Sensor Thermometer Resistance

The ability of metals to change their physical properties under the influence of temperature is widely used in instrument-making. For example, a platinum resistance thermometer uses the property of a metal to increase its electrical resistance when the temperature rises. For example, at t = 0 ^о С, platinum has a resistance of 100 ОМ.

The resistance thermometer is a measuring instrument 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 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 together, and the third is a common one, it has a resistance of 120 Ω. The system of connection is basically three-wire, although there are exceptions. The cable from the instrument to the control must be protected from external interference. It must not be laid with electrical cables and must be shielded.

The working chain is as follows: resistance thermometer - barrier - automatic control system. The device is mounted on pipelines, process 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 maximum by 0.7 degrees.

Since the very design of a device such as a resistance thermometer is directly related to the heat transfer from the medium to the resistor, it is also necessary to take into account the location of the sensor during installation. The pipeline must be thermally insulated, at this point the flow velocity of the medium to be measured must be maximized. The measurement error can be reduced by changing the ratio of the length of the sensor rod to the diameter of the TC. The more it will be, the more accurate the measurement result will be. The depth of immersion should also be calculated from the heat transfer condition of the medium to be measured. 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 resistance thermometer 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 between 200 and 600 degrees Celsius, the protective sleeve is made of stainless steel. When used in corrosive acid or alkaline media, the thermowells are coated with a special polymer compound that is resistant to the medium to be measured.

The resistance thermometer is produced in many countries of 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 at a rapidly changing temperature.