Types of thermometers. Thermometer electronic, digital

In FIG. 75c shows a thermometer that measures the expansion of a gas. A drop of mercury locks a volume of dry air in a capillary with a sealed end. When measuring, the entire thermometer must be immersed in the medium. The movement of a drop of mercury in a capillary indicates a change in the volume of the gas; the capillary has a scale with marks 0 and 100 for the points of melting ice and boiling water, as with a mercury thermometer.

Such a thermometer is not suitable for very accurate measurements. We want to talk about a gas thermometer to clarify the general idea. A thermometer of this type is shown in Fig. 75b. The mercury barometer AB measures the pressure of a constant volume of gas in cylinder C. But instead of marking the height of the mercury column in the barometer in units of pressure, we mark it with 0 when the cylinder is in melting ice and 100 when in boiling water, I plot on them the whole Celsius scale. Using Boyle's law, it can be shown that the scale of the thermometer shown in Fig. 75b should be the same as that of the thermometer in FIG. 75 a.

Application of gas thermometer
When calibrating the gas thermometer shown in Fig. 76, we immerse the cylinder in melting ice and mark 0 on the barometer scale. Then we repeat the whole procedure, replacing the ice with boiling water; we get a mark of 100. Using the scale defined in this way, we build a graph of pressure versus temperature. (If you like, the pressure can be expressed in units of the height of the mercury column.) Then draw a straight line through the points O and 100 and, if necessary, continue it. This will be a straight line that determines the temperature in the gas scale and gives standard values 0 and 100 at the points where ice melts and water boils. Now a gas thermometer will allow us to measure the temperature if we know the pressure of the gas in the cylinder at that temperature. The dotted line in Fig. 76 shows how to find the water temperature at which the gas pressure is 0.6 mHg.

After we have chosen a gas thermometer as a standard, we can compare mercury and glycerin with it. So it was found that the expansion of most liquids, depending on the temperature measured by a gas thermometer, is somewhat non-linear. The readings of the two types of thermometers diverged between the points 0 and 100, agreement in which is obtained by definition. But mercury, oddly enough, gives an almost straight line. Now we can formulate the “dignity” of mercury: “On the gas temperature scale, mercury expands evenly.” This amazing coincidence shows that at one time we made a very good choice - that is why now ordinary mercury thermometers can be used to directly measure temperature.

A thermometer is a high-precision device that is designed to measure the current temperature. In industry, a thermometer measures the temperature of liquids, gases, solids and bulk products, melts, etc. Thermometers are especially often used in industries where it is important to know the temperature of the raw material for proper flow. technological processes, or as one of the controls finished products. These are enterprises of the chemical, metallurgical, construction, agricultural industries, as well as food production.

In everyday life, thermometers can be used in various purposes. For example, there are outdoor thermometers for wooden and plastic windows, room thermometers, thermometers for baths and saunas. You can buy thermometers for water, tea, and even beer and wine. There are aquarium thermometers, special soil thermometers, and incubators. Thermometers are also commercially available. freezers, refrigerators and cellars and cellars.
Installing a thermometer, as a rule, is not technologically difficult. However, do not forget that only the installation of a thermometer performed in accordance with all the rules guarantees the reliability and durability of its operation. It should also be taken into account that the thermometer is an inertial device, i.e. the settling time of its readings is about 10 - 20 minutes, depending on the required accuracy. Therefore, do not expect the thermometer to change its reading the moment it is taken out of the package or installed.
By design features There are the following types of thermometers:

A liquid thermometer is the same glass thermometer that can be seen almost everywhere. Liquid thermometers can be both household and technical (for example, a ttzh thermometer is a technical liquid thermometer). A liquid thermometer works according to the simplest scheme - when the temperature changes, the volume of liquid inside the thermometer changes and when the temperature rises, the liquid expands and creeps up, and vice versa when it decreases. Typically, liquid thermometers use either alcohol or mercury.

Manometric thermometers are designed for remote measurement and registration of the temperature of gases, vapors and liquids. In some cases, manometric thermometers are made with special devices that convert the signal into electrical and allow temperature control.

