Overview and use

Industrial assembled resistance thermometers are commonly used as temperature sensors in conjunction with display instruments, recording instruments, and electronic regulators. It can directly measure the surface temperature of liquids, vapors, gases, and solids in the range of -200 ℃ to 420 ℃ during various production processes.
According to national regulations, our company produces two types of assembled and uniformly designed thermistors: Pt100 platinum thermistors with IEC international standard division numbers and Cu50 copper thermistors with professional standard division numbers.
Product execution standards: IEC751 、IEC1515 JB/T8622-1997、JB/T8623-1997

Working principle

Industrial thermal resistors are divided into two categories: platinum thermal resistors and copper thermal resistors.
Thermistor measures temperature by utilizing the characteristic that a substance's own resistance changes with temperature. The heating part (temperature sensing element) of the thermistor is uniformly double wound with thin metal on a skeleton made of insulating material. When there is a temperature gradient in the measured medium, the measured temperature is the average temperature in the medium layer within the range of the temperature sensing element.
Prefabricated thermistor is mainly composed of junction box, protective tube, terminal block, insulation sleeve box and temperature sensing element, and is equipped with various installation and fixing devices.
The temperature sensing element of WZP platinum resistance is a platinum wire winding, and dual platinum resistance is mainly used in situations where two sets of display, recording or regulating instruments are required to simultaneously detect the temperature at the same location. The temperature sensing element of WZC type copper resistor is a copper wire winding.

platinum

Platinum is the most ideal material for making thermal resistors, with stable physical and chemical properties, particularly strong oxidation resistance, high electrical resistivity, and good processability. The temperature measurement accuracy of platinum resistance thermometer is the highest among existing industrial thermometers, and it is one of the four standard instruments of ITS-90 international temperature scale. It can transmit standard temperatures of 13.8033K~961.78 ℃. Industrial platinum resistance thermometers mainly have two graduation marks, Pt100 and Pt10, with Pt1000, Pt800, and Pt500 being less commonly used.

Copper resistance

Copper is also the most ideal material for making thermal resistors, with low cost, easy purification, high temperature coefficient of resistance, good repeatability, and easy processing into insulated copper wire. The resistance temperature characteristics of copper resistors are almost linear in the range of -50~150 ℃. There are currently two calibration marks for industrial copper resistance thermometers, Cu50 and Cu100. Due to the continuous cost reduction of platinum resistors, copper resistors have mostly been replaced by platinum resistors.

Main technical indicators

The ratio of the resistance value (R100) of the thermistor temperature sensing element at 100 ℃ to its resistance R0 at 0 ℃: （R100/R0）
Division number Pt100: Grade A R0=100 ± 0.06 Ω Grade B R0=100 ± 0.12 Ω R100/R0=1.3850

Temperature measurement accuracy of thermal resistance

Measurement accuracy, also known as allowable deviation or "tolerance", refers to the degree of conformity between the temperature characteristics of a specific thermistor and the standard scale of that type of thermistor. Like thermal resistors, theoretically there are no two thermal resistors with identical materials, organizational structures, and processing states. Therefore, any one thermal resistor deviates from the standard scale, and the two test results of any one thermal resistor are also inconsistent, which can only meet the standard scale to a certain extent. According to the degree of conformity or deviation, thermal resistors are classified into A and B levels, as shown in the table below:

response time

When there is a step change in temperature, the output of the thermal resistor changes to 5% of the step change, and the time required is called the thermal response time, represented by τ 0.5.

Thermistor nominal pressure

Generally refers to the external pressure (static) that the protective tube can withstand at the working temperature without breaking. The allowable nominal pressure is not only related to the material, diameter, and wall thickness of the protective tube, but also to its structural form, installation method, insertion depth, and the type of flow rate box of the measured medium.

Minimum insertion depth of thermistor

Generally not less than 300mm (except for special products)

Self heating effect

When the measured current in the thermistor is 5mA, the measured resistance increment converted to temperature value should not exceed 0.30 ℃.

insulation resistance

The experimental voltage for insulation resistance at room temperature can take any value from 10 to 100V DC, with an ambient temperature range of 15 to 35 ℃ and a relative humidity not exceeding 80%. The insulation resistance value at room temperature should not be less than 100M Ω.

Lead System for Thermal Resistors

The temperature measured by thermal resistance refers to the temperature sensed by the thermal resistance element at the measuring end. The temperature determines the size of the resistance element, but the resistance value output by the measuring element includes the resistance of the lead wire. Therefore, the size, stability, and processing method of the lead wire resistance directly determine the measurement accuracy of thermal resistance. It is known from the division characteristics of thermal resistors that the average resistance change rate per degree of platinum resistors is 0.385 Ω/℃, and the average resistance change rate per degree of copper resistors is 0.428 Ω/℃. The lead resistance must not exceed the allowable temperature measurement deviation of the thermal resistor. The lead resistance of the two-wire system should not exceed 0.1 Ω, otherwise technical treatment is required to deduct the lead resistance. Lead resistance includes two parts: the lead resistance of thermal resistance products (called inner lead resistance) and the lead resistance between thermal resistance products and display instruments (called outer lead resistance). There are three types of lead methods:
Two wire system: Thermistor products only provide two leads, and the measured resistance includes the lead resistance. Generally, the lead resistance is ≤ 0.1 Ω. The two-wire lead method has a large measurement error and is generally used in situations where the lead is not long and the measurement accuracy is not high. The two-wire system only refers to the use of two internal leads for thermistor products, while the external leads installed by users must use three leads.
Three wire system: Thermistor products provide three leads. If the resistance of the three leads is equal, the influence of lead resistance on the measurement results can be eliminated. Both the inner and outer leads use three leads, which is the most widely used wiring method in industrial production. As shown in the following figure, as long as the resistance of the three leads is equal (i.e. R1=R2=R3), the resistance R0 of the temperature measuring element is independent of the size of the lead resistance and can be expressed as: R0=RAC - RAB。

Four wire system: Thermistor products provide four leads, which can completely eliminate the influence of lead resistance on measurement results. This method has high measurement accuracy and is generally only suitable for precision measurements, such as standard platinum resistance thermometers.
As shown in the above figure, regardless of whether the resistances R1, R2, R3, and R4 of the four leads are equal or not, the resistance R0 of the temperature measuring element is independent of the magnitude of the lead resistance and can be expressed as: R0=（RAD+RBC-RAB-RCD）/2。

Structure of Thermal Resistors Product Structure

Prefabricated thermal resistors are mainly composed of junction boxes, protective tubes, terminal blocks, resistance leads, and temperature sensing resistors, and are equipped with various installation and fixing devices.

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