Gas thermometer. Thermometer - a device for measuring air 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 allocate the following types 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. At the same time, 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 the 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.
The gas thermometer operates on the principle of the relationship 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.
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 ArkadyWho 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 the 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 bookIs 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 bookLiquid 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 bookMercury 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 bookResistance 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 bookWho 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
There are many types of thermometers. Each type has its own characteristics and advantages. One of the most popular meters is a gas thermometer. This device is distinguished by its practicality and durability in operation. These devices are made mainly of glass or quartz, so the temperature that it measures should be low or not too high. Modern models differ from their predecessors, but there are no fundamental changes in the operation of new devices.
Peculiarities
A gas thermometer is an analogue of a manometer (pressure gauge). Often, constant volume meters are used. In such devices, the temperature of the gas varies depending on the pressure. The limit of such a thermometer is 1,300 K. The presented types of thermometers are in great demand. Moreover, on modern market new, improved models are presented.
The principle of operation of a gas thermometer is identical to a liquid meter and is based on the effect of expansion of a liquid when heated, only an inert gas is used here as a working substance.
Advantages
The device allows you to measure the temperature in the range from 270 to 1,000 degrees. It is also worth noting the high accuracy of the device. The gas thermometer has forte- reliability. In terms of cost, the devices are quite democratic, but the price will depend on the manufacturer and the quality of the device. When buying a device, it is better not to save and buy really quality option, which will be accurate in operation and will last as long and efficiently as possible.
Scope of application
The gas meter is used to determine the temperature of substances. Can be used in specialized laboratories. The most accurate result is shown when the substance is helium or hydrogen. Also, this type of thermometer is used to measure the operation of other devices.
Often, constant volume gas thermometers are used for the virial coefficient. This type thermometer can also be used for relative measurement with a double instrument.
A gas thermometer is mainly used to measure the temperature of certain substances. This device is widely demanded in the field of physics and chemistry. When using a high-quality gas thermometer, high accuracy is guaranteed. This kind of temperature meter is very easy to use.
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.
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 fixed 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 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 sensitive element, where the thermometric property is realized, and the associated measuring device(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.