Temperature sensors. Part two. Thermistors

The first part of the article briefly talked about history of various temperature scales and their inventors Fahrenheit, Reaumur, Celsius and Kelvin. Now it’s worth getting acquainted with temperature sensors, the principles of their operation, devices for receiving data from these sensors.

The proportion of temperature measurement in technological measurements

In modern industrial production, many different physical quantities are measured. Of these, the mass and volume flow rate is 15%, the level of liquids is 5%, the time is not more than 4%, the pressure is about 10%, and so on. But the temperature measurement is almost 50% of the total number of technical measurements.

Such a high percentage is achieved by the number of measurement points. So, at an average size of a nuclear power plant, the temperature can be measured at about 1,500 points, and at a large chemical plant this number reaches twenty or more thousand.

Such a quantity indicates not only a wide variety of measuring instruments and, as a result, a multitude of primary transducers and temperature sensors, but also constantly increasing demands on the accuracy, speed, noise immunity, and reliability of temperature measuring instruments.

The main types of temperature sensors, the principle of operation

Almost all temperature sensors used in modern production use the principle of converting the measured temperature into electrical signals. Such a conversion is based on the fact that it is possible to transmit an electrical signal at high speed over long distances, while any physical quantities can be converted into electrical signals. Converted to digital code, these signals can be transmitted with high accuracy, and also entered for processing into a computer.

Resistance Thermocouples

They are also called thermistors. Their principle of operation is based on the fact that all conductors and semiconductors have Temperature Resistance Coefficient abbreviated Tks. This is approximately the same as the coefficient of thermal expansion known to everyone: when heated, the bodies expand.

It should be noted that all metals have a positive TCS. In other words, the electrical resistance of the conductor increases with increasing temperature. Here we can recall the fact that incandescent bulbs burn out most often at the moment of switching on, while the coil is cold and its resistance is small. Hence the increased current when turned on. Semiconductors have a negative TCS, with increasing temperature, their resistance decreases, but this will be discussed a little higher.

Metal thermistors

It would seem that it is possible to use any conductor as a material for thermistors, however, a number of requirements for thermistors says that this is not so.

First of all, the material for the manufacture of temperature sensors should have a sufficiently large TCS, and the dependence of the resistance on temperature should be fairly linear in a wide temperature range. In addition, the metal conductor must be inert to environmental influences and provide good reproducibility of properties, which will allow for the replacement of sensors without resorting to various fine-tuning of the measuring device as a whole.

For all these properties, platinum is almost ideal (except for the high price), as well as copper. Such thermistors in the descriptions are called copper (TCM-Cu) and platinum (TSP-Pt).

Thermistors TSP can be used in the temperature range -260 - 1100 ° C.If the measured temperature is in the range 0 - 650 ° C, then the TSP sensors can be used as reference and reference, since the instability of the calibration characteristic in this range does not exceed 0.001 ° C. The disadvantages of TSP thermistors are the high cost and non-linearity of the conversion function in a wide temperature range. Therefore, accurate temperature measurement is possible only in the range indicated in the technical data.

Cheaper copper thermistors of the TSM brand, the dependence of resistance on temperature for which is quite linear, have gained more widespread practice. As a lack of copper resistors, low resistivity and insufficient resistance to high temperatures (easy oxidation) can be considered. Therefore, copper thermistors have a measurement limit of not more than 180 ° C.

A two-wire line is used to connect sensors such as TCM and TSP, if the distance of the sensor from the device does not exceed 200m. If this distance is greater, then a three-wire communication line is used, in which the third wire is used to compensate for the resistance of the lead wires. Such connection methods are shown in detail in the technical descriptions of devices that are equipped with TCM or TSP sensors.

The disadvantages of the sensors considered are their low speed: thermal inertia (time constant) of such sensors ranges from tens of seconds to several minutes. True, low-inertia thermistors are also manufactured, the time constant of which is no more than tenths of a second, which is achieved due to their small dimensions. Such thermistors are made of molded microwire in a glass shell. They are highly stable, sealed, and low inertia. In addition, with small dimensions, they have resistance up to several tens of kilo-ohms.

Semiconductor Thermistors

They are also often called thermistors. Compared with copper and platinum, they have a higher sensitivity and negative TCS. This suggests that with increasing temperature, their resistance decreases. TCS thermistors are an order of magnitude higher than their copper and platinum counterparts. With very small dimensions, the resistance of thermistors can reach up to 1 MΩ, which eliminates the influence on the measurement result of the resistance of the connecting wires.

To measure temperature, the most widely used are semiconductor thermistors KMT (based on oxides of manganese and cobalt), as well as MMT (oxides of manganese and copper). The conversion function of the thermistors is quite linear in the temperature range of -100 - 200 ° C, the reliability of semiconductor thermistors is very high, the characteristics are stable for a long time.

The only drawback is that in mass production it is not possible to reproduce the necessary characteristics with sufficient accuracy. One instance is significantly different from the other, in much the same way as transistors: it seems to be from the same package, but the gain is different for everyone, you cannot find two identical ones out of a hundred. Such a scatter of parameters leads to the fact that when replacing a thermistor, it is necessary to adjust the equipment again.

Most often, a bridge circuit is used to power resistance thermal converters, in which the bridge is balanced using a potentiometer. When the resistance of the thermistor changes due to temperature, the bridge can only be balanced by turning the potentiometer.

A similar scheme with manual adjustment is used as a demonstration in educational laboratories. The potentiometer engine has a scale calibrated directly in units of temperature. In real measuring circuits, of course, everything is done automatically.

The next part of the article will talk about the use of thermocouples and mechanical expansion thermometers - Temperature sensors. Thermocouples

Boris Aladyshkin, i.electricianexp.com

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