1, the nature of the signal, thermocouple itself is the resistance, the change of the temperature, the thermocouple produce positive or negative resistance changes; And thermocouple, it is the change of the induced voltage, he along with the change of temperature change. 2, two kinds of sensors to detect temperature range, 0 - thermocouple general detection The temperature range of 150 degrees, high measurement range of up to 600 degrees ( Of course can detect negative temperature) 。 Thermocouple can detect 0 - Temperature range of 1000 c ( Even higher) So, rosemount 3051 transmitter is low temperature test, the latter is high temperature testing. 3, from material, thermal resistance is a kind of metal materials, has a temperature sensitive metal materials, thermal coupling is bimetallic material, both two dissimilar metals, due to the changes of temperature, in two different electric potential difference at both ends of the wire. 4, PLC corresponding thermocouple and thermocouple input module is also different, this sentence is no problem, but the general PLC are directly connected to the 4 ~ 20 ma signal, and the thermocouple and thermocouple are generally transmitter to access with a PLC. If access to DCS don't have to use the transmitter! Thermocouple is a sign of RTD, thermocouple is tc signal! Installation of thermocouple and the thermocouple, should pay attention to accurate temperature measurement, safety examination and maintenance is convenient, and does not affect the equipment operation and production operations. To meet the above requirements, in the choice of thermocouple and thermocouple installation site and insertion depth should pay attention to the following: 1, in order to make the measurement end of the thermocouple and the thermocouple, and had good heat exchange between the measured medium, should be a reasonable choice of observation, try to avoid the valve, elbow and pipe and equipment near the corner of the furnish thermocouple or thermocouple. 2, thermocouple with a protective casing and thermocouple with heat transfer and heat loss, in order to reduce the measurement error, thermocouple and thermocouple should have enough: (insert depth 1) For the measurement of pipeline fluid temperature thermocouple, generally should be measuring end is inserted into the pipe ( Vertical or inclined installation) 。 As the current body of pipe diameter is 200 mm, the thermocouple or thermocouple insertion depth should choose 100 mm; ( 2) For high temperature and high pressure and high velocity of temperature measurement, Such as main steam temperature) , in order to reduce cases of fluid resistance and prevent cases under the effect of fluid fracturing, the way protecting tube shallow inserted or use hot thermocouple, shallow inserted type thermocouple protection tube, its depth into the steam pipe should be no less than 75 mm; Heat sets the standard of thermocouple insertion depth is 100 mm; ( 3) If is need to measure the temperature of flue gas, although the flue is 4 m in diameter, thermocouple or thermocouple insertion depth of 1 m; ( 4) When measuring the original insertion depth of more than 1 m, shall be installed vertically, as far as possible or add racks and protect casing. The correct use of the correct use of thermocouple can not only get a temperature value accurately, qualified products, but also can save thermocouple materials consumption, both to save money and can guarantee the quality of the product. Installation is not correct, thermal conductivity and time lag error, they are main error of the thermocouple is in use. 1, improperly installed into the error, such as the location of the thermocouples installed and insertion depth cannot reflect the reality of the furnace temperature, etc. , in other words, the thermocouple should not be in too close to the door and heating, the depth of the insert should be at least 8 ~ 10 times the diameter of the protection tube; Thermocouple protection tube and the wall between the interval of missing adiabatic material overflow or cold air invading the furnace heating, so the space between the thermocouple protection tube and furnace wall hole used refractory mortar or insulating material such as asbestos rope jam to avoid cold and hot air convection and affect the accuracy of temperature measurement; Thermocouple cold end is too close to the furnace temperature over 100 ℃; The installation of thermocouples should as far as possible to avoid the strong magnetic field and strong electric field, so should not be thermocouple and the power cable line in order to avoid interference caused by errors within the same catheter; Thermocouple cannot be installed in a few measured medium flowing area, when measured with thermocouple tube gas temperature, thermocouple must be made against the flow direction of installation, and fully contact with the gas. 2, the error of the insulation deterioration and introduce such as thermocouple insulation, protection tube and anchor plate too much dirt or salt slag between the thermocouple interelectrode and wall insulation fault, at high temperatures is more serious, which not only causes the thermoelectric potential loss but also can introduce interference, the resulting error sometimes can reach on baidu. 3, the introduction of thermal inertia error due to thermal inertia of thermocouple instrument behind the change of the measured temperature, indicating value this influence is particularly acute in rapid measurement. So should be used as thermal electrode is small, small thermocouple protection tube diameter. Temperature measuring environmental permits, even protecting tube can be taken. Due to measurement lag, using thermocouples detect temperature fluctuations of the amplitude of the furnace temperature fluctuation amplitude of the small. Measurement lags behind, the greater the fluctuation amplitude of the thermocouple is smaller, and the actual temperature difference. When using the time constant of thermocouple temperature or temperature control, instrument display temperature fluctuation is small, but the actual furnace temperature fluctuations can be quite large. In order to accurately measure temperature, should choose small thermocouple time constant. With thermocouple time constant is inversely proportional to the heat transfer coefficient, heat the density and specific heat is proportional to the diameter, material, such as to reduce the time constant, in addition to the increased heat transfer coefficient, effective way is to try to decrease the size of the hot end. Use, usually USES the material with good thermal conductivity, thin, small diameter of the tube wall to protect casing. In the precision of temperature measurement, using unprotected casing bare wire thermocouple, thermocouple easy damaged, should be timely correction and replacement. 4 error at high temperature, thermocouple, such as protecting tube with a layer of ash, dust attached to the above, the thermal resistance increase, hinder the heat conduction, when the temperature is lower than the true value of measured temperature value. Therefore, we should maintain the cleanness of thermocouple protection tube outside, to reduce the error.