【Daily Question 20090312】What impact does incorrect polarity connection of the compensation wire have on temperature measurement?

If the positive and negative poles of the compensation wire are connected in reverse, it will lead to inaccurate temperature measurement data, with significant errors occurring at higher temperatures. An obvious sign of reversed positive and negative poles is a lower temperature compared to normal conditions. When you don’t remember how to identify the positive and negative wires of a thermocouple, you can try swapping them to measure the temperature on-site; the configuration that results in a higher temperature is the correct one. The type of compensation wire can be identified by the color of its insulation layer: red and blue indicate the positive and negative poles of a K-type thermocouple, red and green indicate those of an S-type thermocouple, while red and brown indicate those of an E-type thermocouple!

An incorrect polarity of the compensation wire has a reverse compensating effect on temperature measurement； It turns out to be compensation for the temperature difference between the ambient temperature and 0°C ； If it’s reversed now, an opposite effect occurs, that is, the actual temperature decreases ； If it’s not possible to distinguish the scale number and polarity of the compensation wire ； The thermocouple can be inserted at a point where the temperature is known, and the actual output in millivolts can be measured ； By determining the positive or negative value of the output in millivolts, its polarity can certainly be known ； Determine the scale number by looking up the corresponding temperature point based on the millivolt value ； Of course, if you remember the color of the compensation wire, you can directly distinguish between positive and negative poles as well as the compensation scale number by color ； But it is still necessary to use the method mentioned above for confirmation. Last edited by yxwch2008 on 2009-3-12 at 10:27.]

Wrong polarity connection will increase the cold-end error. If the scale and polarity of the compensation wire are unknown, twist the ends of the two conductors together and immerse them in boiling water; connect the other end to the instrument that displays this scale. A reading of around 1000°C should be indicated, which shows that the scale values are correct. If the reading differs significantly, then the scale values are incorrect. When using the MV setting on a multimeter, a “+” reading indicates that the red test lead is connected to the positive pole of the compensation wire, while a “–” reading indicates that the red test lead is connected to the negative pole of the compensation wire.

1. What impact does incorrect polarity of the compensation wire have on temperature measurement? Answer: It will cause additional measurement errors. If the temperature difference is zero, the instrument reading has no additional error. If the temperature at the cold end of the thermocouple is higher than the temperature at the instrument’s input, the instrument’s reading will be two times lower than the actual value, by an amount equal to the temperature difference. If the temperature at the cold end of the thermocouple is lower than the temperature at the instrument’s input, the instrument will display a value that is twice the difference in temperature. 2. How to determine when the division number and polarity of the compensation wire are unclear? Answer: Use boiling water to determine it. One end is immersed in boiling water, while the other end is connected to a thermocouple reading instrument; the changes in the instrument’s readings are used to make a judgment. If the pointer reverses direction, the polarity has been reversed. Ruo Yi indicated that a value of around 100 degrees indicates that the wire is compatible with the thermocouple.

It will cause additional measurement errors. If the temperature difference is zero, the instrument reading has no additional error. If the temperature at the cold end of the thermocouple is higher than the temperature at the instrument’s input, the instrument’s reading will be two times lower than the actual value, by an amount equal to the temperature difference. If the temperature at the cold end of the thermocouple is lower than the temperature at the instrument’s input, the instrument will display a value that is twice the difference in temperature.

Reversing the polarity of the thermocouple compensation wire will result in a lower displayed temperature. The division number is determined by checking the markings and color on the cable insulation of the compensation wire. The positive and negative poles of the thermocouple wire can also be identified by color.

It will cause additional measurement errors. If the temperature difference is zero, the instrument reading has no additional error. If the temperature at the cold end of the thermocouple is higher than the temperature at the instrument’s input, the instrument’s reading will be two times lower than the actual value, by an amount equal to the temperature difference. If the temperature at the cold end of the thermocouple is lower than the temperature at the instrument’s input, the instrument will display a value that is twice the difference in temperature. The division number is determined by checking the markings and color on the cable insulation of the compensation wire. The positive and negative poles of the thermocouple wire can also be identified by color.

