Connecting temperature sensors

Temperature sensors are essential elements of many measuring devices. They measure the temperature of the environment and different bodies. These devices are widely used as temperature meters not only in production and industry, but also in everyday life and in agriculture, that is, where people, due to their type of activity, need to measure the temperature. And there is always a question of how to properly connect such a sensor so that its functioning is accurate and error-free?

To connect the temperature sensor, no complicated work is required, the main thing here is to follow the instructions exactly, then the result will be successful, and the most difficult thing that will be required for installation is an ordinary soldering iron.

A typical sensor is, as a complete device, a cable more than 2 meters long, at the end of which the measuring device is attached directly; it differs from the cable in color, usually black. Connect the device to analog to digital converter, which converts the analog signal (current or voltage) from the sensor to digital.

One of the sensor pins is grounded and the other is connected directly to the ADC register with a resistance of 3-4 ohms. After that, the ADC can be connected to the information acquisition module, which can be connected to a computer via a USB interface, where with the help of a special program, one can perform certain actions based on the received data.

The programs allow you to work with the received information and perform many tasks related to temperature measurement. Many modern data acquisition systems are equipped with special displays for monitoring the measurements taken.

Despite the apparent simplicity, temperature sensors have different connection schemes, since it is often necessary to take into account the errors associated with the resistance of the wires.

Let's look at a specific example. The PT100 has a resistance of 100 ohms at a sensor temperature of 0 degrees Celsius. If you connect it according to the classic two-wire circuit, using a copper wire with a cross section of 0.12 sq. Mm, and the connecting cable will be 3 meters long, then the two wires themselves will have a resistance of approximately 0.5 Ohm , and this will give an error , because the total resistance at 0 degrees will already be 100.5 ohms, and this resistance should be at the sensor at a temperature of 101.2 degrees.

We can see that there may be error problems due to the resistance of the connecting wires when connecting to a two-wire circuit, but these problems can be avoided. For this, some devices can be adjusted, for example, by 1.2 degrees.But such an adjustment will not fully compensate for the resistance of the wires, since the wires themselves change their resistance under the influence of temperature.

Suppose that some of the wires are located very close to the heated chamber, together with the sensor, and the other part is far from it and changes its temperature and resistance under the influence of environmental factors in the room. In this case, the resistance of 0.5 Ohm wires during heating to every 250 degrees will become 2 times more, and this must be taken into account.

To avoid a mistake, use a three-wire connection so that the device measures the total resistance together with the resistance of both wires, although you can take into account the resistance of one wire, just multiply it later by 2. After that, the resistance of the wires is subtract from the sum and the reading of the sensor itself remains. With this solution, quite high accuracy is achieved, even if the resistance of the wires can be significantly affected.

However, even a three-wire circuit cannot correct the error associated with a different degree of resistance of the wires due to the inhomogeneity of the material, different cross-sections along the length, etc. Of course, if the length of the wire is small, then the error will be negligible, and even with a two-wire circuit, the deviations in the temperature readings will not be significant. But if the wires are long enough, then their influence is very significant. Then you should use a four-wire connection when the device measures the resistance of the sensor exclusively, without taking into account the resistance of the wires.

So a two-wire circuit is applicable in cases where:

• The measurement range is not higher than 40 degrees, and high accuracy is not required, an error of 1 degree is acceptable;

• The connecting wires are large and short enough, then their resistance is relatively small, and the error of the device itself is approximately commensurate with them: let the resistance of the wires be 0.1 Ohm per degree, and the accuracy required is 0.5 degrees, that is , the resulting error is smaller than the allowable one. The three-wire circuit is applicable in cases where measurements are made at distances from 3 to 100 meters from the sensor, and the range is up to 300 degrees, with a permissible error of 0.5%.

For more accurate and precise measurements, where the error should not exceed 0.1 degrees, a four-wire circuit is used.

A conventional tester can be used to test the device. The range for sensors that have a resistance of 100 ohms at 0 degrees is only suitable from 0 to 200 ohms, this range is available for any multimeter.

The test will be generated at room temperature, while determining which of the device's wires are shorted and which are connected directly to the sensor, then they measure whether the device shows a resistance that should be according to the passport at a certain temperature. At the end, you need to make sure that there is no short circuit on the housing. thermal converter, this measurement is performed in the megohm range. To fully comply with safety measures, do not touch the cables and the box with your hands.

If during the test the tester shows an infinitely high resistance, this is a sign that grease or water has accidentally been found in the housing of the sensor.Such a device will work for a while, but its readings will be floating.

It is important to remember that all work on connecting and checking the sensor should be done with rubber gloves. The device should not be disassembled, and if something is damaged, for example, there is no insulation of the power cables in some places, then such equipment should not be installed. During installation, the sensor may interfere with other devices operating nearby, so they must be turned off first.

If you have any difficulties, then entrust the work to professionals. In general, according to the instructions, everything can be done independently, but in some cases it is better not to risk it. After completing the installation, make sure that the device is firmly fixed in the right place, this is very important. Remember that the sensor is extremely sensitive to moisture. Do not carry out installation work during a thunderstorm.

Perform preventive checks from time to time to ensure that the sensor is working well. In general, its quality should be high, do not save when buying a sensor, a high-quality device cannot be very cheap, this is not the case when you have to try to save money.