The operation of manometric thermometers is based on the dependence of the pressure of the working substance in a closed volume on temperature. Depending on the state of the working substance, gas, liquid and condensation thermometers are distinguished.

Structurally, they are a sealed system consisting of a cylinder connected by a capillary to a pressure gauge. The bulb is immersed in the measurement object and when the temperature of the working substance changes, the pressure changes in closed system, which is transmitted through the capillary tube to the manometer. Depending on the purpose, manometric thermometers are self-recording, indicating, scaleless with built-in transducers for remote transmission of measurements.

The advantage of these thermometers is the possibility of their use in explosive objects. The disadvantages include a low temperature measurement accuracy class (1.5, 2.5), the need for frequent periodic verification, the complexity of repair, big sizes bulb.

The thermometric substance for gas manometric thermometers is nitrogen or helium. A feature of such thermometers is quite big size bulb and, as a result, a significant inertia of measurements. The temperature measurement range is from -50 to +600°C, the scales of thermometers are uniform.

For liquid manometric thermometers, the thermoelectric substance is mercury, toluene, propyl alcohol, etc. Due to the high thermal conductivity of the liquid, such thermometers are less inertial than gas ones, but with strong temperature fluctuations environment the error of the instruments is higher, as a result of which, with a significant length of the capillary, for liquid manometric thermometers, compensation devices are used. Temperature measurement range (with mercury filling) is from -30 to +600°С, thermometer scales are uniform. In condensation manometric thermometers, low-boiling liquids propane, ethyl ether, acetone, etc. are used. The filling of the bulb occurs at 70%, the rest is occupied by steam of the thermoelectric substance.

The principle of operation of condensation thermometers is based on the dependence of the saturated vapor pressure of a low-boiling liquid on temperature, which excludes the influence of changes in ambient temperature on the readings of thermometers. The bulbs of these thermometers are quite small, as a result, these thermometers are the least inertial of all manometric thermometers. Also, condensation manometric thermometers are highly sensitive, due to the non-linear dependence of saturated vapor pressure on temperature. The temperature measurement range is from -50 to +350°C, the scales of thermometers are not uniform.

The resistance thermometer works due to the well-known property of bodies to change electrical resistance with a change in temperature. Moreover, in metal thermometers, the resistance increases almost linearly with increasing temperature. In semiconductor thermometers, on the contrary, the resistance decreases.

Metal resistance thermometers are made from a thin copper or platinum wire placed in an electrically insulating case.

The principle of operation of thermoelectric thermometers is based on the property of two dissimilar conductors to create a thermoelectromotive force when the place of their connection, the junction, is heated. In this case, the conductors are called thermoelectrodes, and the entire structure is called a thermocouple. In this case, the value of the thermoelectromotive force of a thermocouple depends on the material from which the thermoelectrodes are made, and the temperature difference between the hot junction and cold junctions. Therefore, when measuring the temperature of the hot junction, the temperature of the cold junctions is either stabilized or corrected for its change.

Such devices allow you to measure temperature remotely - at a distance of several hundred meters. At the same time, only a very small temperature-sensitive sensor is located in the controlled room, and an indicator is located in the other room.

are intended for signaling the set temperature, and when it is reached, for turning on or off the corresponding equipment. Electrocontact thermometers are used in systems for maintaining a constant temperature from -35 to +300°C in various laboratory, industrial, power and other installations.

Electrocontact thermometers are made to order, according to specifications enterprises. Such thermometers are structurally divided into 2 types:

— Thermometers with manually adjustable contact temperature,

— Thermometers with constant or preset contact temperature. These are the so-called thermal contactors.

Digital thermometers are highly accurate, high speed modern appliances. The basis of a digital thermometer is an analog-to-digital converter that operates on the principle of modulation. The parameters of a digital thermometer are completely dependent on the installed sensors.

Condensation thermometers work using the dependence of the saturated vapor pressure of a low-boiling liquid on temperature. These instruments are more sensitive than other conventional thermometers. However, since the vapor pressure dependences for the liquids used, such as ethyl ether, methyl chloride, ethyl chloride, acetone, are non-linear, as a result, the thermometer scales are plotted unevenly.

gas thermometer operates on the principle of dependence between temperature and pressure of a thermometric substance, which is deprived of the possibility of free expansion when heated in a confined space.

