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There are those who understand the thermocouple problem worth noting
9/23/2013 10:37:00 AM
One for thermocouple thermoelectric power, should pay attention to the following questions :
1 : the thermocouple thermocouple thermoelectric power working end and the cold end of the function of the temperature difference between both ends , rather than the thermocouple cold junction and the working end , a function of the temperature difference at both ends ;
2 : thermoelectric power generated by the thermocouple size , when the thermocouple material is uniform and independent of length and diameter of the thermocouple ,
3: When the two thermocouple wire thermocouple material composition is determined, the size of the thermocouple thermoelectric power , the temperature difference between the thermocouple only relevant ; if the thermocouple cold junction temperature is kept constant, the thermocouple thermoelectric power is only working end single-valued function of temperature . The two different materials A and B conductors or semiconductors welded together to form a closed loop, two different components of the conductor ( wire thermocouple or called hot electrode ) connected across the synthesis loop , when the engagement point temperature is not the same , electromotive force is generated in the loop , a phenomenon known thermoelectric effect , and this force is called thermoelectric power .
Thermocouple temperature measurement requires its cold end ( measuring end to the hot end , the measuring circuit is connected by wire end is called cold end ) the temperature remains constant , the size of the thermoelectric power and temperature measurement was only a proportional relationship . If the measurement , the cold side of the ( ambient ) temperature changes, will seriously affect the accuracy of measurement. Take certain measures in the cold side cold junction compensation due the impact of temperature changes is called the thermocouple cold junction compensation .
Attachment: thermocouple cold junction compensation calculation method: from millivolts to temperature: cold junction temperature measurement , converted to the corresponding millivolt values , with the thermocouple millivolt values together , converted the temperature . From the temperature to millivolts : measure the actual temperature and the cold junction temperature , respectively, converted to millivolt value obtained by subtracting the value of millivolts , that was the temperature .
eg. If the indicating instrument with cold junction compensation , the actual temperature is 520 degrees ; if indicating instrument without cold junction compensation , first check the K-type thermocouple indexing table drawn: 520 degrees corresponding voltage 21.497mv, 25 degrees corresponds voltage 1mv; 21.497mv +1 mv = 22.497mv; further investigation K-type thermocouple indexing table can be drawn , 22.497mv corresponding temperature value 543.47 degrees, ie, the actual temperature of 543.47 degrees .
PS: indexing number is used to reflect the temperature sensor in the measurement temperature range temperature sensor voltage or resistance changes corresponding to the standard series , namely thermal resistance, thermocouple , resistance, electric potential corresponding temperature value .
Second, the difference between the thermocouple and thermal resistance :
First, the nature of the signal , the thermal resistance is the resistance itself , temperature changes, the resistance generated positive or negative change in resistance ; the thermocouple is to generate an induced voltage changes, he is changed with temperature . Although all contact Thermometer , but their different temperature range , thermocouple used in high temperature environments, such as platinum and rhodium platinum and rhodium 30 --- 6 (B -type ) measurement range of 300 degrees ~ ~ 1600 degree of short-term measurable 1800 degrees . S-type measuring a 20 ~ ~ 1300 ( Short 1600), K -type measuring a 50 ~ ~ 1000 , short-term 1200). XK type a 50 ~ ~ 600 (800), E -type a 40 ~ ~ 800 ( 900 ) . There J-type , T type. Such instruments are generally used for more than 500 degrees of high temperature, low temperature thermoelectric power output when it is , when the electric potential of hours , the interference measure and the second table and the requirements are high, otherwise the measurements are not allowed , and, in the lower temperature region , the cold junction temperature and ambient temperature changes caused by the relative error becomes very prominent and difficult to obtain full compensation . Then at low temperature, is generally used as a heat resistance temperature range from 200 to 500 , and even lower temperatures measured ( as measured by carbon resistor temperature of about 1K ) . Now normal use platinum RTD Pt100, ( also Pt50, 100 , and 50 represents the thermal resistance of the resistance at 0 � . indexing using the old BA1, BA2 to that , BA1 at 0 degrees to 46 ohms resistance , is also useful in the industrial copper resistance , dividing number is CU50 and CU100, but the temperature range is small, in a 50 ~ ~ 150, and in some special occasions there is resistance indium , manganese resistance, etc. ) .
