In our daily work, we often encounter the use of temperature sensors. Although both thermocouples and resistance temperature detectors (RTDs) are temperature sensing elements, their principles and functions are different. For the same temperature measurement location, should we choose an RTD or a thermocouple? Today, we will analyze this in detail.
1. Differences in working principles
A thermocouple is made by welding or twisting together two different conductors or semiconductor materials. It consists of a hot junction and a cold junction. The hot junction is inserted into the equipment to be measured, while the cold junction is placed outside the equipment. If the two ends are at different temperatures, a thermoelectric potential is generated in the thermocouple circuit. Since the thermoelectric potential is a function of the measured temperature, the measured value of the electromotive force can be converted into a temperature value. A resistance temperature detector (RTD) works by converting the change in resistance of a conductor with temperature into an electrical signal, thereby measuring the temperature.
2. Structural differences
Thermocouple structure
There are three types of thermocouple tip joint shapes, as shown in the figure below. Depending on the thermocouple type, wire diameter, and operating temperature, the joint can be formed using methods such as gas welding, butt welding, resistance welding, arc welding, or silver soldering.
In industrial applications, external sheaths are typically used to facilitate installation and extend the lifespan of thermocouples. Sheaths are generally classified as protective sheaths or armored sheaths.
Structure of a thermal resistor
There are three types of RTD (Resistant Temperature Detector) element shapes, with ceramic-encapsulated types currently dominating. Ceramic-encapsulated types are used for RTDs with protective tubes and armored RTDs. The bare platinum wire diameter for ceramic and glass-encapsulated types is around tens of micrometers, while that for mica-plate types is approximately 0.05 mm. The leads use platinum alloy wire that is much thicker than the element wire.
In industrial applications, thermocouples and resistance temperature detectors (RTDs) protective sheaths look almost identical. How can they be identified without a nameplate or known signal information?
First, examine the leads of the temperature sensing element. Thermocouples typically have only two leads; three leads indicate a resistance temperature detector (RTD). For devices with four leads, you need to measure the resistance value to determine if it's a dual-wire or four-wire RTD. If the resistance is infinite, it's a dual-wire RTD; the pair of leads with almost zero resistance is a single-wire RTD. If the resistance of the two pairs of leads is between 10 and 110 ohms, it's a single-wire four-wire RTD. The resistance value should be close to the RTD's coefficient; that indicates it's a RTD of that coefficient.
If there are only two leads, you can use a digital multimeter to measure the resistance value to determine if it is a thermocouple. Since the resistance value of a thermocouple is very small, the thermal resistance is almost zero. If the resistance value is very small when measured, it is likely a thermocouple.
Even at room temperature, the minimum resistance of a resistance temperature detector (RTD) will be greater than 10 ohms. Commonly used RTDs include four types: Pt10, Pt100, Pt1000 platinum RTDs, and Cu50 and Cu100 copper RTDs. At room temperature (20°C), their resistance values are: Pt10 10.779 ohms, Pt100 107.794 ohms, Cu50 54.285 ohms, and Cu100 108.571 ohms. The resistance values are even higher at temperatures above 20°C; comparing these values usually allows for identification. If it's an RTD, then the RTD's type (e.g., its coefficient of resistance) can be determined.
How should one make a judgment at the work site?
Thermocouples: Thermocouples have positive and negative terminals, and so do compensating leads. First, ensure the connection is correct. Common problems during operation include short circuits, open circuits, poor contact (which can be diagnosed with a multimeter), and deterioration (identified by surface color). During inspection, the thermocouple should be separated from the secondary instrument. My method for troubleshooting, provided for your reference: Short-circuit the compensating lead on the secondary instrument with a tool; the meter should indicate room temperature (if not, the meter is faulty). Then short-circuit the thermocouple terminals; the meter should indicate the ambient temperature where the thermocouple is located (if not, the compensating lead is faulty). Finally, use a multimeter in mV mode to roughly estimate the thermocouple's thermoelectric potential (if normal, check the manufacturing process).
Resistance temperature detectors (RTDs): Problems are mainly short circuits and open circuits, which can be determined using a multimeter. If a short circuit is suspected during operation, simply disconnect one wire from the resistor end and observe the display. If it reaches maximum, the RTD is short-circuited; if it returns to zero, the wire is short-circuited. If the meter reading is low or unstable when properly connected and configured, the protective tube may have water ingress. Maximum reading indicates an open circuit; minimum reading indicates a short circuit. Generally, RTDs are used for temperatures below 300 degrees Celsius, while thermocouples are used for temperatures above 300 degrees Celsius. The resistance of an RTD changes with temperature, and the thermoelectric potential of a thermocouple changes.
In conclusion, how do you choose between thermocouples and resistance temperature detectors (RTDs)?
Based on the temperature measurement range: thermocouples are generally chosen for temperatures above 500℃, while resistance temperature detectors (RTDs) are generally chosen for temperatures below 500℃.
Choose based on measurement accuracy: select resistance temperature detectors (RTDs) for high accuracy requirements, and select thermocouples for lower accuracy requirements;
Choose according to the measurement range: thermocouples generally measure "point" temperature, while resistance temperature detectors (RTDs) generally measure the average temperature of space.
