How does a mercury thermometer work? A thermometer is a device for measuring temperature.

The oldest devices for measuring temperature - liquid-in-glass thermometers - use the thermometric property of the thermal expansion of bodies. The action of thermometers is based on the difference in the coefficients of thermal expansion of the thermometric substance and the shell in which it is located (thermometric glass or less often quartz).

The liquid thermometer consists of a glass cylinder 1, capillary tube 3 and spare tank 4 (Figure 3-1). thermometric substance 2 fills the balloon and partially capillary tube. Free space in the capillary tube and in the reserve tank is filled with an inert gas or can be kept under vacuum (at temperatures below +100°C). The spare tank or the part of the capillary tube protruding beyond the upper division of the scale serves to protect the thermometer from damage during excessive overheating.

The temperature is judged by the magnitude of the apparent change in the volume of the thermometric substance. The temperature is measured from the height of the level in the capillary tube. The degree scale is applied either directly to outer surface massive thick-walled capillary (stick thermometer), or on a special scale plate located inside the outer glass shell of the thermometer (embedded scale thermometer), or on an applied scale plate to which the capillary tube is attached.

Chemically pure mercury is most often used as a thermometric substance. It does not wet glass and remains liquid over a wide temperature range. Some disadvantage of mercury is the low value of its expansion coefficient. The lower measurement limit is limited by the solidification temperature of mercury and is equal to minus 35°C. The upper limit of measurement with a mercury thermometer is determined allowable temperatures for glass: 600°C for standard thermometers and 500°C for technical ones (GOST 2823-59). When replacing glass with quartz, the upper limit of measurement is slightly increased.

Since the boiling point of mercury at normal atmospheric pressure is 356.58 ° C, for thermometers designed to measure high temperatures, the space above the mercury in the capillary tube is filled with an inert gas under pressure. For thermometers with a scale up to 500°C, the gas pressure reaches 20 bar(20- 10 5 n / m 2).

Rice. 3-1. Diagram of a liquid glass thermometer

In addition to mercury, other liquids, mainly of organic origin (ethyl alcohol, methyl alcohol, kerosene, toluene), are also used as a thermometric substance in glass thermometers.

The main advantages of glass liquid thermometers are ease of use and sufficient high accuracy measurements even for series-produced thermometers.

The disadvantages of glass thermometers include: poor visibility of the scale (if you do not use special magnifying optics) and the inability to automatically record readings (if you exclude the use of slow motion filming), transmit readings over a distance (if you do not use television) and repair (a broken thermometer cannot be restored! ).

Glass liquid thermometers are very wide application and are available in the following main varieties:

1. Technical mercury, with an embedded scale, with a lower part immersed in the measured medium, straight and angled. Thermometers are manufactured with scales from -35 to +50°С and from 0°С to 50; 100; 150; . . .; 500°C. Price smallest division scale within the measurement range up to +50°C is 0.5 or 1°C and, gradually increasing, reaches 5 or 10°C at the upper measurement limits of 450 and 500°C.

2. Laboratory mercury, stick or embedded scale , immersed in the measured medium up to the measured temperature mark, straight, small outer diameter (5-11 mm). Thermometers are divided into four groups according to the limits of measurement and the price of division of the scale. The most accurate thermometers with a scale division value of 0.1°C have a measurement interval of 50°C, for example from +150 to +200°C (not higher than + 350°C). The upper limit of measurement for scales starting from 0°C is 500°C at a scale interval of 2°C.

3. Liquid (non-mercury) thermometers are available in various designs, including those with an applied scale plate , for measuring temperatures from -190 to +100°С.

4. High-precision and exemplary mercury thermometers with an upper measurement limit of 600°C are characterized by a small scale division value - up to 0.0 GS.

5. Electrocontact mercury thermometers with an embedded scale, with contacts soldered into the capillary tube for breaking (or closing) a column of mercury electrical circuit. They are made to measure either a constant contact temperature or an arbitrarily variable one within the range from 0 to 300°C.

