What are the related problems of using thermocouple to measure surface temperature
Although the thermocouple is the most common method of surface temperature measurement, because the reading of the thermocouple is actually the measurement of its own current temperature, the challenge of measurement is always how to make the thermocouple correctly match the heat of the measured surface. However, when relying on the measured value of the thermocouple as a reference value for determining the emissivity, few infrared thermographers will consider the uncertainty of this measured value.
This article will explain the principle behind the thermocouple, and through demonstration, explain the many problems in its use. In addition, we will also focus on the priority to use the combination of thermal imaging cameras and thermocouples, and the thermal imaging camera itself as an excellent method for measuring surface temperature.
A large number of commercial and industrial processes rely on accurate temperature measurement. But is the measurement performed accurately? Temperature measurement methods and temperature measurement accuracy are two extremely important questions that must be answered in all applications. We will discuss this topic in the full text.
The core theme of this article revolves around the biggest temperature measurement problem, "using thermocouples to accurately measure surface temperature". The author frankly stated that although thermocouples can provide accurate temperature readings for liquids and gases, there are many unique problems with using thermocouples for surface temperature measurement.
"If we want to measure temperature, why can't we just use thermocouples?" This is a question often asked by infrared imaging lecturers, which makes students who use infrared cameras in class have interesting thinking. When asked about thermocouple installation, many students suggested using electrical tape because it is cheap, easy to install and easy to disassemble. A student from the HVAC industry said that he usually uses electrical tape to install thermocouples on compressors. Compared to other meters, he prefers to rely on thermocouple temperature readings.
Temporary installation of thermocouples may be the worst method, because it does not achieve consistent and accurate results for measuring surface temperature. Permanent installation by bonding is a preferred method for those who need to obtain consistent measurement results. When the permanent installation method is inconvenient or feasible, infrared imaging technology will be the first choice, but it is not the only one.
The physicist Thomas Seebeck discovered the "thermoelectric effect" in 1821, that is, any conductor affected by a temperature gradient generates a voltage. Seebeck misinterpreted this effect, believing that current has a magnetic effect rather than an electrical effect. In fact, in the reports submitted to the Prussian Academy of Sciences in 1822 and 1823, his observations were described as follows: "The temperature difference caused the magnetic polarization of metals and ores."
Leopoldi Nobili and Macedonio Melloni, two Italian physicists, continued Seebeck's work to create thermoelectric batteries. This kind of thermoelectric battery is now called "thermopile". When Nobili and Melloni coupled the thermopile with a galvanometer, they became the first physicists capable of measuring infrared radiation.
The basic structure of thermocouple
The diagram in Figure 1 shows a complete thermocouple circuit. Two dissimilar metals are connected to a circuit, and voltage changes are caused by the temperature gradient.
Changes in temperature gradient will cause voltage changes. These voltages are generally in the "microvolt/℃" range. The higher the temperature gradient (indicating higher temperature), the higher the voltage generated.
When the temperature change is small, the voltage change of Seeback is linearly proportional to the temperature. The traditional formula is expressed as:
In order to measure the voltage change caused by the temperature gradient, a voltmeter must be installed in the circuit. This adds two electrical contacts: one is a copper-to-copper contact, and the other is a copper-to-dissimilar metal contact. Since we have two thermocouples in the circuit, how does the voltmeter distinguish between these two thermocouples? Please note that the ice bath temperature in Figure 1 is assumed to be 0°C, which is used as the "known reference junction" or known temperature. Once the temperature of one junction is known, the temperature of the other junction—that is, the temperature we intend to measure—can be determined by the calculation of a mathematical formula.
When you buy and install a thermocouple, where should you add an ice bath? For factory-made thermocouples, such as Extech EA10, the manufacturer uses hardware compensation and internal temperature sensing resistors instead of ice baths. This is usually called an electronic freezing point reference circuit, which is different from various types of thermocouples.
Voltage to temperature
The voltage of the thermocouple must eventually be converted to temperature. The thermoelectric potential generated by the thermocouple is a function of the temperature difference between the two ends of the thermocouple, which is very close to linear over a very wide range. The curve in the figure below is the "standard response curve" of the K-type thermocouple. This is a calibration process.
However, the relationship between temperature and voltage of thermocouples is not always linear. The formula introduced earlier shows an ideal temperature-voltage relationship, in which Seebeck's coefficient α is a constant. But it does not match the actual situation. α should be a variable represented by a polynomial.
The calibration process of the thermocouple will generate an ideal curve, as shown in Figure 2. The blue line represents the actual output (millivolt) versus temperature, and the dashed line is the "best fit" line of the data. Although there may be obvious nonlinear data at some points when checking the actual data, the output voltage of this type of thermocouple has a relatively linear relationship with temperature changes. This curve is for illustrative purposes only.
The polynomial coefficients representing the calibration curve are combined with the millivolt input value to determine the temperature reading of the thermocouple.
Like many other industrial revolution inventions in the nineteenth century, thermocouples have many everyday uses.