temperature sensor

Temperature sensor is a device that allows to evaluate the value of a parameter and, if necessary, transfer information further along the control circuit. Today, individual testers are equipped with this kind of fixtures, which makes use convenient. Temperature sensors vary in design and functionality. Others are designed to assess the condition of milk, others are suitable for molten metals.

History of Thermometers

Researchers disagree on who first invented the thermometer. Candidates for the role:

1. Galileo Galilei.
2. Cornelis Drebbel.
3. Robert Flood.
4. Santorio Santorio.

More Philo of Byzantine and Heron of Alexandria were aware of the changing properties of substances under the action of temperature. In particular, the ancients interested in the air. It is noticed that when the temperature of the sealed flask, partially filled with water, the level of the separation of media moves. This is very similar to modern mercury devices. Galileo Galilei called the inventor of this class of devices - the scientist designed thermoscopes. The difference is the lack of scale.

Forced to recognize the pioneer of Robert Flud, who was the first to quantify the measurement of the shift in 1638.The design came out extremely successful. Something similar is used in industry and today. In 1613 and 1611, Santorio Santorio and Francesco Sagredo have already experimented with the scale. The term "thermometer" is first mentioned in the 1624 La Récréation Mathématique edition.

It quickly became clear that the thermal coefficient of water expansion was not high, already in 1654 an analogue with alcohol appeared, by 1730 the design acquired a practically modern appearance( the scale of the Reaumur physics is still used in France).Scientists have actively experimented with other liquids. In parallel, work was underway on the scale: in 1665, Christian Huygens proposed standard boiling points and freezing points for water.

There was no single notion of the degree of a degree until, in 1742, Celsius divided the distance between the two points mentioned above into one hundred equal parts( in the original version, the boiling point of water was taken as zero, 100% of ice melted).Appeared unit in the current view. In 1848, William Thomson( Lord Kelvin) proved the possibility of creating an absolute scale with zero below which the temperature would no longer fall( minus 273.16 degrees Celsius — zero on the Kelvin scale).The magnitude degrees Celsius and Kelvin are equal.

The final form of the composition of the thermometer took in 1714, thanks to Daniel Fahrenheit, who determined that the maximum coefficient of thermal expansion is characterized by mercury. In 1724, the glassblower offered his own scale, the name of the device is Ray Bradbury's story( the temperature of a mixture of water, salt and ice was taken as a reference point).The story does not end there, and in 1999 the first temporal non-contact thermometer appeared. Similar are used, for example, to bring up the milk, intended for food.

How to measure temperature

Thermometric properties of substances are used for measurements. It sounds trivial, like the phrase “oil oily”, but this is the reality. The substances depend on temperature:

1. Geometrical dimensions. The mentioned quality was noted by the ancients on the example of air and water. In today's world, the ability of different thermal expansion of two dissimilar metals is used more often. They are connected in a strip, "back to back", it turns out the sensor. The thermometer is called bimetallic. Similar properties in a pair show, for example, iron and zinc. The two strips, being joined together by rivets together, will bend when heated.
2. Electrical Resistance. Quality is actively used in semiconductor technology. All cheap refrigerators, where it is irrational to use a thermocouple, are supplied with similar resistances. The property works in practice. Of course, the properties of materials are different, the rate of change of parameters is not the same.
3. Electromotive force. Scientists have discovered that individual semiconductors are able to form a potential when heated. Similar qualities are characterized by minerals. For example, the famous tourmaline, named for its ability to attract ash( when heated, the surface of the crystal acquired a charge that caused the specified phenomenon).
4. Radiation spectrum. A body placed in a cold environment emits waves of electromagnetic nature. And on the graph of radiation density, it looks like a hump with a vertex shifted to the left. The higher the temperature, the stronger the mountain is shifted up the frequency scale. For example, the sun is so hot that the maximum of solar radiation falls on the visible spectrum in the green area. Similarly, a blacksmith sees red-hot metal changing shades while the furs fan the fire. Spectral thermometers allow remote measurements.

