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 how to find flaws, analyze the situation and clarify a number of controversial points.
First, a little theory.
A thermocouple consists of a compound of different alloys, which, due to their physicochemical properties, upon contact, begin to be a source of a weak electric current. When the temperature is applied to a thermocouple, the potential difference in it changes. This difference is measured in several millivolts. This potential difference depends on the type of materials used to make the thermocouple. A thermocouple, in its principle of operation, is similar to an electric battery, where a potential arises when two materials and an electrolyte interact.
The principle of operation of the measuring channel of the "thermocouple" type is shown below.
The junction point of the metals in the thermocouple itself is the "hot junction". "Cold junction" are all other connections in this measurement channel. To determine the temperature of the zone in which the thermocouple is installed, it is sufficient to measure the number of millivolts at the entrance to the measurement channel.
The connection between a thermocouple and a voltage meter (such as a millivoltmeter or I / O module) is usually made with temperature compensation cables or extension cables. In the first case, the materials of the conductors have characteristics similar to those of a thermocouple; in the second, they are made of the same material.
Despite the use of temperature-compensated 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 electromotive force (distorting the reading).
Let's take an example.
There is a Cromel-Alumel type thermocouple. When the Cromel and Alumel conductors are in contact with copper, two new thermocouples are formed: "Cromel-Copper" (electromotive force will be generated = V1) and Copper-Alumel (electromotive force will be generated = V2). Therefore, two additional electromotive forces that are opposite to each other in polarity will be generated, which will add up to the electromotive force of the Cromel-Alumel connection (electromotive force = V will be generated).
Ultimately, we will have the following electromotive force in the measurement circuit:
Vgeneral = V+(V1-V2),
where V1-V2 is usually negative. If all the "cold junctions" were at 0 ° C, they would have no effect on the overall measuring circuit. All the electromotive force generated by them would be equal to zero. However, most often it is necessary to neutralize the influence of "cold junctions" by adding additional electromotive force of the measuring circuit according to the following formula:
Vcompensation = V2-V1
To remove the electromotive force of the "cold junction" (V1-V2), it is necessary to add the calculated part of the electromotive force (V2-V1). Thus, the subtraction of numbers close in magnitude will improve the quality of the line measurement.
Below is the formula for calculating the correct thermocouple voltage:
Vgeneral = V + V1-V2 + V2-V1, где:
V1-V2 – this is the cold junction voltage;
V2-V1 – this is the thermal compensation coefficient.
Thermal compensation can be carried out through the hardware settings of the controller (Hardware) or through software with special programs (Software).
For those who have patiently read up to this point, a little more theory, but more in-depth.
For example, installation is complete and temperature readings are not as expected.
What do you need to check first:
1. Grounding the extension cable. Accidental interference can affect the measurement result, distorting the readings.
2. You need to be sure that the hardware configuration of the PLC has the correct RTD settings in Hardware. The fixed reference temperature is set to 0 °C.
3. The choice of the thermocouple definition is best set in manual mode, because this gives a large number of options for adjustment.
4. It is necessary to check which parameters of the "cold junction" compensation are set in HW (hardware) or SW (software). They should not be duplicated. If nothing is set, then the error can reach the value of the ambient temperature.
Usually, during factory tests, before sending the cabinets to production, the measurement channels are tested with special voltage generators (calibrators). So, for example, if you apply 50mV to the K-type thermocouple channel, the temperature value should be 1233 °C. The connections of the temperature compensation cables to the controller form a "compensated type" field and "parasitic", "cold" connections can form in the lines. It is necessary to make sure that extension cables or compensation cables are not connected through a lead-through metal (terminal block) of another type of metal, as this will increase "cold junction" errors.
It is advisable to pay attention to the polarity of the conductors in the measurement channel. It is important to observe the colors of the conductors according 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 the temperature compensation cables by the magnetic core (the cores in the cable are magnetised). For a K-type thermocouple, the magnetic core is usually the white negative lead.
What do you need to do to check a thermocouple measurement line? It is necessary to sequentially break the communication line from the thermocouple, junction box, controller, taking measurements and keeping records of measurements in the table. If the difference across the various thermocouples is relatively constant, then the "cold junction" compensation procedure in the controller will need to be revised. 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 as a correction factor in the software or hardware configuration.
In addition, I would like to note that engineers use instruments of insufficient accuracy class at the facilities. To measure the voltage of thermocouples, it is necessary to measure with an accuracy of hundredths, or better than thousandths of a volt. If such accurate instruments are not used, then the error in the verification of readings can reach 30 ° C.
In 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 usually about 300 m. The connection between the thermocouple connector and the controller I / O module 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 the external wiring diagrams.
To check the integrity of the thermocouple, it is necessary to disconnect the plug that goes from the sensor to the junction box, after making a measurement 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 a replacement of the same type.
Caution: The temperature compensation cables have polarity and must not be reversed. The pairs of conductors are colored according to the international standard STD, with one color being positive and the other negative. The color of the conductors for all temperature compensation cables can vary from type to type. The image below shows the color and polarity of the conductors in accordance with the accepted standard.
It is important that the connection between the thermocouple and the controller is made with respect to the signal polarity (mV). Below is a table of correspondence / conversion of mV to temperature, for type "K" thermocouples (NiCr-Ni) according to IEC 584-1 (values in bold are temperature, values in the table are mV).
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