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Temperature sensors are used to measure temperature. In industry, two types of sensors are mainly used - thermocouples and resistance thermometers.

Thermistors change their resistance depending on the temperature Copper or platinum wires are used as thermistors. The standardized static characteristics of industrial thermistors are recorded in GOST 50342-92. The nominal statistical characteristics for this GOST standard are laid out in the resistance thermometer program. Most Russian- and foreign-made thermistors having the appropriate calibration characteristics and electrical isolation from the housing can be connected to the ZET 210 and ZET 220 ADC boards as well as the ZET 017-T8 strain gauge station.

Both copper and platinum thermistors can be connected to the ZET 210 and ZET 220 ADC boards as well as the ZET 017-T8 strain gauge station. When configuring the program, you must set the type of thermal resistance and its calibration (offset at 0°C) in the main panel of the program. In multi-channel measurements, thermistors of the same type must be connected for all the channels.

The thermistors can be connected to the ADC boards according to a three-wire or a two-wire circuit. The two-wire connection scheme gives satisfactory results when the sensor is removed a short distance from the instrument. For example, suppose that you are using a copper thermistor of 100 Ohms. The resistance of this sensor is changed to dR = 0.39%R = 0.39 Ohm when the temperature changes by one degree. This means that if the resistance of the wires connecting the sensor with the device are equal to 0.4 Ohms, the temperature measurement error will be equal to one degree. The table gives the reference values of the resistance of copper wires of different cross sections and the allowable length of the wires in a two-wire connection scheme.

Effect of the length of the copper cable in the measurement of thermal resistance
Cross-section of the cable	Resistivity of the cable	Maximum cable length with an error of 1 degree for thermal resistance of 50 Ohms	Maximum cable length with an error of 1 degree for thermal resistance of 100 Ohm
0.25 mm²	82 Ohm/km	1.25 m	2.5 m
0.5 mm²	41 Ohm/km	2.5 m	5 m
1 mm²	20.5 Ohm/km	5 m	10 m

When the thermistor is at placed far away, you must use a 3-wire connection scheme. The third wire is used to compensate for the resistance of the connecting wires. All three of the wires must be made of the same copper cable with a cross-section of at least 0.5 mm² and have the same length. The maximum length of the wires must not exceed 200 m. When working with spark protection barriers, you need a 4-wire connection system for the thermistor. For a four-wire connection, you need to use differential inputs of the ADC boards and strain gauge station.

To make sure the program and thermal sensor are in working order and the circuit connection and settings are correct, place the connected sensor in boiling water (or melting ice). The temperature measured by the program must not deviate from 100°C or 0°C by more than 2°C. Without the sensor you can perform a test by connecting a precise constant 100 Ohm (with accuracy better than 0.5%) or 50 Ohm resistor to the input instead of the thermistor. Set the thermal sensor type as copper or platinum. Then the program should indicate a temperature of 0±2°C. You can use a precise resistor in the same way to check the quality of a long line, connecting the resistor on the long line instead of a thermistor.

There are several types of thermocouples. The most common thermocouples are Chromel-Alumel and Chromel-Kopel. Other types—platinum/platinum-rhodium (S and R), iron-constantan (J), copper-constantan (T) and tungsten-rhenium—are less common. The ZETLab program and ADC boards can work with all thermocouple types. The program incorporates the standardized static characteristics in accordance with the requirements of GOST 50432-92 for the following thermocouples:

• R - (platinum - 13% rhodium/platinum);
• S - (platinum - 10% rhodium/platinum);
• B - (platinum - 30% rhodium/platinum - 6% rhodium);
• J - (iron/copper - nickel (iron/constantan));
• T - (copper/copper - nickel (copper/constantan));
• E - (nickel - chromium/copper - nickel (chromel/constantan));
• K - (nickel - chrome/nickel - aluminum (chromel/alumel));
• N - (nickel - chromium - silicon/nickel - silicon (Nichrosil/nisil));
• A-1 - (tungsten - rhenium/tungsten - rhenium);
• l - (Chromel/Kopel).

Before measuring, you must set the type of the thermocouple being used and the channel of the cold junction compensator In multi-channel measurements, different types of thermocouples can be connected to the channels. A thermocouple works on the principle of measuring temperature between the “hot junction” (measuring junction) and the free ends (“cold junctions”) of the thermoelectrodes. Therefore, the thermocouple must be connected to the device directly or through extension wires made from the same thermoelectrode materials. The temperature of the “cold junctions" is measured in the zone where the thermocouples are connected (near the terminal block) by a special temperature sensor (cold junction compensator) and is taken into account when calculating the temperature. To achieve the most accurate and correct measurement of the temperature of the cold junctions, make sure there are no large temperature gradients, convective flows (blowing, wind, drafts) or radiant heat from hot bodies near the contact pads.

If you switch on the thermocouple thermometer program, but you connect a jumper instead of a thermocouple to the input of the device (i.e., if you short circuit the input), the program should show the temperature being measured in the zone of the contact pads (the temperature of the cold junction). Immediately after you turn it on, the temperature is close to the ambient temperature, then increases slightly as the device warms up. This is normal, because the thermocompensation sensor is not supposed to measure the temperature of the environment, but rather the temperature of the cold junctions. If necessary, a cold junction compensator can be adjusted. Tuning should be performed in accordance with the instructions for calibrating the cold junction compensator. To verify that the program, thermocouples and compensating wire work, you must immerse the thermocouple in boiling water .

The readings must not deviate from 100° by more than 1-2°. The ZET 017-T8 strain gauge station, ZET 220 ADC board and ZET 410/411 amplifier have high input impedance, so the resistance of the thermocouple and the compensating wires and their length do not affect measurement accuracy in principle. However, the shorter the thermocouple wires, the less they will be affected by interference. In any case, the length of the thermocouple wires must not exceed 50 m. If you want to measure the temperature at large distances, it is better to use distributed systems with an ZET 410/411 external amplifier. In this case, the distance between the amplifier and the ADC board can be up to 200 m. Some thermocouples are isolated from the housing (the hot junction is isolated or welded to the protective cover), while others are not. ZET 411 amplifiers can be used with any thermocouple, while the other ones can be used only with thermocouples isolated from the housing.