The ability of metals to change their physical properties under the influence of temperature is widely used in instrument making. So, for example, a platinum resistance thermometer uses the property of a metal to increase its electrical resistance with increasing temperature. For example, at t=0oC, platinum has a resistance of 100 ohms.
Resistance thermometer is a control and measuring device that transmits a signal to the controller about the current resistance between the sensor contacts. The controller processes the data and sends it to the process control system, where the operator already sees the current state of the temperature in the production process. The resistance thermometer is used in both chemical and engineering industries.
The design of the sensor is quite simple. The copper resistance thermometer consists of three contacts, two of which are closed to each other, and the third is common, it has a resistance of 120 ohms. The connection system is mostly three-wire, although there are exceptions. The cable from the device to the control must be protected from third-party pickups. It must not be laid together with electrical cables and must beshielded.
The working circuit is as follows: resistance thermometer - barrier - automatic control system. The device is mounted on pipelines, technological columns, used to measure the temperature in the bearings of working pumps. In operation, the resistance thermometer is unpretentious and quite accurate, its readings differ from the real ones by a maximum of 0.7 degrees.
Since the very design of such a device as a resistance thermometer is directly related to heat transfer from the measured medium to the resistor, the location of the sensor should also be taken into account during installation. The pipeline must be thermally insulated, at this point the flow rate of the measured medium must be maximum. The measurement error can be reduced by changing the ratio of the length of the sensor rod to the diameter of the vehicle. The larger it is, the more accurate the measurement result will be. The immersion depth must also be calculated from the condition of heat transfer of the measured medium. For example, in environments with a high degree of heat transfer, such as steam, or liquids, it should be 1.5 times the active length of the RTD device.
During installation, a “thermowell” is first installed in the sampling site. This is a kind of protective sleeve designed to facilitate the dismantling of the device so that the sensor can be replaced without stopping the process. Since the range of measured temperatures is in the range from 200 to 600 degrees Celsius, the protective sleevemade of stainless steel. When used in aggressive acidic or alkaline environments, the sleeves are coated with a special polymer composition that is resistant to the measured medium.
Resistance thermometer is produced in many countries around the world. The most famous models are American "Wika", Russian "Metran" and European "Endress Hauser". These devices have a high degree of reliability. They are able not only to transmit a constant temperature, but also to measure rapidly changing temperatures.