Expanded Classification of Thermometers

Let's make a reservation that within the framework of this review we do not separate pyrometers from topics. This is a slightly different class of devices, is actively used for similar purposes as temperature sensors. So, it is customary to distinguish between:

• Expansion thermometers. Based on the ability of bodies to change the geometric dimensions:

1. Glass liquid thermometers - outside the window. Already considered temperature sensors. Mercury is often used as a liquid for a number of reasons: it preserves the state of aggregation in a wide range of environmental conditions, does not wet the glass, and is easily extracted from natural components. The disadvantages include toxicity, a small coefficient of temperature expansion and freezing already at minus 35 degrees Celsius. This is reminiscent of the benefits of alcohol thermometers.
2. Gauge thermometers are based on the temperature dependence of the vapor pressure of a substance in the working chamber. Such systems are readily used as thermostats of old refrigerators, where there is no electronics. Pros: the system does not need electric power supply, which greatly simplifies the design of the device. These temperature sensors are located in the area of ​​the evaporator, through a tube connected to the controller( located in the refrigeration compartment), where there is a relay.

• Thermometric sensors and resistance thermometers include thermocouples and thermistors. This is a hackneyed topic, let's touch a little below. The materials used for these temperature sensors are metals, semiconductors, and other classes of the periodic table.

Thermometer Designs

The device that displays the interface between two media is recognized as the first and most widely used in everyday life. This is not the only model. Previously used weight thermometers. This consisted of a hollow platinum ball, partially filled with mercury and with a capillary hole in the bottom. The higher the temperature rose, the more the air in the sphere expanded. As a result, more mercury droplets flowed out. As a result, equilibrium ensued, the temperature was judged by the remaining mass.

A platinum resistance thermometer is used as a reference( from 13.81 to 903.89 degrees Kelvin), and a germanium one is used below( up to 4.2 K).Above the specified limit already used platinum. Finally, a quasi-monochromatic pyrometer is used above 1337.58 degrees Kelvin. With the help of these tools obtained data about the world. It is logical to use these devices for taring. The quasi-monochromatic pyrometer is already working on the basis of an estimate of the spectrum; it has no relation to resistance. At a temperature of 6300 K, most alloys are already drawn into steam, and the pyrometers of microwave radiation are used above the above-mentioned mark and up to 100,000 K.

The principle of operation of the construction of a quasi-monochromatic optical pyrometer is based on comparing the spectrum of the body under study with the spectrum of the reference( tungsten) filament. The device includes a lens, the viewfinder is equipped with a filter that transmits a predominantly visible spectrum of waves. It is possible to regulate the heating of the thread with a rheostat, to see it against the background of the body under study. When objects become indistinguishable( merge), the desired temperature of tungsten is reached. Accuracy strongly depends on the experimenter: the method is counter-indicative to color blindness. The upper limit of measurement is limited by the melting point of the yarn.

Other quasi-monochromatic pyrometers using filters separate some components of the spectrum. For example, red and blue, and then by their intensity determine the temperature. Photometric sensors are already used here: incident light changes the properties of semiconductor materials. Known devices that evaluate the full range of emission. We are talking about the integral brightness, when the image of the object is focused on the sensitive element.

Below the 4.2 K point, a number of reference scales are applied. For ultralow temperatures from 0.01 to 0.8 K, the dependence of the magnetic susceptibility of a substance on the degree of heating is used( it is more appropriate to speak of cooling).In the rest of the range, the dependences of the pressure of saturated helium vapor( 3 and 4) are used.

In addition to the principles listed in the previous section, alternatives are known that are not used in everyday life. If you do not take into account the devices from the construction theme. We are now talking about thermal imagers, where a visual assessment of the overall terrain is used. In this regard, devices resemble optical pyrometers. The builder just by eye finds areas that are very distracting from the overall picture, takes appropriate corrective measures. The rest of the imager operates on the basis of a matrix of photosensitive elements. The instrument does not include temperature measurement( only a qualitative assessment).

We are not going to talk about thermocouples and resistances, the information to most readers is already known. We only mention that two classes of devices are often used in everyday life. Including the probes of the above testers. The temperature dependence of the resistance is usually linear, the angle depends on the material. As for thermocouples, sensors consist of two dissimilar semiconductors. A change in temperature leads to the formation of the potential at the conclusions of the structure.

Today elementary sensors are often included in the composition of microchips. This is not news that integrated solutions are much easier to use. Similarly, the motion sensor is supplied with electronic filling to amplify the original signal to an acceptable value. Enter the scope of capabilities of integrated temperature sensors and other functions. Principles of temperature measurement are few, if you do not take into account the exotic, as magnetic susceptibility, they are all simple. In household appliances, for example, bimetallic plates are often used.