 ° F. Anders Celsius put his scale in 1742 concludes on the basis of melting point and boiling point of water, but the other way round as the named today after him scale.

Gustav Robert Kirchhoff in 1859 formulated the law of radiation named after him, which laid the foundation for based on thermal radiation thermometer.

Types of thermometers

Contact thermometers

Contact thermometers require a thermal contact to the test object. Measurement errors occur here mainly due to insufficient thermal contact to the test object or excessive heat dissipation by the thermometer.

• Dilatometric thermometers ( liquid thermometers, bimetal thermometers )
• Thermometer for measuring body temperature, usually sooner mercury thermometer, now increasingly electronic thermometer or non-contact pyrometer
• Bimetallic thermometer ( evaluation of different expansion coefficients of two successive attached different metal plates )
• Electronic thermometer with semiconductor temperature sensor ( eg PTC resistor or sensor circuits )
• Resistance thermometers with platinum ( Pt100) or silicon
• Thermometer with thermocouple (NiCr / Ni, Pt / PtRh, Fe / constantan, etc.)
• Gas thermometer ( evaluation of a pressure measurement)
• Vapor pressure thermometer ( relationship between vapor pressure of a liquid and the absolute temperature - Clausius -Clapeyron equation)
• Liquid crystal thermometer (e.g. thermometer as wine bottles ) are based on the thermochromic properties of the liquid crystals
• Heating thermostat valves and thermostatic mixer taps work with a wax- filled expansion element.

Noncontact thermometer

Noncontact thermometers use the property that objects send one of the characteristic temperature proportional electromagnetic characteristic radiation. A portion of this emitted radiation is infrared radiation which is used for contactless temperature measurement.

• Raman thermometer, based on Raman spectroscopy ( see also Fiber-optic temperature measurement), use a frequency-stable measuring beam and evaluate the backscatter from. You can measure spatially resolved along one dimension.
• Radiation thermometer ( pyrometer) is measured by the thermal radiation of the object and were divided in the past into the following categories: Niedertemperaturpyrometer (approx. -20 ... 200 ° C ), measuring wavelength around 5 microns 15 microns ...
• Hochtemperaturpyrometer (about 400 ... 3000 ° C ), measuring 1 micron wavelengths 1.5 microns ...

• Wavey measured pyrometer ( -50 ... 1600 ° C), a wave-length 3.43 microns 14 microns ...
• Short Wavy measured pyrometer ( 50 ... 3000 ° C), a wave-length 0.8 microns 2.7 microns ...

A special case of the radiation thermometers are the thermographic cameras. They provide two-dimensional temperature profiles ( thermal images ) which are, and used in mechanical engineering, automation in the R & D sector, in medicine, security / surveillance technology in the construction industry in the. For the presentation of the images produced by thermography software is usually used. The false-color representation often used assigns each color to a temperature. In the picture often a color wedge is to be displayed with a temperature scale.

Other

A system based on gravity and temperature-dependent density of a liquid thermometer Galileo thermometer was not invented by Galileo Galilei, but only named after him.

Calibration

For the calibration of thermometers there is the international standard ITS -90. Based on this standard calibrates the Physikalisch- Technische Bundesanstalt (or the German Calibration Service or the United Kingdom UKAS - United Kingdom Accreditaion Service ) platinum thermometers, which are then taken as a reference for manufacturers. The following temperature points are used:

Calibration of reference thermometer takes place in so-called fixed-point cells. Are the Dewar vessels, is realized in which firstly the temperature fixed point by, for example, indium is heated to its melting point. On the other hand, allows a tube to introduce the sensor of the reference thermometer.

Accuracy and measurement error in the temperature measurement

The accuracy of a thermometer is on the one hand limited by the display: neither one can observe the level more accurate than a millimeter in a conventional liquid thermometer, still is possible with a digital thermometer to determine the temperature accurately, as shown on the display. On the other hand, the manufacturer guarantees of industrially manufactured thermometers only a certain agreement with the calibration, which is indicated on the device or in the manual.

Independently of one always measures all thermometers only the temperature of the thermometer (position of the sensor) that matches only by heat exchange with the temperature of the object to be measured. With contact thermometers the necessary period of time the measurement is done in particular is dependent on the thermal conductivity of the object and the sensor.

In liquid thermometers, there are further sources of error: the glass tube itself expands with increasing temperature, thus affecting the inner diameter of the capillary; the capillary itself could be made inaccurate; Also, the expansion coefficient of the liquid temperature. These effects, a manufacturer can partially compensate thereby that he attaches the scale only in the calibration. A common reading errors in liquid thermometers is the parallax error. Also, make sure that the liquid has possibly distributed on storage in the capillary and must first be brought back by knocking or skidding into the measuring ball.