His work is based on the differences in the thermal expansion of the substances from which the plates of the applied sensitive elements are made. Bimetal thermometers widely used on sea and river vessels, industry, nuclear power plants, for temperature measurement in liquid and gaseous media.

A bimetallic thermometer is made up of two thin metal strips, for example, copper and iron, when heated, their expansion occurs unequally. The flat surfaces of the tapes are tightly fastened to each other, while the bimetallic system of two tapes is twisted into a spiral, and one of the ends of such a spiral is rigidly fixed. When the coil is cooled or heated, ribbons made of different metals contract or expand to varying degrees. As a result, the spiral either twists or unwinds. A pointer attached to the free end of the spiral displays the measurement results.

QUARTZ THERMOMETERS

Quartz thermometers work based on temperature dependence resonant frequency of the piezoquartz. A significant disadvantage quartz thermometers is their inertia, which reaches several seconds, and instability when working with temperatures above 100oC.

Liquid and gas thermometers.

Liquid thermometer - a device for measuring temperature, the principle of operation of which is based on thermal expansion liquids. The liquid thermometer is a direct reading thermometer.

It is widely used in engineering and laboratory practice to measure temperatures in the range from –200 to 750 °C. A liquid thermometer is a transparent glass (rarely quartz) tank with a capillary (of the same material) soldered to it.

The °C scale is applied directly to a thick-walled capillary (the so-called stick liquid thermometer) or to a plate rigidly connected to it (a liquid thermometer with an external scale, Fig. a). A liquid thermometer with an embedded scale (Fig. b) has an external glass (quartz) case. The thermometric fluid fills the entire reservoir and part of the capillary. Depending on the measurement range, a liquid thermometer is filled with pentane (from -200 to 20 ° C), ethyl alcohol (from -80 to 70 ° C), kerosene (from -20 to 300 ° C), mercury (from -35 to 750 ° C). C) etc.

The most common mercury liquid thermometers, since mercury remains liquid in the temperature range from -38 to 356 ° C at normal pressure and up to 750 °C with a slight increase in pressure (for which the capillary is filled with nitrogen). In addition, mercury is easy to clean, does not wet glass, and its vapor in the capillary creates low pressure. Liquid thermometers are made from certain types of glass and subjected to special heat treatment(“aging”), which eliminates the shift of the zero point of the scale associated with repeated heating and cooling of the thermometer (the correction for the shift of the zero point of the scale must be entered for accurate measurements). Liquid thermometers have scales with different price divisions from 10 to 0.01 °С. The accuracy of a liquid thermometer is determined by the value of its scale divisions. To ensure the required accuracy and convenience, liquid thermometers with a shortened scale are used; the most accurate of them have a point of 0 ° C on the scale, regardless of the temperature interval marked on it. The measurement accuracy depends on the depth of immersion of the liquid thermometer in the measured medium. The thermometer should be immersed up to the counted division of the scale or up to a line specially marked on the scale (liquid tail thermometers). If this is not possible, a correction for the protruding column is introduced, which depends on the measured temperature, the temperature of the protruding column and its height. The main disadvantages of a liquid thermometer are significant thermal inertia and dimensions that are not always convenient for work. Liquid thermometers of special designs include meteorological thermometers (of a special design, intended for meteorological measurements mainly at meteorological stations), metastatic (Beckmann thermometer, a mercury thermometer with an embedded scale used to measure small temperature differences), medical, etc. Medical mercury thermometers have a shortened scale (34-42 °С) and a scale interval of 0.1 °С. They operate on the principle of a maximum thermometer - the mercury column in the capillary remains at the level of the maximum rise when heated and does not fall until the thermometer is shaken.

Gas thermometer.