6. Special thermometers, including maximum (medical, etc.), minimum, meteorological and other purposes.

The indisputable and most significant advantage of an electronic thermometer is its absolute safety of use: the absence of toxic mercury; comfortable, soft tip, not capable of injuring; the electronic thermometer is made of durable plastic, which is important when using it for children, as you can not be afraid that the child will break it. The time for measuring temperature with mercury thermometers takes about 10 minutes, while with an electronic thermometer you can accurately measure the temperature in just 1-2 minutes. Using an electronic (digital) thermometer is much easier and more enjoyable. You no longer have to strain your eyesight and turn the thermometer to determine its readings, as is usually the case with a mercury thermometer, which requires a certain skill from the user. Indications are displayed on the big liquid crystal display, and for some models the choice of units of measurement (°С or °F) is possible. In addition, they are equipped with additional functions: a sound signal when elevated temperature, memory for several measurements, etc.

With proper operation, the measurement error of electronic thermometers is only 0.1 ° C. The disadvantages include the need for batteries to work. But when using high-quality batteries, they will have to be replaced only after a few years. Many models also have an auto-off feature to extend battery life. It can be concluded that there are practically no minuses.

The principle of operation of an electronic thermometer.
Electronic or digital thermometers work on the basis of a built-in metal sensor. The principle of operation is based on a change in resistance. They look about the same as ordinary ones, with the only difference that their case is made of high-quality plastic or rubber, with a built-in tip and display.
Most of the models are shockproof, that is, in case of an accidental fall on the floor, absolutely nothing will happen to the device. For the thrill-seekers, there are even waterproof options.
The digital thermometer will tell you the result in a maximum of a minute! He will notify you of this by giving a sound signal. True, when measuring temperature in the armpit, the time can increase up to 3 minutes. But still it is much faster than mercury thermometers.
Many thermometers have many additional features. Such as remembering the last measurements, display backlight, the ability to select temperature scale(Celsius or Fahrenheit), interchangeable caps for hygiene purposes, etc.

On a note.
It should be remembered that the measurement indicators are more accurate, the tighter the device fits the skin. If the temperature is measured in the armpit, after the signal that the results are ready, you need to hold the thermometer a little more.
How to "measure" the temperature. When using the thermometer orally, the person's mouth must be closed.
In addition, do not forget that the digital thermometer is battery operated. They need to be replaced from time to time. Therefore, it is always better to have a couple of spares on hand so that the thermometer does not stop working at the most inopportune moment.

In pharmacies and specialized stores can be found for babies. They are made in the form of a nipple, which solves the problem of an open mouth.

True, such a device will be useless for the parents of those children who are not accustomed to a pacifier.

Infrared thermometer.

Infrared thermometer - the most modern model thermometers. It measures infrared radiation emitted from the surface of the human body. The data obtained is transformed into degrees familiar to us.
True, they can measure the temperature only in certain places (usually auricles, forehead, temples).
There are also non-contact devices. In terms of functions, these thermometers do not differ much from electronic ones, and the speed of their measurement is even faster.

So, why should you opt for an electronic thermometer.
1. It is equipped with a digital screen.
2. Each electronic thermometer is equipped with a sound signal. The device emits a sound when turned on, which confirms its performance. At the end of the temperature measurement, a signal also sounds - this means that the procedure is over.
3. In the memory of the latest digital technology, the readings of the last few examinations are stored. This allows you to control the slightest deviations in the results.
4. Plastic utensils are safe for children to use on their own. You can not be afraid that the thermometer will break.
5. The thermometer turns off automatically if there is no need for its further use.
6. Electronic device runs on batteries. It is enough to replace the old power supply with a new one, after which you can use the device for several more years. Even if the device is disposable (in the sense that the manufacturer does not provide for the replacement of batteries), it will still have a service life of up to several thousand hours.
7. Some models of electronic thermometers are equipped with a backlight, which allows them to be used at night without turning on the electricity.
8. The electronic children's thermometer is equipped with a flexible tip, which makes it possible to measure the temperature in a rectal or oral way.
9. Some manufacturers have equipped their products with protective covers, which saves the case from mechanical damage, and therefore the storage issue disappears by itself.
10. All electronic thermometers have a special button to turn on. Thanks to her, you do not need to shake the thermometer to throw off the last temperature reading.

Good to know.
What is normal temperature?
Normal body temperature is 36.6 degrees. So it is written in the instructions for a mercury thermometer. But in the annotations to electronic devices indicators are different. If you measure the temperature orally, the temperature is considered to be from 35.7 to 37.3 degrees, rectally - 36.2 - 37.7. In the usual way, that is, under the armpit, a healthy body temperature is considered to be from 35.2 to 36.7 degrees. In children, the figures may be slightly higher. How younger child- the higher the temperature. For example, in infants it can be up to 37.4 degrees - provided that they have normal behavior and well-being. In children of this age, the system of thermoregulation has not yet been established.

