What is the reason for the large variation in the temperature range of thermocouples?

According to you, the situation is open. Measure the two signal lines from the thermocouple device to see if the MV signal is normal or abnormal That is the problem of hot couples. If the signal is normal, you go to check your entire circuit, see if there is a temperature transmitter in the middle, and see if the 4-20mA of the temperature change output is normal.

Some thermocouples have temperature transmitters installed on the equipment to see if the output of 4-20mA is normal).

Then there is the short circuit to see where it is, the line is not connected, etc., it is very simple. Check it step by step.

The thermocouple should be connected with a compensation line as much as possible, and the graduation number should be consistent, check whether it is falsely connected, whether the wiring is correct, and the thermometer will display the maximum value in the state of open circuit.

1, The combination of two different conductors or semiconductors is called a thermocouple, and if the combination is not good, these situations will occur when the time is broken;

2. This will also occur if the compensation wire or lead wire is not in good contact or there is a short circuit;

3. Generally, the thermocouple is put into the protective sleeve after passing through the porcelain tube, if it is not properly assembled, the two metal arcs inside will be twisted together, causing irregular short circuit.

Are the two connected to the same table?

Is there a compensation line in the middle?

Maybe there is a short circuit in a certain position, take it apart and check it, and replace the compensation line and sheath.

In the production process of thermocouples, the surface of the even wire is always polluted after multiple shrinkage stretching, and at the same time, from the internal structure of the even wire, there is inevitably stress and lattice inhomogeneity. The stress introduced by quenching or cold working can be basically eliminated by annealing, and the error caused by unqualified annealing can reach a few tenths to a few degrees. It is related to the temperature to be measured and the size of the temperature gradient on the thermocouple electrode.

If a high-temperature thermocouple needs to be annealed, the annealing temperature should be higher than the upper limit of the operating temperature and the insertion depth should be greater than the actual depth used. Thermocouples must be carefully cleaned (pickled and cleaned with sodium tetraborate) and annealed to remove contamination and stress from the thermocouple.

Generally speaking, if the thermocouple is made of homogeneous conductors, its thermoelectric potential is only related to the temperature at both ends, if the thermoelectrode material is not homogeneous, and the thermoelectrode is in a temperature gradient field, the thermocouple will produce an additional thermoelectric potential, that is, "inhomogeneous potential". Its magnitude depends on the state of the temperature gradient distribution along the length of the thermoelectrode, the inhomogeneous form and degree of inhomogeneity of the material, and the position of the thermoelectrode at the temperature site. The main causes of uneven thermoelectrodes are:

In terms of chemical composition, such as uneven distribution of impurities, segregation of components, local metal volatilization on the surface of the thermoelectrode, oxidation or selective oxidation of a metal element, thermal diffusion at high temperature at the measuring end, and contamination and corrosion of thermocouples in harmful atmospheres. Instability refers to the change of the index value of a thermocouple with different use times and conditions. In most cases, it can be the main cause of inaccuracies.

Factors that affect instability are: fouling, volatilization of the thermal electrode at high temperatures, oxidation and reduction, embrittlement, radiation, etc. If the change of the graduation value is relatively slow and uniform, then a supervised check is often performed or a periodic check is scheduled according to the actual usage, which can reduce the error introduced by instability.

Errors in the temperature of thermocouple display Thermocouple element deterioration introduces errors, such as when the thermoelectrode is chemically attacked and deteriorates, the deteriorated thermoelectrode is cut or replaced. The thermoelectric performance of the compensation wire adopted by the enterprise is not exactly the same as the thermoelectric performance of the thermocouple, and the matching compensation wire should be replaced. The error caused by the heat exchange difference is caused by the fact that the hot end of the thermocouple is not in sufficient contact with the measured object or the thermal equilibrium is not reached during temperature measurement.

If there is a lot of ash on the surface area of the protective pipe, the ash or crust should be removed; If the thermocouple installation position is improper or the depth is not enough, it can not be measured in the place where the medium flow rate is the largest, then the measurement point should be selected, and the installation position should be adjusted to make it in the position where the temperature needs to be measured. When the insulation of the thermocouple measurement line is damaged, causing a short circuit or grounding, check it with a universal meter to find out the fault and repair the insulation.

Under the conditions of correct installation and use, the temperature measurement error of thermocouple has the following aspects: the basic error of thermocouple, the temperature change error of the cold junction and the error of the display and control instrument. These errors, although random, can be calculated as allowable errors.

Indexing error of thermocouples. This is because thermocouples are uniformly indexed by product lot and inevitably have errors, expressed in ET. Thermocouple cold junction temperature compensation error.

It is not possible to keep the cold junction at 0 degrees Celsius in industrial applications, even under laboratory conditions. Compensation wires are used, which are denoted as EF. The cold-junction compensation bridge is used, which is expressed in EC.

Displays the meter error. Displays the error generated by the meter, expressed in ED. The error of the display meter is determined according to the accuracy class of the meter.