A device for measuring temperature, the operation of which is based on the dependence of the pressure or volume of an ideal gas on temperature. The most commonly used gas thermometer is a constant volume rice.), which is a gas-filled balloon 1 constant volume, connected by a thin tube 2 with device 3 for pressure measurement. In such a gas thermometer, the change in temperature of the gas in the cylinder is proportional to the change in pressure. Gas thermometers measure temperatures in the range from ~2K to 1300 K. The maximum achievable accuracy of a gas thermometer depending on the measured temperature is 3 10 -3 - 2 10 -2 deg. A gas thermometer of such high accuracy is a complex device; when measuring temperature, they take into account: deviations of the properties of the gas filling the device from the properties of an ideal gas; changes in the volume of the cylinder with a change in temperature; the presence of impurities in the gas, especially condensing ones; sorption (absorption by a solid or liquid of a substance from the environment) and desorption of gas by the walls of the cylinder; diffusion (mutual penetration of contacting substances into each other due to the thermal movement of particles of the substance) of gas through the walls, as well as temperature distribution along the connecting tube.

Thermal resistance.

Resistance thermometers (otherwise called resistance thermometers) are devices for measuring temperature. The principle of operation of the device is to change the electrical resistance of alloys, semiconductors and pure metals (i.e. without impurities) with temperature. The sensitive element of a thermometer is a resistor, which is made of a film or metal wire, and has a dependence of electrical resistance on temperature. The wire is wound on a rigid frame made of quartz, mica or porcelain, and enclosed in a protective metal (glass, quartz) sheath. The most popular thermal resistors are made of platinum. Platinum is oxidation-resistant, high-tech, and has a high temperature coefficient. Sometimes copper or nickel thermometers are used. Resistance thermometers are usually used to measure temperatures in the range from minus 263 C to plus 1000 C. Copper resistance thermometers have a much smaller range - only from minus 50 to plus 180 C. The main requirement for the design of the thermometer is that it must be sufficiently sensitive and stable, those. sufficient for the required measurement accuracy in the specified temperature range under appropriate conditions of use. The conditions of use can be both favorable and unfavorable - aggressive environments, vibrations, etc. Typically, resistance thermometers work in conjunction with potentiometers (a resistive element whose resistance value changes mechanically; a device for measuring EMF, voltages by the compensation method), logometers (a device designed to measure the ratio of two electrical quantities), measuring bridges. The accuracy of measurements of the resistance thermometer (thermal resistance) itself largely depends on the accuracy of these devices. Resistance thermometers can be various: surface, screw-in, plug-in, with bayonet connection or connecting wires. Thermal resistors can be used to measure temperature in liquid and gaseous media, in climatic, refrigeration and heating technology, furnace building, mechanical engineering, etc.

Thermocouples.

Thermocouple is a thermoelement used in measuring and converting devices. Its principle of operation is based on the fact that heating or cooling contacts between conductors that differ in chemical or physical properties, is accompanied by the appearance of thermoelectromotive force (thermopower). A thermocouple consists of two metals welded at one end. This part of it is placed in the place where the temperature is measured. Two free ends are connected to the measuring circuit (millivoltmeter). The most common thermocouples are platinum-platinum-rhodium (PP), chromel-aluminum (XA), chromel-copel (XK) (kopel - copper-nickel alloy ~ 43% Ni and ~ 0.5% Mn), iron constant (LC).

Thermocouples are used in a wide variety of temperature ranges. So, a thermocouple made of gold alloyed with iron (2nd thermoelectrode - copper or chromel) covers the range of 4-270 K, copper - constantan 70-800 K (constantan is a thermostable alloy based on Cu (59%) with the addition of Ni (39 -41%) and Mn (1-2%)), chromel - kopel 220-900 K, chromel - alumel 220-1400 K, platinum-rhodium - platinum 250-1900 K, tungsten - rhenium 300-2800 K. Eds of metal thermocouples conductors usually lies in the range of 5-60 mV . The accuracy of determining the temperature with their help is, as a rule, several K, and for some thermocouples it reaches ~0.01 K. The emf A thermocouple made of semiconductors can be an order of magnitude higher, but such thermocouples are characterized by significant instability.

Thermocouples are used in devices for measuring temperature and in various automated systems management and control. In combination with an electrical measuring device (millivoltmeter, potentiometer), a thermocouple forms a thermoelectric thermometer.

