The ability of metals to change their physical properties under the influence of temperature is widely used in instrumentation. For example, a platinum resistance thermometer uses the property of a metal to increase its electrical resistance with increasing temperature. For example, at t = 0 ° C, platinum has a resistance of 100 OM.
A resistance thermometer is a
control and measuring device that transmits to the controller a signal 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 temperature state in the production process. Resistance thermometer is used in both chemical and engineering industries.
The design of the sensor is quite simple. A 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 mainly three-wire, although there are exceptions. The cable from the device to the control must be protected from external interference. It must not be laid with electrical cables and must be shielded.
The working circuit is as follows: resistance thermometer - barrier - automatic control system. The device is mounted on
pipelines, process columns, used to
measure temperature in the bearings of working pumps. In operation, the resistance thermometer is unpretentious and quite accurate, its readings differ from real ones by a maximum of 0.7 degrees.
Since the design of such a device as a resistance thermometer is directly related to the heat transfer from the medium to the resistor, it is worth considering the location of the sensor during installation. The pipeline must be thermally insulated; at this point, the flow rate of the medium to be measured 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 conditions 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 device with a resistance thermometer.
During installation, a “thermowell” is first installed in the sampling point. 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 thermowell is made of stainless steel. When used in aggressive acid or alkaline environments, the liners are coated with a special polymer composition that is resistant to the medium being measured.
Resistance thermometer is produced in many countries of the world. The most famous models: American "Wika", Russian "Metran" and European "Endress Hauser". These devices have a high degree of reliability. They are capable of not only transmitting a constant temperature, but also taking measurements at rapidly changing temperatures.