How to choose between thermocouple temperature sensor and thermistor temperature sensor?

In daily work, it is common to encounter the use of temperature sensors, thermocouples, and thermal resistors. Although they are both used as temperature sensing elements, their principles and functions are different. The same temperature measuring ground

Should we choose a thermal resistor or a thermocouple? Today we will comprehensively analyze it.

1. Difference in working principle

A thermocouple is made up of two different conductors or semiconductor materials welded or twisted together, divided into a hot end and a free end. The hot end is inserted into the device that needs to be measured, and the cold end is placed outside the device,

If the temperatures at both ends are different, a thermoelectric potential will be generated in the thermocouple circuit. Since thermoelectric potential is a function of the measured temperature, the measured value of the electromotive force can be converted into temperature

Value. Thermal resistance is the property that the resistance value of a conductor changes with temperature, converting the change in resistance into an electrical signal for temperature measurement.

2. Structural differences

Structure of thermocouple

There are three types of shapes for thermocouple front-end bonding, as shown in the following figure. According to the type, wire diameter, and operating temperature of the thermocouple, gas welding, butt welding, resistance welding, arc welding, and silver can be used

Welding and other methods are used for joining.

In industrial applications, in order to facilitate installation and extend the service life of thermocouples, an external sleeve is usually used. Sleeve is generally divided into protective tube type and armored type.

The structure of thermal resistance

There are three types of component shapes for thermal resistors, with ceramic packaging currently dominating. Ceramic encapsulated type is used for thermal resistors with protective tubes and armored thermal resistors. Ceramic and glass encapsulation type

The diameter of bare platinum wire is about tens of micrometers, while that of mica plate type is about 0.05mm. The lead wire uses platinum alloy wire that is much thicker than the component wire.

In industrial use, the appearance of thermocouple and thermistor protective sleeves is almost the same. How should they be identified without nameplates or signals?

Firstly, look at the lead wires of the temperature measuring element. Usually, thermocouples only have two lead wires, and if there are three lead wires, it is a thermal resistor. But for those with four lead wires, measurement is required

Determine whether it is a dual branch thermistor or a four wire thermistor based on the resistance value. If it is infinite, then it is a double thermistor, and a pair of lead wires with almost zero resistance value is a thermocouple

Resistance. If the resistance of two pairs of lead wires is between 10 and 110, it is a single four wire thermal resistor. To determine which division mark of the thermal resistor is closest to its resistance value, it is the appropriate division

The thermal resistance of the temperature sensor.

If there are only two lead wires, a digital multimeter can be used to measure the resistance value to determine. Due to the small resistance value of the thermocouple, the thermal resistance is almost zero; If the resistance value is very small during measurement

It could be a thermocouple.

At room temperature, the minimum resistance value of a thermistor will also be greater than 10. The commonly used thermistors include Pt10, Pt100, and Pt1000 platinum thermistors, There are four types of copper thermistors: Cu50 and Cu100

At room temperature of 20 ℃, the resistance values of Pt10 are 10.779, Pt100 are 107.794, and Pt100 are 1077.94, Cu50 is 54.285 and Cu100 is 108.571. room temperature

When the temperature is greater than 20 ℃, its resistance value is larger, and comparing the resistance values of the two can mostly determine it. If it is a thermistor, you can also know what division mark it is.

How should one make judgments in the workplace?

Thermocouple: Thermocouples have positive and negative poles, and compensating wires also have positive and negative poles. Firstly, ensure the connection and accurate configuration. During operation, common issues include short circuits, open circuits, and poor contacts (with versatile applications)

During inspection, it is necessary to separate the thermocouple from the secondary meter to determine whether it has deteriorated (identified by surface color). My method of judgment in practice, for reference: short circuiting with tools

The compensation line on the secondary meter indicates room temperature (if not, the meter is broken), and then short-circuit the thermocouple terminal to indicate the ambient temperature where the thermocouple is located (if not, the compensation line is faulty)

)Use a multimeter to roughly estimate the thermoelectric potential of the thermocouple in MV mode (if normal, please check the process).

Thermal resistance: It can be determined by short circuit, open circuit, and multimeter. During operation, if there is a suspicion of a short circuit, simply remove one end of the resistor and check the display instrument. If it reaches the maximum value, the thermoelectric sensor will be activated

When short circuiting, returning to zero, and short circuiting of wires are ensured to ensure normal connection and configuration, the meter value may display low or unstable, and the protective tube may have been flooded. Display maximum, thermoelectric blocking circuit, display minimum,

Short circuit. Generally speaking, thermal resistors are used for temperatures below 300 degrees Celsius, and thermocouples are used for temperatures above 300 degrees Celsius. As the temperature changes, the resistance value of the thermocouple will change, and the thermal resistance of the thermocouple will also change

The electric potential will change.

To summarize, how to choose thermocouples and thermistors?

According to the temperature measurement range, thermocouples are generally selected for temperatures above 500 ℃, while thermistors are generally selected for temperatures below 500 ℃;

Select based on measurement accuracy: choose a thermistor for high accuracy requirements and a thermocouple for low accuracy requirements;

According to the measurement range selection: thermocouples generally measure the "point" temperature, while thermal resistors generally measure the spatial average temperature.