The measuring device is connected either to the ends of thermoelectrodes (contacts (usually junctions) of conductive elements forming a thermocouple) ( rice. , a) or into the discontinuity of one of them ( rice. , b) . When measuring temperature, one of the junctions is tactilely thermostated (usually at 273 K). Depending on the design and purpose, thermocouples are distinguished: immersed and surface; with an ordinary, explosion-proof, moisture-proof or other sheath (hermetic or non-hermetic), as well as without a sheath; ordinary, vibration-resistant and shock-resistant; stationary and portable, etc.

Raising the temperature ceiling raises the problem of measuring high temperatures. Accurate measurements require careful standardization of measuring instruments, which provides an assessment of the accuracy of the results and their comparability with the data of other authors. For standardization, the melting (freezing), boiling and triple points of certain "reference" substances are used. Primary reference points are defined in the 1968 International Practical Temperature Scale (IRTS-68).

For very high temperatures (exceeding 3000 K), various tungsten alloys are used. The most commonly used pair is tungsten with the addition of 3% rhenium - tungsten with the addition of 25% rhenium with a thermoEMF close to 40 mV at a limiting temperature of 2573 K. molybdenum is operable up to 3300 K, but has a very low thermoEMF (8.24 mV at 3273 K). All of these thermocouples can only operate in hydrogen, pure inert gases, or vacuum.

Lecture 3

Optical pyrometers.

At very high temperatures, measurements with optical pyrometers are the most reliable and often the only possible method. This method applicable at temperatures below 1200 K, but its main area of ​​​​use is the measurement of temperatures above this value. The advantages of the pyrometer are measurements without physical contact with the object and high speed, disadvantages - problems associated with radiation: the sample must either be a black body (emissivity equal to 1), or be in thermal equilibrium with a black body, or the emissivity of the sample must be known.

Pyrometry requires the measurement of radiation flux, which is either by visual comparison of the unknown flux with that from a lamp of known characteristics (visual or subjective pyrometers), or by using a physical receiver for this purpose (photoelectric or objective pyrometers).

Taking into account the laws of radiation, pyrometers can be divided into the following types:

1. Spectral pyrometers operating in such a narrow band of the spectrum that the effective wavelength almost independent of temperature. Knowing the spectral emissivity, the true temperature can be calculated. Because the measured radiation follows Planck's law, these pyrometers can be calibrated at one fixed point.

Rice. 1. Visual brightness pyrometer,

1 - radiation source

2 – optical system, pyrometer lens

3 - standard incandescent lamp

4 - filter with a narrow bandwidth

5 - eyepiece

6 - a rheostat that regulates the filament current

7 – measuring device

An example is a brightness pyrometer, which provides the highest accuracy of temperature measurements in the range of 103-104 K. In the simplest visual brightness pyrometer with a disappearing filament, the lens focuses the image of the body under study on the plane in which the filament (ribbon) of the reference incandescent lamp is located. Through an eyepiece and a red filter that allows you to select a narrow spectral region near the wavelength λe = 0.65 μm (effective wavelength) , the thread is viewed against the background of the image of the body and, by changing the filament filament current, the brightness of the thread and the body is equalized (the thread at this moment becomes indistinguishable). The scale of the device that records the filament current is usually calibrated in ° C or K, and at the moment the brightness is equalized, the device shows the so-called brightness temperature ( Tb) body. true body temperature T is determined on the basis of the laws of thermal radiation of Kirchhoff and Planck according to the formula:

T \u003d T b C 2 /(C2+λ eIn α λ ,T) , (1)

where C 2= 0,014388 m×K , α λ , T - absorption coefficient of the body, λ e - effective wavelength of the pyrometer. The accuracy of the result primarily depends on the strictness of the fulfillment of the measurement conditions (α λ , T , λ ee, etc.). In this regard, the observed surface is given the shape of a cavity. The main instrumental error is due to the instability of the temperature lamp. A noticeable error can also be introduced individual characteristics the eyes of the observer.