Those who work in temperature measurement know that the working principle of a thermocouple involves two different conductors forming a closed circuit; when the temperatures at its two ends are equal, the thermoelectrical potential of this circuit is zero. When the working end of the thermocouple and the accumulated temperature at that end are not equal, a heat potential is generated in the closed circuit. The magnitude of the thermoelectromotive force is related to the temperature difference between the two ends. The temperature at the free end of the thermocouple is not a constant value. Manufacturers use special wires for thermocouples – compensation wires. Extending the thermocouple to a constant temperature compensates for the thermoelectric potential of the thermocouple, that is, E = eAB(t) – eCD(t0). For example, when the process requirements for the workpiece dictate a temperature of 920°C. The operator also sets the values according to the process requirements, and the instrument readings are normal. After the liquid fire treatment, striations appeared on the workpieces, rendering an entire batch of them unusable. An analysis of the accident revealed that the compensation wire was connected in reverse; after correction, the temperature inside the furnace was 973°C. At that time, the temperature at the free end of the phase-change thermocouple was 45°C, while the indoor temperature was 18°C. The temperature error resulting from the reversed calculation is: EAB at 45.0 degrees = 1.817 mV; ECD at 18.0 degrees = 0.718 mV. ΔE = -2 × EAB × (45.0 – 18.0) = -2.198 mV, which corresponds to 53.13°C. 　 The above examples show that reversing the thermocouple compensation wire can cause a considerable temperature error. Therefore, do not connect the compensation wire reversely. Reposted from the hq0769 blog: Compensation wires for R and S type thermocouples. Thermocouples also known as platinum-rhodium-platinum types come in R and S classifications, representing platinum-rhodium13-platinum and platinum-rhodium10-platinum thermocouples respectively. The former is less used in China, but it has a higher electromotive force (at 1600°C, the electromotive forces of R and S type thermocouples are 18.849 mV and 16.777 mV respectively). At low temperatures of 100°C, the values for both types are almost identical (the electromotive forces are 0.647 mV and 0.646 mV respectively), while at 200°C there is a slight difference (the electromotive forces are 1.467 mV and 1.441 mV respectively). Therefore, compensation wires for R and S type thermocouples are commonly used in the Chinese market today. If a compensation wire with S classification, which is commonly available on the market, is used with a thermocouple of R classification, there is no error at temperatures below 100°C. Even at the maximum temperature of 200°C for heat-resistant compensation wires, when the temperature at the hot end of the thermocouple is 600°C, 1000°C, or 1300°C respectively, the resulting error is only 2.5°C, 2.2°C, and 2.0°C. However, among the commonly used thermocouples, the type B bimetallic (platinum-rhodium 30-platinum-rhodium 6) thermocouple is an exception; it does not have a dedicated compensation wire, or in other words, it generally does not require the use of a compensation wire in practical applications. Double platinum-rhodium thermocouples are commonly used for temperature measurement in the range of 1300–1600 °C (platinum-rhodium-platinum thermocouples are usually used for temperatures up to 1300 °C). Their electromotive force at low temperatures is surprisingly low; for example, it is only 0.033 mV at 100 °C and 0.178 mV at 200 °C. This is a significant difference compared to the average electromotive force of 0.700 mV per 100 °C across the entire temperature range of 0–1800 °C. Therefore, even without compensation, the resulting error is very small. Let’s talk more about the classification of compensation wires. In principle, they are divided into extension type and compensation type. For the extension type, the nominal chemical composition of its alloy wire is the same as that of the thermocouple it is used with, so the thermoelectric potential is also the same; this type is indicated by “X” in the model designation. For the compensation type, the nominal chemical composition of its alloy wire differs from that of the thermocouple it is used with, but within its operating temperature range, its thermoelectric potential is close to the nominal value of the thermoelectric potential of the associated thermocouple; this type is indicated by “C” in the model designation. 　　Based on compensation accuracy, there are ordinary grades and precision grades; the error after compensation in the precision grade is roughly half that of the ordinary grade, and it is typically used in applications where high measurement accuracy is required. For compensation wires of the S and R grades, the tolerance for the precision grade is ±2.5°C, while that for the standard grade is ±5.0°C; for compensation wires of the K and N grades, the tolerance for the precision grade is ±1.5°C, and the tolerance for the standard grade is ±2.5°C. For models, the standard grade is not indicated, while the precision grade is marked with an “S”. 　　Based on operating temperature, they are generally divided into those for general use and those for high-temperature use. Those for general use have an operating temperature of 0 ~ 100°C (with a few having a range of 0 ~ 70°C); those for high-temperature use can operate at temperatures up to 0 ~ 200°C. 　　Furthermore, compensation wires can be classified as single-core or multi-core (flexible wires) based on the number of conductors; they can also be divided into ordinary type and shielded type depending on whether they have a shielding layer. There are also compensation wires designed for intrinsically safe circuits used in explosion-proof environments. Author: Fang Yuanbai. Last edited by wopale3 on 2009-3-12 13:00]

