The RTD temperature sensor is a sensor thermometer. It uses the principle that the resistance of a conductor or semiconductor varies with temperature to measure temperature.
The RTD temperature sensors are divided into two main categories: metal RTDs and semiconductor thermistors. RTDs are widely used to measure temperatures in the range of -200 to +850°C. In a few cases, low temperatures can be measured up to 1K and high temperatures up to 1000°C.
The RTD sensor consists of an RTD, connecting leads, and a display instrument. The RTD can also be connected to a temperature transmitter to convert the temperature into a standard current signal output. The material used to manufacture the RTD should have the most stable resistance temperature coefficient and resistivity possible, preferably with linear work, stable physical and chemical properties, good rewiring, etc. Currently, the most commonly used RTDs are platinum RTDs and copper RTDs.
Working method
An RTD is a primary element that converts temperature changes into resistance changes and usually requires the resistance signal to be passed through leads to a computer control unit or another primary instrument. Industrial RTDs are installed on production sites at a distance from the control room, so the leads of the RTD can have a significant impact on the measurement results.
There are three main types of leads for National Standard RTD’s:
- Two-wire system:Â Connected to a wire to lead the resistance signal is called a two-wire system: this lead method is very simple,but due to the connection of the wire there is bound to lead resistance r, r size and wire material and length of the factors related to this lead method is only suitable for measuring the lower accuracy of the occasion.
- Three-wire system:Â The way to connect one lead at one end of the root of the RTD and two charges at the other end is called a three-wire system. This way is usually used with the bridge, can better eliminate the influence of lead resistance, is the most commonly used in industrial process control.
- Four-wire system: The way of connecting two wires at each end of the root of the RTD is called a four-wire system, where two leads provide a constant current I for the RTD, convert R into a voltage signal U and then lead U to the secondary instrument through the other two leads. It can be seen that this lead type can eliminate the influence of the resistance of the leads and is mainly used for high precision temperature detection.
The RTD is connected in a three-wire system. The three-wire system is used to eliminate measurement errors caused by the resistance of the connecting wires. This is because the circuit for measuring the RTD is generally an unbalanced bridge. The RTD as a bridge arm resistance, its connection wire (from the RTD to the central control room) also becomes part of the bridge arm resistance. This part of the resistance is unknown and changes with the ambient temperature, resulting in measurement errors. Using a three-wire system, one wire is connected to the power side of the bridge and the remaining two are connected to the bridge arm where the RTD is located and to its adjacent bridge arm, thus eliminating the measurement error caused by the wireline resistance.
Advantages
The RTD temperature sensor is a commonly used temperature sensor product that can measure temperature using the principle that the resistance value of a conductor or semiconductor changes with temperature and has the advantages of stable performance, flexibility, and reliability.
Factors affecting measurement
Insertion depth
The selection of the RTD temperature measurement point is the most important. The location of the temperature measurement point must be typical and representative of the production process. Otherwise, the significance of measurement and control will be lost. When the thermocouple is inserted into the measured site, a heat flow will be generated along the length of the sensor. When the ambient temperature decreases, there will be heat loss. To make the thermocouple temperature sensor and the measured object temperature inconsistency and produce temperature measurement error. In summary, the error caused by heat conduction is affected by the insertion depth. The insertion depth is related to the protection tube material.
Metal protection tube because of its good thermal conductivity, its insertion depth should be more profound, ceramic material insulation performance is good can be inserted shallowly. For engineering temperature measurement, the insertion depth is also related to the measurement object is static or flowing, such as the flow of liquid or high-speed airflow temperature measurement, will not be subject to the above restrictions, the insertion depth can be shallow, the experiment should determine the specific value.
Response time
The basic principle of the contact method of temperature measurement is that the temperature measuring element should be in thermal equilibrium with the object to be measured. Therefore, a certain amount of time is required for the temperature to be measured for the two to reach thermal stability. The length of the holding time is close to the thermal response time of the temperature measurement element. The thermal response time depends mainly on the structure of the sensor and the measurement conditions and varies greatly.
For gaseous media, especially static gases, at least 30min should be maintained to reach equilibrium; for liquids, the fastest should also be more than 5min. For the constantly changing temperature of the measured place, especially the instantaneous change process, the whole process is only 1 second. The response time of the sensor is required in milliseconds. Therefore, the ordinary temperature sensor can not keep up with the measured object temperature change speed lag and the failure to achieve thermal equilibrium and measurement errors. Consequently, it is best to choose a sensor with a fast response. In addition to the impact of the protection tube, the thermocouple measurement end diameter is also its main factor for thermocouples. The thinner the coupling wire, the smaller the diameter of the measurement end, the shorter the thermal response time.
Increased thermal impedance
RTD temperature sensor is used at high temperatures if the measured medium is gaseous. This is because the dust is deposited on the surface of the protection tube etc. It will be burned and melted on the surface so that the thermal impedance of the protection tube increases; if the measured medium is molten, there will be slag deposits in the process of use, which not only increases the response time of the thermocouple but also makes the indicated temperature low. Therefore, in addition to the regular calibration, frequent sampling is also necessary to reduce the error. For example, an imported copper melting furnace was installed with a continuous temperature measurement thermocouple sensor and equipped with a consumable thermocouple temperature measurement device, used to calibrate the accuracy of constant temperature measurement with thermocouples promptly.
Thermal radiation
The RTD temperature sensor inserted into the furnace for temperature measurement will be heated by the thermal radiation emitted by the hot object. Assuming that the gas inside the furnace is transparent and that the temperature difference between the thermocouple and the furnace wall is significant, errors in temperature measurement will occur due to energy exchange. In general, to reduce the thermal radiation error, the heat transfer should be increased. The temperature of the furnace wall should be made as close to the temperature of the thermocouple as possible. In addition, the thermocouple installation location should be as far as possible to avoid the heat radiation from the solid so that it can not be radiated to the thermocouple surface; thermocouple preferably with heat radiation shield.
According to the actual situation, these are the four factors that affect the thermocouple temperature sensor measurement in the use of the time we should pay attention to, according to the actual situation, to ensure the best effect size.