A thermocouple is a simple and extensive component for measuring temperature.
A thermocouple, shown in the figure below, is made up of two different metal wires connected at one end, called a measuring connection (“hot”).
At the other end, where the wires are not connected, they are connected to the rows of the signal ventilation circuit, which are usually made of copper. This connection between thermocouple metals and copper traces is referred to as the reference (“cold”) connection.
Types of thermocouples
Advantages of using thermocouples
Temperature range: Most practical temperature ranges, from cryogenic to jet engine exhaust, can be provided using thermocouples. Depending on the metal wires used, a thermocouple can measure temperatures in the range of -200 ° C to + 2500 ° C.
Durable: Thermocouples are rugged devices that are protected from shock and vibration and are suitable for use in hazardous environments. Quick response: Because they are small and have a low heat capacity, thermocouples react quickly to temperature changes, especially if the test joint is exposed. They can respond to rapidly changing temperatures in a few hundred milliseconds.
Self-cooling: Because thermocouples do not need to stimulate energy, they are not prone to heating and are inherently safe.
Signal Complex Ventilation: Significant signal betting is necessary to convert the thermocouple voltage to a usable temperature reading. Traditionally, signal ventilation requires a large investment of design time to avoid errors that reduce accuracy.
Accuracy: In addition to the inherent inaccuracy of thermocouples due to their metallurgical properties, thermocouple measurements are only as accurate as the degree of connection of reference joints, traditionally at 1 to 2 ° C.
Sensitivity to corrosion: Because thermocouples are made of two different metals, in some environments corrosion over time may impair accuracy. Hence, they may need support. Care and maintenance is essential.
Noise Sensitivity: Noise from electrical and magnetic fields can be a problem when measuring the microwave surface signal. Twisting a pair of thermocouple wires can greatly reduce the magnetic field pickup. Using a protective cable or running wires in a metal duct and guard can reduce the electric field pickup. The measuring device must provide the signal filter as hardware or software with severe rejection of the line frequency (50 Hz / 60 Hz) and its harmonics.
Problems measuring temperature with thermocouples
For many reasons, converting the voltage from a thermocouple to an accurate temperature reading is not easy: the voltage signal is low, the temperature-voltage relationship is nonlinear, the connection compensation is required, and the thermocouples may cause ground problems. Let’s consider these issues one by one.
The voltage signal is small
The most common types of thermocouples are J, K and T. At room temperature, their voltages vary at 52 μV / ° C, 41μV / ° C and 41 μV / ° C, respectively. Other less common types have even smaller voltage changes with temperature. This small signal requires a large boost step before converting analog to digital. Table 1 compares the sensitivities of different thermocouples.
Because the signal voltage is low, the signal conditioning circuit typically requires about 100 or more gains – relatively simple signal conditioning.
What becomes more difficult is to distinguish the actual signal from the noise picked up on the thermocouple axes. Thermocouple leads are long and often run through noisy electrical environments. Noise in selected parts can easily overshadow the small thermocouple signal.
The two methods for extracting the signal from the noise are usually combined. The first use of a differential input amplifier is as a tool amplifier for signal amplification. Because a lot of noise appears on both wires (normal mode), different measurements eliminate it. The second is the low-pass filter, which eliminates off-band noise. The low-pass filter should eliminate both radio frequency interference (above 1 MHz) that may cause the amplifier to modify and eliminate the 50Hz / 60Hz (power supply).
It is important to place the filter in front of the amplifier for radio frequency interference (or use an amplifier with filtered inputs). The 50Hz / 60Hz filter location is often not critical. It can be combined with an RFI filter, placed between the amplifier and the ADC, as part of the Sigma Delta ADC, or programmed in software. As a medium filter.
Compensation for reference connections:
The reference junction temperature of the thermocouple must be obtained for accurate readability with absolute temperature. When thermocouples were first used, this was done by holding the reference junction in an ice bath. The following is a figure of a thermocouple circuit with one end at an unknown temperature and the other end in an ice bath (0 ° C). This method was used for a comprehensive description of the different types of thermocouples, so in almost all thermocouple tables, 0 ° C is used as the reference temperature.
But keeping the thermocouple reference junction in an ice bath is not practical for most measurement systems. Instead, most systems use a technique called connection compensation (also known as cold connection compensation). The reference connection temperature is measured with another temperature-sensitive device – typically an IC, thermistor, Diode or RTD (Temperature Detector).
The thermocouple voltage reading is then compensated to reflect the reference connection temperature. It is important to read the reference connections as accurately as possible – with an accurate temperature sensor maintained at the same temperature as the reference connection. Any error in reading the reference connection temperature is indicated directly in the final thermocouple reading.
:Various sensors are available to measure the reference temperature
- Thermistors: They have a quick response and a small package. But they need to be linear and have high accuracy, especially in the wide temperature range. They also need current to stimulate, which can heat up and lead to drift. The overall accuracy of the system can be poor when accompanied by signal ventilation.
- Temperature Detectors (RTDs) Resistance: RTDs are accurate, stable, and reasonable, but control the size and cost of the package used to control applications.
- Remote thermal diodes: A diode is used to feel the temperature near the thermocouple connector. A ventilation chip converts the diode voltage, proportional to the temperature, to an analog or digital output. Its accuracy is limited to about 1 degree pound
- Integrated Temperature Sensor: An integrated temperature sensor, a standalone IC that senses temperature locally, must be carefully installed close to the reference connection, and can combine connection compensation and signal ventilation. Accuracy is achieved in small fractions of 1 ° C.
The voltage signal is nonlinear
The slope of the thermocouple curve response changes over temperature. For example, at 0 ° C a T-type thermocouple output changes at 39 μV / ° C, but at 100 ° C the slope increases to 47 μV / ° C.
There are three common ways to compensate for the nonlinearity of a thermocouple. Choose the part of the curve that is relatively flat and use this slope as a line in this approximate area – an approach that works especially well for measurements over a limited temperature range. No need for complicated calculations. One of the reasons for the popularity of K- and J thermocouples is that they both have high temperature elasticity, with the sensitivity gradient (Seebeck coefficient) remaining relatively constant (see figure below).
From the figure above – change the sensitivity to the thermocouple with temperature. Note that the Seebeck K-type coefficient is approximately constant at about 41 μV / ° C from 0 ° C to 1000 ° C. Another method is to store the table in memory, which matches each of the thermocouple voltage sets to the corresponding temperature.
Then use linear interpolation between the two nearest tables in the table to get other temperature values. The third approach is to use higher order equations which is a model of thermocouple behavior. While this method is the most accurate, it is also the most compact in terms of computation. There are two sets of equations for each thermocouple. Converts a temperature set to a thermocouple voltage (to compensate for the reference connection). Another set converts the voltage of the thermocouple to temperature.
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