Calibration of Thermocouple

Transcription

1 Calibration of Thermocouple 1. Purpose of this experiment Understand the working principle of the thermocouple thermometers. Learn the method of welding nickel chromium copper or bronze and copper copper thermocouple, and calibrate it. 2. The principle of thermocouple, welding and calibration 1) The Principle of the thermocouple thermometer Thermometer with measuring temperature can generally be divided into the following five categories: expansion thermometer (such as mercury thermometer), the pressure gauge thermometer (such as nitrogen filling thermometer), the resistance thermometer (such as platinum resistance thermometer, PRT), thermocouple thermometers (such as platinum and rhodium 10 thermocouple platinum, nickel chromium nickel silicon TC), radiation thermometer (such as optical pyrometer). Thermocouple thermometers are widely used in industrial and agricultural production and research work because of their high degree of accuracy on measuring temperature. Two Distinctness character wire and alloy wire A and B, connect to a closed loop circuit. This is called thermocouple, as shown in Fig.1. A, B are called hot electrode. If the two contacts 1 and 2 at different temperature, there have a thermoelectric circuit potential E. The phenomenon is known as the thermoelectric effect, found by the Seebec, as early as 1821, also known as Seebeck effect. Thermocouple is based on this effect to measure temperature.

2 Fig.1 thermocouple circuit As shown in Fig. 1, the thermoelectric potential E12 in the closed loop thermocouple is only relate with the nature of thermocouple and the temperature at the end point, has nothing to do with the length of wire and its cross section size. When TC is certain materials, thermoelectric potential E12 is only with the thermocouple temperature at both ends of the t and t0, E12 = (t, t0). If the reference terminal (also known as the cold side) temperature t0 remain the same, between THERMOELECTRIC POWER E12 at both ends of the size that can be used to measure client (also known as the hot side) of a high and low temperature. Usually,the thermocouple cold with ice water on the coexistence of vacuum flask, the temperature t0 is equivalent to 0. 2) Thermocouple Welding The cold and hot side of thermocouple is made of two metals welding. In order to reduce heat transfer error and delay, welding points should be small and its diameter should be no more than twice the wire diameter. The welding method can be used spot welding, welding, as shown in Fig.2 a and b. It can also be wrapped around; the thermocouple can be able to re weld together, known as twist spot welding, and as shown in Fig.2c. But the twist circle should be no more 2 3. a) b) C) Fig.2 thermocouple hot tap The two hot electrode of the thermocouple should be insulated, in order to prevent short circuit. If its wire is naked, thermocouple, usually used to set the insulation on the wire insulation, there must be insulated tube on the wire. The

3 polyethylene or PTFE are widely used as insulated materials within a range of room temperature. The hot electrode polarity is determined by measuring gain or loss electron. The thermocouple losing electron is positive; the thermocouple gaining electron is negative. In the thermoelectric potential EAB(t,t0)list, the positive is at the top, the second is position is the negative. For example, at the copper konstantan thermocouple, the copper polarity is anode. The konstantan is cathode. The platinum platinum rhodium 10 thermocouple, the platinum and rhodium alloy is the cathode, the anode is pure platinum. 3) The calibration of thermocouple thermometer The thermocouple material in the laboratory may be not consistent with the provisions of standardized documents and the chemical composition of materials. So its thermoelectric nature and allowed deviation don t agree with the standard thermocouple. For this reason, the general used thermocouple in laboratory is a non standard thermocouple. The scale mark must be calibrated by measuring temperature workers themselves. Calibration of the thermocouple is placed in the same heat with the thermometer at the standard thermocouple reflected in the thermoelectric potential match; plot the mv t diagram. 3. Circuit diagram The thermocouple and its connections wires between the anode and cathode are shown as Fig.3. Laboratory with manual potentiometer is used DC partial pressure difference lines. The principle of the manual potentiometer is shown as Fig.4.In the Fig.4, the standard battery EN, the standard resistance RN and the galvanometer G circuit are used to calibrate the working current I1. Changeover Switch K connect to the "standard" position, and then adjust Rs to change the current I1 when current calibration. The galvanometer s pointer is at zero degree when I1RN=EN. The process is called current standardization work.

4 Therefore, using the potentiometer, the first step is standardized current work"; namely, keep the changeover switch at the "standard" position. Adjust Rs, and make the galvanometer s pointer is at zero. And then switch to the measured position, regulate B location, and make the galvanometer s pointer is at zero again. The electric potential at B location is the measured. Fig.3 the measuring temperature circuit of thermocouple Fig.4 the principle route map of the manually potentiometer 4. Steps of the Experiment 1) Scrape off the insulation paint of the thermometer with zero emery paper. The thermocouple then twisted into a hinge at end of the chain, welded in the potassium chloride solution. The voltage is not being too high; about 100volts.The welding voltage can be control until appearing the electric spark by regulating the transformer. So that the two metal ends of welding materials form a small ball. 2) Put the thermocouple into calibration device (see Fig.3 and 4),The measured ends in the constant temperature heat source in which inserted a standard mercury thermometer to read the exact temperature heat source temperature and enable the

5 potentiometer current standardization. And then switch K access to the "measured" position to read the mv thermoelectric potential value. 3) Change the constant temperature of heat source and repeat the steps 2, make the measured temperature changed at a certain temperature from room temperature. The overall count out is not less than five points. Record the temperature of heat source and mv numerical values at every time 4) Calculate the corresponding temperature and calculation error through the graduated scale of thermocouple. 5. Experimental record data The type and measuring range of equipment The metal material s name and its diameter d used the thermocouple. Table 1 Experimental data recording and data processing Original Record Mercury thermometer readings( 0 C) Potentiometer reading (mv) Calculation thermocouple calculated temperature ( 0 C) Absolute error( 0 C) 6. The contents of the report 1) Experimental record 2) Experimental data processing 3) Try to discuss the following question (1) Whether the thermocouple temperature is consistent with the mercury thermometer readings? If not, why?

6 (2) Which produced higher thermoelectric potential between Bronze constantan thermocouple and chromel constantan thermocouple?