Translated from Greek, it means "to measure heat." The history of the invention of the thermometer dates back to 1597, when Galileo created a thermoscope - a ball with a soldered tube - to determine the degree of heating of water. This device did not have a scale, and its readings depended on atmospheric pressure. With the development of science, the thermometer has changed. The liquid thermometer was first mentioned in 1667, and in 1742 the Swedish physicist Celsius created a thermometer with a scale where 0 was the freezing point of water and 100 was the boiling point.

We often use a thermometer to determine the temperature of the air outside or body temperature, but the use of a thermometer is by no means limited to this. Today, there are many ways to measure the temperature substances, and modern thermometers are still being improved. Let us describe the most common types of temperature meters.

Operating principle of this type thermometers is based on the effect of liquid expansion when heated. Thermometers that use mercury as a liquid are often used in medicine to measure body temperature. Despite the toxicity of mercury, its use allows you to determine the temperature with greater accuracy compared to other liquids, since the expansion of mercury occurs according to a linear law. In meteorology, alcohol thermometers are used. This is primarily due to the fact that mercury thickens at 38°C and is not suitable for measuring lower temperatures. The range of liquid thermometers is on average from 30 ° C to +600 ° C, and the accuracy does not exceed one tenth of a degree.

gas thermometer

Gas thermometers work on the same principle as liquid thermometers, only they use an inert gas as a working medium. This type of thermometer is analogous to a manometer (a device for measuring pressure), the scale of which is graduated in units of temperature. The main advantage of a gas thermometer is the ability to measure temperatures around absolute zero(its range is from 271 °С to +1000 °С). The maximum achievable measurement accuracy is 2*10 -3 °C. Obtaining a high accuracy gas thermometer is challenging task, so these thermometers are not used in laboratory measurements, but are used for the primary determination of the temperature of a substance.

This type of thermometer works by analogy with gas and liquid. The temperature of the substance is determined depending on the expansion of the metal spiral or bimetal strip. The mechanical thermometer is highly reliable and easy to use. As independent devices, such thermometers have not received wide distribution and are currently used mainly as devices for signaling and controlling temperature in automation systems.

Electric thermometer (resistance thermometer)

The operation of an electric thermometer is based on the dependence of the conductor resistance on temperature. The resistance of metals increases linearly with increasing temperature, which is why metals are used to create this type of thermometer. Semiconductors, compared with metals, provide greater measurement accuracy, however, thermometers based on them are practically not produced due to the difficulties associated with grading the scale. The range of resistance thermometers directly depends on the working metal: for example, for copper it ranges from -50 °C to +180 °C, and for platinum - from -200 °C to +750 °C. Electric thermometers are installed as temperature sensors in production, in laboratories, on experimental stands. They are often bundled with other measuring devices.

Also called thermocouple. A thermocouple is a contact between two different conductors that measure temperature based on the Seebeck effect, discovered in 1822. This effect consists in the appearance of a potential difference at the contact between two conductors in the presence of a temperature gradient between them. Thus, through the contact with a change in temperature, begins to pass electricity. The advantage of thermocouple thermometers is the ease of execution, a wide range of measurements, the possibility of grounding the junction. However, there are disadvantages: the thermocouple is subject to corrosion and other chemical processes with time. Thermocouples with electrodes made of noble metals and their alloys - platinum, platinum-rhodium, palladium, gold. The upper limit of temperature measurement using a thermocouple is 2500 °C, the lower limit is about -100 °C. The measurement accuracy of the thermocouple sensor can reach 0.01 °C. A thermocouple-based thermometer is indispensable in industrial control and monitoring systems, as well as in measuring the temperature of liquid, solid, granular and porous substances.

Fiber Optic Thermometer

With the development of optical fiber manufacturing technologies, new opportunities for its use have arisen. Fiber-optic sensors are highly sensitive to various changes in external environment. The slightest fluctuation in temperature, pressure or tension of the fiber leads to changes in the propagation of light in it. Fiber optic temperature sensors are often used to ensure industrial safety, for fire warning, monitoring the tightness of containers with flammable and toxic substances, leak detection, etc. The range of such sensors does not exceed +400 °C, and the maximum accuracy is 0.1 °C.