2. The most sensitive (but also the least accurate) are radiation pyrometers or total radiation pyrometers that record the total radiation of the body. Total radiation pyrometers cover the entire effective spectral range emitted by a sample, regardless of wavelength. The measured radiation obeys the Stefan-Boltzmann law [the law of blackbody radiation: blackbody radiation power is directly proportional to the surface area and the fourth power of body temperature P=ST 4 ] and the true temperature can be calculated from the total emissivity of the sample. The lens of radiation pyrometers focuses the observed radiation onto a receiver (usually a thermopillar or bolometer), the signal of which is recorded by a device calibrated against black body radiation and showing the radiation temperature T r. The true temperature is determined by the formula:

T=α t -1/4 *T r , (2)

where α T is the total absorption coefficient of the body. Radiation pyrometers can measure temperature starting from 200°C. In industry, pyrometers are widely used in monitoring and control systems. temperature conditions various technological processes.

3. Spectral band pyrometers operating in a wider spectrum band. They have a strongly temperature dependent effective wavelength. Corrections for temperature are possible only by numerical integration of the experimental curve of the spectral emissivity.

4. Two-color (color or ratio) pyrometers. These are spectrum or spectral band pyrometers that use the ratio of measured radiation in two different bands of the spectrum to determine temperature. For narrow spectral bands, temperature corrections can be calculated from the ratio of the spectral emissivities for the two effective wavelengths. These pyrometers determine the ratio of brightnesses, usually in the blue and red regions of the spectrum b 1(λ1, T)/ b 2(λ2, T) (for example, for wavelengths λ1= 0.48 µm and λ2= 0.60 micron). The scale of the device is calibrated in °C and shows the color temperature Tc. true temperature T body is determined by the formula

(3)

Color pyrometers are less accurate, less sensitive, and more complex than brightness pyrometers; used in the same temperature range.

The sensitivity of colored pyrometers in the range from 1300 to 4000 K is 2 to 10 K. If there is a strong absorption of radiated radiation, colored pyrometers outperform all other types of pyrometers. However, the assumption of equal emissivity for two different wavelengths is very often not true.

At optimal conditions Experimental accuracy provided by a standard pyrometer is 0.04 K at 1230 K and 2 K at 3800 K. It is obvious that achieving such an accuracy in conventional studies is impossible. The upper measurement limit of pyrometers can be raised by using neutral density filters. The literature describes a precision instrument that allows measurements at temperatures up to 10,000 K.

To compare the radiation fluxes from the sample and from the lamp, a physical receiver (sensor) can be used instead of the human eye. This increases the speed and accuracy of measurements, and also expands their range in the direction of more low temperatures due to the sensitivity of the sensor to infrared radiation.

A very accurate spectral pyrometer is an instrument based on the principle of photon counting. It provides measurements in the range from 1400 to 2200 K with an accuracy of 0.5 to 1.0 K, respectively, in accordance with the requirements of IPTS-68. In most pyrometers, the unknown (measured) radiation flux is compared with the lamp radiation flux and the measurement accuracy depends on the characteristics of the lamp, with the main source of error being the shift of its radiation parameters. In a photon-counting pyrometer, the radiant flux of the sample is measured directly and only one fixed point (the melting point of gold) and an adjustable, but not calibrated, radiation source are needed for calibration.

There are also a number of non-traditional measurement methods that are used when conventional methods are not possible or the errors are too large. This is the use of the temperature dependence of line broadening in the emitter and in the absorber (the upper temperature limit is only 1300 K). It is also a noise thermometer based on the temperature dependence of electrical resistance noise voltage (practical limit 1800 K). Thermometers of this type are successfully used to measure cryogenic temperatures. Measurement accuracy is 1Ka best result in the range from 300 to 1300 K it is even ±0.1 K. These are also acoustic or ultrasonic thermometers using the dependence of the speed of sound on temperature.

An interesting indirect way of measuring temperatures is based on determining the heating curve of the corresponding thermometer over a certain time without the need to reach the final equilibrium temperature, which may be unacceptable for this thermometer.

gas thermometer

A gas thermometer is a device for measuring temperature, the operation of which is based on the dependence of pressure or volume of an ideal gas on temperature. The most commonly used gas thermometer is a constant volume, in which the change in temperature of the gas in the cylinder is proportional to the change in pressure. The temperature scale of a gas thermometer coincides with the thermodynamic temperature scale. A gas thermometer is used to measure temperatures up to 1300 K (Kelvin).

