What is the difference between a thermocouple and a thermometer?

The principle of thermocouple thermometer

is to connect "galvanometer-copper wire-iron wire-copper wire" in series into a loop. At this time, two "junctions" will be formed at the two ends of the iron wire and the connection of the copper wire. If the temperatures of the two junctions are different, voltage will be generated between them. The microammeter can measure the weak current flowing through the iron wire and the copper wire. To use the thermocouple as a thermometer, the following calibration must be done first. Put one junction into ice water and the other junction into boiling water, and record the current intensity at this time. This is the current value when the temperature difference is 100℃. For two known metal conductors, the current value is proportional to the temperature difference between the two junctions. The measurement range is very large, that is, from -200℃ to 1700℃, and the sensitivity is very high.

Mercury thermometer

Due to the relationship between the freezing point (-38.87℃) and boiling point (356.7℃) of mercury, its measurement can only be within this range, and it can measure high temperatures. If it is used to measure low temperatures, it must be limited.

Thermocouples are two different metal materials connected together. At different temperatures, different currents will be generated at both ends, which is called thermoelectric potential. Other thermometers, such as glass thermometers, are made based on the principle of thermal expansion and contraction of temperature-sensitive liquids; platinum resistance thermometers are guided by the change in resistance of platinum at different temperatures. There are also semiconductors and bimetallic wires, but the principles are different.

Is it necessary to use cold-end compensation when wiring thermocouples? The mV value output by the thermocouple corresponds to the difference between the hot-end temperature and the cold-end temperature. If there is no cold-end compensation, the temperature corresponding to the mV value output by the thermocouple must be added to the temperature value of the cold-end of the thermocouple to obtain the true temperature of the hot end. If your thermocouple is connected to the control box through a compensation wire, the error caused by this is the ambient temperature of the control box; if you use ordinary copper wire to connect to the control box, the error is the ambient temperature of the thermocouple junction box. The collected temperature is low. What is the difference between a thermocouple and a thermometer?

There are many types of transmitters in temperature measurement applications, and thermocouples are a commonly used type, which can be widely used in automobiles, homes and other fields. Compared with RTDs, thermistors, and temperature sensing integrated circuits (ICs), thermocouples can detect a wider temperature range and have a higher cost-effectiveness. In addition, the ruggedness, reliability, and fast response time of thermocouples make them the first choice in various working environments.

Of course, thermocouples also have some drawbacks in temperature measurement, such as poor linearity. Although they can measure a wider temperature range than RTDs and temperature sensor ICs, their linearity is greatly reduced. In addition, RTDs and temperature sensor ICs can provide higher sensitivity and accuracy, which are ideal for precision measurement systems. Thermocouple signal levels are very low and often require amplification or high-resolution data converters for processing.

If the above issues are excluded, the low price, ease of use, and wide temperature range of thermocouples make them widely used.

Thermocouple Basics

Thermocouples are differential temperature measurement devices composed of two different metal/alloy wires, one for the positive terminal and the other for the negative terminal. Table 1 lists four common thermocouple types, the metals used, and the corresponding temperature measurement range. Each thermocouple has unique thermoelectric properties within its specified temperature range.