With correct connection, the total thermoelectromotive force received by the instrument is EZ = EK(T1,T3) + EKX(T3,T2) = EK(T1,T3) + EK(T3,T2) = EK(T1,T2). Due to incorrect connection, according to the law of the intermediate conductor, the total thermoelectromotive force received by the instrument becomes E’Z = EK(T1,T3) + EKX(T3,T2). For KX-type compensation wires, E’KX(T3,T2) = -EKX(T3,T2) = -EK(T3,T2). As a result, an error occurs in the instrument’s measurement, given by EZ’ – EZ = EK(T1,T3) – EK(T3,T2) – EK(T1,T3) – EK(T3,T2) = 2EK(T3,T2). EK(T1,T2) represents the thermoelectromotive force between junction temperatures T1 and T2 for thermocouples made of materials A and B, with T3 being the intermediate temperature. Therefore, when the positive and negative poles of the thermocouple compensation wire are connected in reverse, not only is no compensation effect achieved, but the error also doubles compared to when no compensation wire is used； To determine the type of thermocouple and the polarity of the compensation wire, it is necessary to understand the physical properties of the thermocouple material as well as those of the compensation wire: (1) Check the color ； (2) Test hardness and softness ； (3) Whether it is paramagnetic or not can be easily identified.

1. Answer: If the cold end of the thermocouple is at the same temperature as the gauge head, the instrument will display a normal value. If the cold junction temperature of the thermocouple is higher than the temperature at the instrument’s input, the instrument’s reading will be lower than the actual value. If the cold junction temperature of the thermocouple is lower than the temperature at the instrument’s input, the instrument’s reading will be higher than the actual value. 2. Answer: The grading codes are not clear; one should look at the color of the insulation covering on the cable cores. For K-type compensation wires, the positive terminal is red and the negative terminal is blue (black). For E-type wires, the positive terminal is red and the negative terminal is brown. For S-type wires, the positive terminal is red and the negative terminal is green ； If the polarity is unclear, boiling water can be used for testing. This post was last edited by zhaohh3211 on 2009-3-13 08:10]

It can be checked using a temperature calibrator: short-circuit one end of the compensation wire and connect the other end to the calibrator; if the ambient temperature is displayed, the polarity is correct, otherwise it is reversed.

Compensation wires operate within a certain temperature range. A pair of insulated wires with a thermoelectromotive force nominal value matching that of the thermocouple serve merely to extend the thermal electrode, moving the cold end of the thermocouple to the instrument terminals in the control room. It cannot on its own eliminate the effect of changes in the cold-end temperature on temperature measurement, and it does not serve a compensating function. When using thermocouple compensation wires, it is essential to ensure that the models match, that the polarity is correct, and that the temperature at the connection point between the compensation wire and the thermocouple does not exceed 100°C. If the polarity is connected incorrectly, the error increases.

Temperature measurement errors can occur due to the wiring and type of compensation wires, which may result in overestimations or underestimations; unclear scale markings, confusion regarding the color of the cable insulation, and ambiguous polarity can all be detected by using boiling water for testing.