From the book All About Everything. Volume 1 the author Likum Arkady

Who Invented the Thermometer? Have you ever wondered, “I wonder how hot this is?” Or, “I wonder how cold it is?” If you are interested in heat, then imagine the range of questions related to this phenomenon that scientists want to clarify! But

From the book Great Soviet Encyclopedia (BE) of the author TSB

From the book Great Soviet Encyclopedia (GA) of the author TSB

From the book Great Soviet Encyclopedia (VO) of the author TSB

From the book Great Soviet Encyclopedia (ME) of the author TSB

From the book Great Soviet Encyclopedia (TE) of the author TSB

From the book All About Everything. Volume 4 the author Likum Arkady

From book Big Encyclopedia technology author Team of authors

From the book Who's Who in the World of Discoveries and Inventions author Sitnikov Vitaly Pavlovich

From the author's book

From the author's book

From the author's book

Is there a thermometer without mercury? We are so accustomed to the fact that thermometers consist of a thin tube filled with mercury that we rarely think about why this mercury is needed in this tube, that is, how this device works. A thermometer, or thermometer, is just a device

From the author's book

Liquid thermometer A liquid thermometer is the simplest instrument used very widely in almost all branches of the economic complex of Russia, in medical institutions, in everyday life for measuring the air temperature in rooms (including industrial,

From the author's book

Mercury thermometer A mercury thermometer is a device that is a liquid thermometer designed to measure temperatures in the range of 35-750 °C. For high-temperature mercury thermometers filling the space above mercury with pressurized nitrogen,

From the author's book

Resistance thermometer Resistance thermometers are made from pure metals and from metals of the semiconductor series. Resistance thermometers are designed for measurements formed on the characteristics of conductors and semiconductors, showing the possibility

From the author's book

Who Invented the Thermometer? Have you ever wondered, “I wonder how hot this is?” Or, “I wonder how cold it is?” If you are interested in heat, then imagine the range of questions related to this phenomenon that scientists want to clarify! But

Ideal gas equation of state

allows us to take as a thermometric quantity either p, or V, which can be measured with great accuracy.

As the experiment shows, sufficiently rarefied gases are very close to ideal. Therefore, they can be taken directly as a thermometric body.

In this way, one arrives at the ideal-gas temperature scale. The ideal gas temperature is the temperature measured by a gas thermometer filled with a rarefied gas. The advantage of the ideal gas temperature scale over all other empirical temperature scales is that, as experience shows, the temperature T, determined by formula (4), very weakly depends on the chemical nature of the gas with which the tank of the gas thermometer is filled. The readings of various gas thermometers when measuring the temperature of the same body differ very little from each other.

In practice, a gas thermometer is usually implemented in the following way: volume of gas V is maintained constant, then the measured pressure serves as an indicator of temperature p.

Charles's law for reference points in this case will have the form:

where p 1 - pressure of a certain mass of gas, close to ideal, at the temperature of melting ice T 1 ; R 2 - pressure at the boiling point of water T 2 .

The degree of temperature, by definition, can be chosen so that the difference between the indicated temperatures is equal to 100, i.e.

It has been experimentally established that the pressure R 2 is 1.3661 times greater than R one . Therefore, to calculate T 2 and T 1 we have two equations: K and . Their solution gives T 1 = 273.15 K; T 2 \u003d 373.15 K.

To determine the temperature of a body, it is brought into contact with a gas thermometer and, after thermal equilibrium is established, the pressure is measured. R gas in a thermometer. In this case, the body temperature is determined by the formula

It follows from this that when T=0 R=0. The temperature corresponding to zero pressure ideal gas is called absolute zero, and the temperature measured from absolute zero is called absolute temperature. Here the concept of absolute zero temperature is introduced on the basis of extrapolation. In reality, as we approach absolute zero, there are more and more noticeable deviations from the laws ideal gases the gases begin to condense. A rigorous proof of the existence of absolute zero temperature is based on the second law of thermodynamics.