Hello, dear, I am glad to answer for you: What happens if the temperature of the cold junction of the thermocouple is higher than the temperature of the hot junction? Answer: Old:

Yes, its actual temperature value will change. The thermocouple thermometer adopts cold junction temperature compensation, the formula is limb source liter e(t,0) = e(t,t0) + e(t0,0), t0 is the actual cold junction temperature, and the cold junction temperature after compensation is 0. The effects of cold junction temperature changes on temperature measurement are:

The thermoelectric potential is related to the temperature at both ends of the thermocouple, and the temperature-thermoelectric potential relationship curve is indexed when the temperature of the cold junction is 0. In practical application, because the cold junction of the thermocouple is exposed to space and is affected by the ambient temperature, the temperature of the cold junction in the temperature measurement cannot be kept at 0 or fixed at a certain temperature, and the thermocouple electric potential is determined by both the hot and cold junction temperatures. Therefore, if the temperature of the cold junction changes freely, it will inevitably cause measurement errors.

In order to eliminate this error, the cold junction temperature must be compensated.

Summary. The temperature of the cold junction of the thermocouple is higher than the temperature of the hot junction will change the actual temperature value The thermocouple thermometers all use the temperature compensation of the cold junction, the formula is e(t,0)=e(t,t0)+e(t0,0), t0 is the actual cold junction temperature, and the temperature of the cold junction after compensation is 0. The effects of cold junction temperature changes on temperature measurement are:

The magnitude of the thermoelectric potential of a thermocouple is related to the temperature at both ends, and the temperature-thermoelectric potential relationship curve is indexed when the temperature of the cold junction is 0. In practical application, because the cold junction of the thermocouple is exposed to space and is affected by the ambient temperature, the temperature of the cold junction in the temperature measurement cannot be kept at 0, nor can it be fixed at a certain temperature, and the thermocouple potential is determined by both the hot and cold junction temperatures. Therefore, if the temperature of the cold junction changes freely, it will inevitably cause measurement errors.

In order to eliminate this error, the cold junction temperature must be compensated. If my answer is helpful to you, please give a like, look forward to your like, your efforts are very important to me, and your support is also the motivation for my progress. Finally, I wish you good health and a happy <>

The temperature of the cold junction of the thermocouple is higher than the temperature of the hot junction will change the actual temperature value The thermocouple thermometers all use the temperature compensation of the cold junction, the formula is e(t,0)=e(t,t0)+e(t0,0), t0 is the actual cold junction temperature, and the temperature of the cold junction after compensation is 0. The influence of the temperature change of the cold junction on the temperature measurement is as follows: The thermoelectric potential of the thermocouple is related to the temperature at both ends, and the temperature-thermoelectric potential relationship curve is indexed when the temperature of the cold junction is 0.

In practical application, because the cold end of the thermocouple is exposed to the ambient temperature between the empty standby judgment and the balance of the balance trembling, the cold junction temperature in the temperature measurement cannot be kept at 0, nor can it be fixed at a certain rolling temperature, and the thermocouple potential is determined by both the hot and cold junction temperatures. Therefore, if the temperature of the cold junction changes freely, it will inevitably cause measurement errors. In order to eliminate this error, the cold junction temperature must be compensated.

If my answer is helpful to you, please give a like, look forward to your like, your efforts are very important to me, and your support is also the motivation for my progress. Finally, I wish you good health and a happy <>

If the temperature of the cold junction is higher than that of the hot junction, will the detected temperature show a negative number?

Hello dear<>

The cold junction temperature is higher than the hot junction temperature, and the detected temperature will show a negative number.

Commonly used thermocouples can be measured from -50 +1600, and some special thermocouples can measure as low as -269 (e.g. gold-iron-nickel-chromium) and up to +2800 (e.g. tungsten-rhenium).

Thermocouple is one of the most commonly used temperature sensing components in the industry, and the working principle of thermocouple is based on the Seeback effect, that is, two conductors of different compositions are connected at both ends to form a loop, and if the temperature of the two connecting ends is different, a thermal current is generated in the loop.

Thermocouple considerations.

Generally, the cold junction temperature is 0 as the standard for thermocouple calibration. For this reason, the cold end is often placed in an ice-water mixture to keep its temperature at a constant 0. Under laboratory conditions, the cold end is usually placed in a test tube containing a simple insulating oil, and then placed in an insulated container filled with an ice-water mixture, which is the cold end to keep 0.

Because the temperature index gauge of the thermocouple is obtained under the condition that the temperature of the cold junction is kept at 0, and the measuring circuit or display instrument used with it is scaled according to this relationship curve, so when the temperature of the cold junction is not equal to 0, the indication value of the instrument needs to be corrected by silver.

271 (2k), depending on which thermocouple.

270 can be used for K, T, N, and E types of thermoelectric fighting on the indexing table, but the level of acceptance and manufacturing in practical applications is often not so low.

Thermocouples commonly used in low-temperature measurements are:

T-type thermocouples (copper-copper-nickel thermocouples), also known as copper-constantan thermocouples, are also the best thermocouples for measuring low temperatures of cheap metals. The positive electrode (TP) of the T-type thermocouple is pure copper, and the negative electrode (TN) is a copper-nickel alloy, which is often called constantan, which is common with nickel-chromium-constantan's constantan EN, and cannot be used with iron-constantan's cornstone JN, although they are both called constantan.

The T-type thermocouple covers the measuring temperature zone of -200 350. Cavity wax.

Gold-iron-nickel-chromium low-temperature measurement, can be used in 2 273K Van Wu silver slip circumference, the sensitivity is about 10 V.