Infrared thermometer (pyrometer)

Unlike all previous types of thermometers, it is a non-contact device. You can read more about pyrometers and their characteristics in a separate one on our website. A technical pyrometer is capable of measuring temperature in the range from 100 °C to 3000 °C, with an accuracy of several degrees. Infrared thermometers are convenient not only in production conditions. Increasingly, they are used to measure body temperature. This is due to many advantages of pyrometers compared to mercury analogs: safety of use, high accuracy, minimum time for temperature measurement.

In conclusion, we note that now it is difficult to imagine life without this universal and irreplaceable device. Simple thermometers can be found in everyday life: they are used to maintain the temperature in the iron, washing machine, refrigerator, ambient temperature measurement. More sophisticated sensors are installed in incubators, greenhouses, drying chambers, in production.

The choice of a thermometer or temperature sensor depends on the scope of its use, the measurement range, the accuracy of the readings, overall dimensions. And the rest - it all depends on your imagination.

Description and principle of operation of an electronic thermometer.

Temperature 36.6 degrees as temperature reference healthy person is nothing more than a convention. In reality, the normal temperature of a healthy person depends on characteristic features body, time of day, age, previous physical exertion, food intake, sleep ... So, in particular, in the morning a person’s temperature is several tenths of a degree lower than in the evening, on an empty stomach is lower than after eating, and in children the temperature is slightly higher than in people of age. The norm is the temperature from 36 to 37 degrees Celsius, with its fluctuations during the day within one degree, but not more than 37.2 degrees.

The temperature of the patient can be measured by placing the thermometer in the mouth (orally), in the rectum (rectally), in the armpit (axillary), etc. In our country, the latter option is preferred. The method of measuring the temperature in the armpit is quite convenient for the patient, but the longest, since it is necessary to ensure close contact of the measuring zone of the thermometer with the muscles for a sufficiently long time.

In addition, depending on the location of the temperature measurement, its values ​​\u200b\u200band will vary. Thus, the temperature measured in the armpit, on average, is half a degree lower than that measured orally, and one degree lower than that measured rectally. This does not mean that one temperature or another is "correct" and others are not. Just be sure to let your doctor know how you usually take your temperature.

Mercury thermometers or, as they are also called by the people, thermometers are familiar to every person in our country. Thermometers of this type have a number of undoubted advantages- low cost, accuracy, independence from the power source, memory of the maximum measured temperature due to the special device of the mercury capillary (as a result, such thermometers are also called maximum ) - and one significant drawback: mercury used as a filler in such a device is very toxic.

Mercury thermometers require careful handling, although they are not exactly fragile. According to the state standard regulating their production "Medical maximum glass thermometers" (GOST R), the thermometer must withstand a load of up to 50 N (5.1 kg). The minimum trouble-free service life of such thermometers is 450 cycles (one cycle is temperature measurement and subsequent shaking), the manufacturer's warranty, according to GOST, must be at least one year. Maximum term the service of such a thermometer with careful handling is not limited, which is quite logical, since in such a thermometer there are neither mechanical components nor electronic parts aging over time.

About 10-15 years ago appeared on the market electronic thermometers , which are based on a thermistor - an element that changes its resistance depending on temperature. Such thermometers are easy to use, allow you to read the temperature well, store the latest measurement results, and are absolutely harmless to humans.

It is inconvenient to measure the body temperature of infants and young children with classic oblong-shaped thermometers. A conventional thermometer can be a cause of discomfort for them, because of which the constant contact of the thermometer with the body will not be ensured. Therefore, especially for children under three years of age, electronic thermometers in the form of a pacifier are designed.

The device is not a full-fledged replacement for a pacifier, its silicone part may become unusable from prolonged sucking. The device weighs less than 40 grams. The thermometer is waterproof, making it easy to disinfect. Due to the tightness of the case, battery replacement is not provided, but the battery reserve should be enough for 2000 measurements.

Another type of medical thermometer, which can now be found in pharmacies, is based on measuring the intensity of a special electromagnetic radiation- infrared rays. Accordingly, they are called infrared (IR) thermometers . The human body, like any other physical object whose temperature is different from absolute zero, is a source of infrared radiation. The more intense the infrared radiation, the higher the person's temperature - it is on this principle that night vision devices work, highlighting the contour of a person against a colder environment.

Infrared thermometers have a significant advantage over mercury and electronic ones. They can work without contact with the surface of the body, which reduces discomfort for sleeping people and children and facilitates the disinfection of thermometers.