Kelvin scale

(absolute thermodynamic temperature scale)

In SI, it was agreed to determine the temperature scale by one reference point, which was taken as the triple point of water. In the so-called absolute thermodynamic temperature scale, or the Kelvin scale, it is assumed by definition that the temperature of this point is exactly 273.16 K.

Such a choice numerical value made so that the interval between the normal melting points of ice and the boiling point of water is, as accurately as possible, 100 K, using an ideal gas thermometer. This establishes the continuity of the Kelvin scale with the previously used scale with two reference points. The measurements showed that the temperatures of the normal melting points of ice and the boiling points of water in the described scale are approximately 273.15 and 373.15 K, respectively.

The temperature scale defined in this way does not depend on the individual properties of the thermometric substance.

Absolute thermodynamic temperature T, counted on this scale, is a measure of the intensity of the chaotic motion of molecules and is a monotonic function of internal energy. For an ideal gas is directly related to the internal energy ().

It received the name "thermodynamic" because it can be completely independently derived from purely thermodynamic calculations on the basis of the second law of thermodynamics.

The absolute thermodynamic scale is the main temperature scale in physics. In the temperature range where a gas thermometer is suitable, this scale practically does not differ from the ideal gas temperature scale.

Celsius temperature ( t, ) connected with T(in K) equality

And K.

Types of thermometers

The temperature cannot be measured directly. Therefore, the action of thermometers is based on various physical phenomena, depending on temperature: on thermal expansion of liquids, gases and solids, changes in gas or saturated vapor pressure with temperature, electrical resistance, thermal emf, magnetic susceptibility, etc.

The main nodes of all devices for measuring temperature are the sensing element, where the thermometric property is realized, and the measuring device associated with it (pressure gauge, potentiometer, measuring bridge, millivoltmeter, etc.).

The standard of modern thermometry is a gas thermometer of constant volume (pressure is a thermometric quantity). With the help of gas thermometers, the temperature is measured in a wide range: from 4 to 1000 K. Gas thermometers are usually used as primary instruments, according to which secondary thermometers used directly in experiments are calibrated.

Of the secondary thermometers, liquid thermometers, resistance thermometers, and thermoelements (thermocouples) are most widely used.

In liquid thermometers, the thermometric body is usually mercury or ethanol. Typically, liquid thermometers are used in the temperature range from 125 to 900 K. The lower limit of the measured temperatures is determined by the properties of the liquid, the upper limit - by the properties of the capillary glass.

In resistance thermometers, the thermometric body is a metal or semiconductor whose resistance changes with temperature. The change in resistance with temperature is measured using bridge circuits (see Fig.). Resistance thermometers from metals are used in the temperature range from 70 to 1300 K, from semiconductors (thermistors) - in the range from 150 to 400 K, and carbon ones - up to liquid helium temperatures.

Widespread in temperature measurements received thermometers based on thermocouples. Here, two junctions of dissimilar metals serve as a thermometric body. If two conductors are connected according to the scheme (see Fig.), Then the voltmeter in the circuit will register voltage, meaning

which is proportional to the temperature difference between junctions 1 and 2. If the temperature of one of the junctions is maintained constant, then the voltmeter readings will depend only on the temperature of the second junction. Such thermometers are especially convenient to use in the region of high temperatures - about 700-2300 K.

At very high temperature materials melt and the described types of thermometers are not applicable. In this case, the body itself, the temperature of which must be measured, is taken as the thermometric body, and the electromagnetic energy emitted by the body is taken as the thermometric quantity. According to the known laws of radiation, a conclusion is made about the temperature of the body. The International Committee for Weights and Measures established the thermodynamic scale at temperatures above 1064 precisely on the basis of the laws of radiation. Instruments that measure radiation energy are called pyrometers.

At very low temperatures (> 1 K), it is also not possible to apply the usual methods of measuring temperatures, since the equalization of temperatures upon contact occurs very slowly and, in addition, the usual thermometric values ​​​​become unsuitable (for example, the gas pressure becomes very low, the resistance is practically independent of temperature ). Under these conditions, the body itself is also taken as a thermometric body, and the characteristics of its properties, for example, magnetic ones, are taken as a thermometric quantity.