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Thermocouples. Problems encountered during commissioning

Thermocouples. Commissioning experience

During commissioning, there are cases when it is difficult to understand the reasons for incorrect temperature measurement of thermocouples. In this case, it is necessary to find out the reasons for unreliable measurements. In this article, I would like to share ways to find flaws, analyze the situation and clarify a number of controversial points..
First a little theory.
A thermocouple consists of a combination of different alloys, which, due to their physical and chemical properties, upon contact, begin to be a source of a weak electric current (EMF). When temperature is applied to a thermocouple, the potential difference in it changes, measured in several millivolts. This potential difference depends on the type of materials used to make the thermocouple. A thermocouple, by its principle of operation, is similar to an electric battery, where the interaction of two materials and an electrolyte creates a potential (emf).
The principle of operation of the measuring channel of the "thermocouple" type.

Схема соединения термопар 

The junction point of the metals in the thermocouple itself is referred to as the "hot junction". "Cold junction" is all other connections in this measurement channel. To determine the temperature of the zone in which the thermocouple is installed, it is enough to measure the number of millivolts at the entrance to the measurement channel.

The connection between the thermocouple and a voltage meter (such as a millivoltmeter or I/O module) is usually made using temperature compensation cables or extension cables. In the first case, the conductor materials have characteristics similar to those of a thermocouple, in the second case, they are made of the same material.
Despite the use of thermal compensating materials, there will always be a point at which the thermocouple will come into contact with another material, such as the copper traces of electronic circuits. In this case, new connections ("cold junction") will be created, which will contribute to the total measured EMF (distorting the readings).
Let's look at an example.

There is a “Cromel-Alumel” type thermocouple. When Cromel and Alumel conductors are in contact with copper, two new thermocouples are formed: "Cromel-Copper" (EMF = V1 will be generated) and Copper-Alumel (EMF = V2 will be generated). Consequently, two additional and opposite in polarity EMFs will be generated, which will be added to the EMF of the Cromel-Alumel connection (EMF = V will be generated).
Ultimately, we will have such an EMF in the measurement circuit:
V общий = V+(V1-V2),
where V1-V2 is usually a negative value. If all "cold junctions" were at 0 °C, then they would not have any effect on the overall measurement circuit, since the entire EMF generated by them would be equal to zero. However, most often it is necessary to level the influence of "cold junctions" by adding an additional EMF to the measuring circuit according to the following formula:
V compensatory = V2-V1
To remove the EMF of the "cold junction" (V1-V2), you must add the calculated part of the EMF (V2-V1). Thus, the subtraction of numbers close in magnitude will improve the quality of the line measurement.
The formula by which the correct thermocouple voltage should be calculated is:
Vcom = V + V1-V2 + V2-V1, where:
V1-V2 – is the cold junction voltage;
V2-V1 – is the temperature compensation coefficient.
Temperature compensation can be carried out through the hardware settings of the controller (Hardware) or through software with special programs (Software).
For those who patiently read up to this point, a little more theory, but already more in-depth.
For example, the installation is completed at the site, and the temperature readings are not as expected.
What to check first:
1.    Grounding the extension cable. Random interference can affect the measurement result, distorting the readings.
2.    You need to be sure that the RTD parameters in Hardware are correctly set in the PLC hardware configuration. Fixed reference temperature set to 0 °C.
3.    The choice of thermocouple definition is best set in manual mode, as this gives a large number of possible options for setting.
4.    Check which cold junction compensation parameters are set in HW (hardware) or SW (software). They must not be duplicated. If nothing is set anywhere, then the error can reach the value of the ambient temperature.
Usually, when carrying out factory tests, before sending the cabinets for production, the measurement channels are tested with special voltage generators (calibrators). So, for example, if you apply 50mV to the channel of a K-type thermocouple, then the temperature value should be 1233 °C. It should be noted that the connections of thermal compensation cables to the controller form a “compensated type” field and “parasitic”, “cold” connections can form in the lines. It must be ensured that extension cables or compensating cables are not connected through a lead-through metal (terminal block) of another type of metal, as all this increases cold junction errors.
It is advisable to pay attention to the polarity of the conductors in the measurement channel. It is important to match the conductor colors to the polarity of the thermocouple. If they are confused, it will be impossible to obtain correct temperature readings. Sometimes experts evaluate the polarity of the cores of thermal compensation cables by the magnetic core (the cores in the cable are magnetized). For a K-type thermocouple, the magnetic conductor is usually a white negative wire.
What should be done to test the thermocouple measurement line? Break in turn the communication line from the thermocouple, junction box, controller, taking measurements and keeping records of the measurements in the table. If the difference across different thermocouples is relatively constant, then the cold junction compensation procedure in the controller will need to be re-evaluated. So, for example, if we see from the entries in the table that there is a deviation at all measurement points, then we can sum them up and take them into account in the form of a correction factor in the software or hardware configuration.
In addition, I would like to note that at the facilities engineers use devices of insufficient accuracy class. To measure the voltage of thermocouples, it is necessary to measure with an accuracy of hundredths, and preferably thousandths of a volt. If you do not use such accurate instruments, then the error in reconciliation of readings can reach 30 ° C.

Калибратор это качественный задатчик напряжений

At industrial facilities (furnaces, columns, etc.), all thermocouples are connected to the controller after installation through special junction boxes. The length of the temperature compensation cable is typically around 300 m. The connection between the thermocouple connector and the I/O module of the controller is made via a bipolar temperature compensation cable. For example, there are cables for type "K" with 24 pairs each.
The connection between each thermocouple and the junction box is made by inserting the plugs of each thermocouple into the corresponding pre-wired socket. These connections must strictly comply with external wiring diagrams.
To check the integrity of the thermocouple, it is necessary to disconnect the plug from the sensor to the junction box, having previously measured in millivolts. This value should approximately correspond to the temperature of the environment in which the sensor is installed. If this is not the case, the thermocouple and its extension cable must be replaced with one of the same type of spare..
Attention: temperature compensation cables have polarity and must not be reversed. Pairs of conductors are colored in accordance with the international STD standard, with one color corresponding to positive and the other to negative. Conductor colors for all thermal compensation cables may vary from type to type. The following figure shows the color and polarity of the conductors in accordance with the accepted standard.

Цветовая схема жил проводников 

It is important that the connection between the thermocouple and the controller is done with regard to signal polarity (mV). Below is a millivolt to temperature conversion/conversion table for "K" (NiCr-Ni) thermocouples according to IEC 584-1 (values in bold are temperature, values in the table are millivolts).





An article where the features of the use of thermal resistances are described in sufficient detail

#Thermocouple, #thermocouples, #coldjunction, #hotjunction, #thermalcompensation, #commissioning, #measuring line, #measuring channel, #EMF

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