One example of an IR thermometer is the WF-2000 forehead infrared thermometer from the British company B. Well. This thermometer is designed to measure the temperature in the temple area, since the temporal artery is located almost under the skin. Before measuring, the forehead should be wiped from sweat and the thermometer should be slowly moved around the temple, either over the skin or in close proximity to it. During measurement, the thermometer will beep shortly. After the measurement is completed (it takes from 5 to 30 seconds), a long beep will sound. After that, it does not make sense to keep the thermometer at the forehead to increase the accuracy of the measurement, since after a minute the thermometer automatically turns off. If the temperature is above 37.5 degrees, the device will draw your attention to this with a special sound signal. An interesting feature of the WF-2000 thermometer is the ability to measure water temperature or room temperature. The temperature measurement range is from minus 22 to plus 80 degrees Celsius. To do this, just bring the thermometer to water or any object in the room that does not heat itself and is in the room for at least half an hour. The thermometer cannot be submerged in water itself, it is not waterproof.

The thermometer is equipped with a memory for the last 25 measurements. The battery is an ordinary "hour" battery (CR-2032), the device itself gives a signal about its replacement.

Temperature, along with pressure, is the most important parameter in the operation of heating systems. After all, it is temperature that is the determining factor in the operation of heating devices. It is by temperature that the Sanitary Norms and Rules are normalized ( SNiP), according to which the air temperature in the room should not be below 18°С. What will be the pressure in the system is not so important, the main thing is that it is enough to reach the highest point of the system, but every consumer feels the temperature in the room. Boiler houses have special temperature charts that display the dependence of the temperature of the water supplied to the heating system on the outside temperature.

That is why it is important, along with pressure, to control the temperature of the coolant as well. That's what thermometers are for. According to the principle of action, they are divided into:
— expansion thermometers , which are based on the expansion of liquids or solids(metals) when heated. These include liquid thermometers, manometric and bimetallic;
— electrical , which include thermoelectric thermometers and resistance thermometers.

Let us dwell in more detail on only a few types of thermometers, most common in boiler rooms and central heating stations.

Liquid thermometers consist of glass capillary tube and graduated scales, which represent degrees. The lower end of the tube is connected to reservoir, which is filled with liquid, the upper end is sealed. When heated, the liquid, as is known, expands, and the height of its column in the capillary corresponds to a certain temperature. According to the shape of the bottom of the tube, liquid glass thermometers are classified into straight and corner. This is indicated in the notation as type A and type B.
In industry, mercury thermometers are the most common. They are used to measure temperatures in the range from -35 to + 600°C. The correct readings of a liquid thermometer largely depend on the correctness of its installation on the pipeline. When measuring temperatures up to +150°C the sleeve in which the thermometer is placed is filled with engine oil. To protect the thermometer, a frame must be screwed onto the sleeve, which is made of metal with low thermal conductivity.

Principle of operation manometric thermometers based on an increase in pressure in closed system liquids or gases as their temperature increases. Depending on the working substance used, the limits of the measured temperatures also change. In general, they fluctuate in the range from -200 to +1000°С.

The main parts of a manometric thermometer are thermal bulb and flexible capillary copper tube long up to 6 m which connects the balloon to tubular spring. When the liquid is heated, the pressure increases, which is transferred to the spring. The spring, at the moment of straightening, acts on the thrust and turns the arrow of the indicating pressure gauge.

When using manometric thermometers Special attention needs to be addressed for safety capillary, as well as the density of its connection with the bulb and secondary device. The capillary is fixed to the walls with special brackets. If necessary, the capillary can be laid in a pipe or under a metal square. When installing, it should also be remembered that the capillary should not be laid near heated surfaces and heating devices.

Work bimetal thermometer is based on the use of two metals with different coefficients of linear expansion in the design of its sensitive element. Metal plates are firmly connected to each other (most often by welding) and together form bimetallic element(plate or spiral), which, when heated, expands and rotates the thermometer needle connected to it.

The design of a typical bimetallic thermometer includes round case, which has dial, kinematic mechanism with an arrow and a sensitive bimetallic element in a protective tube.

The accuracy class of the instruments is 2,5 , measurement range - from -70 to +600°С. The error at low ranges is about 1 degree, at higher ranges the error reaches 10 degrees.

The advantages of bimetallic thermometers compared to liquid ones are mechanical strength and insensitivity to pressure surges. Therefore, the use of bimetallic thermometers in boiler rooms, if possible, is more preferable.