Second, the two sensors detect the temperature range is not the same, the general resistance temperature detector 0-150 degrees ( of course, negative temperature can be detected ) , the thermocouple can detect the temperature range of 0-1000 ( or higher ), so that the former the low-temperature test , which is the temperature detected .
Third , from the material of resistance, which is a metal material with changes in temperature-sensitive metal thermocouple bimetallic materials , both of two different metals , due to temperature changes in the two wires of different metals potential difference generated across .
Fourth , the work of field judgment thermocouple
Thermocouple with positive and negative, compensation wire has positive and negative points , first ensure connect, configure correctly. During operation . Common short circuit , open circuit, poor contact ( with a multimeter can be judged ) and metamorphism ( depending on the surface color to identify ) . Inspection , to make the thermocouple and the secondary table separately , with the tools on the table shorted secondary compensation line, the table indicates room temperature and then shorted thermocouple terminals, table indicates where the ambient temperature of the thermocouple ( not compensate Line fault ) , then multimeter mv file roughly estimate thermocouple thermoelectric power ( such as normal , check the process ) . Thermal resistance short circuit and open circuit with a multimeter can be judged , in operation, the suspect short, just remove one end of the resistor thread see display instruments, such as the maximum thermal resistance short circuit back to zero, wire short circuit, to ensure the normal connection and configuration , the table values indicate low or unstable , the possibility of flooded protective tube display max.
1 : the thermocouple thermocouple thermoelectric power working end and the cold end of the function of the temperature difference between both ends , rather than the thermocouple cold junction and the working end , a function of the temperature difference at both ends ;
2 : thermoelectric power generated by the thermocouple size , when the thermocouple material is uniform and independent of length and diameter of the thermocouple ,
3: When the two thermocouple wire thermocouple material composition is determined, the size of the thermocouple thermoelectric power , the temperature difference between the thermocouple only relevant ; if the thermocouple cold junction temperature is kept constant, the thermocouple thermoelectric power is only working end single-valued function of temperature . The two different materials A and B conductors or semiconductors welded together to form a closed loop, two different components of the conductor ( wire thermocouple or called hot electrode ) connected across the synthesis loop , when the engagement point temperature is not the same , electromotive force is generated in the loop , a phenomenon known thermoelectric effect , and this force is called thermoelectric power .
Thermocouple temperature measurement requires its cold end ( measuring end to the hot end , the measuring circuit is connected by wire end is called cold end ) the temperature remains constant , the size of the thermoelectric power and temperature measurement was only a proportional relationship . If the measurement , the cold side of the ( ambient ) temperature changes, will seriously affect the accuracy of measurement. Take certain measures in the cold side cold junction compensation due the impact of temperature changes is called the thermocouple cold junction compensation .
Attachment: thermocouple cold junction compensation calculation method: from millivolts to temperature: cold junction temperature measurement , converted to the corresponding millivolt values , with the thermocouple millivolt values together , converted the temperature . From the temperature to millivolts : measure the actual temperature and the cold junction temperature , respectively, converted to millivolt value obtained by subtracting the value of millivolts , that was the temperature .
eg. If the indicating instrument with cold junction compensation , the actual temperature is 520 degrees ; if indicating instrument without cold junction compensation , first check the K-type thermocouple indexing table drawn: 520 degrees corresponding voltage 21.497mv, 25 degrees corresponds voltage 1mv; 21.497mv +1 mv = 22.497mv; further investigation K-type thermocouple indexing table can be drawn , 22.497mv corresponding temperature value 543.47 degrees, ie, the actual temperature of 543.47 degrees .
PS: indexing number is used to reflect the temperature sensor in the measurement temperature range temperature sensor voltage or resistance changes corresponding to the standard series , namely thermal resistance, thermocouple , resistance, electric potential corresponding temperature value .
Second, the difference between the thermocouple and thermal resistance :
First, the nature of the signal , the thermal resistance is the resistance itself , temperature changes, the resistance generated positive or negative change in resistance ; the thermocouple is to generate an induced voltage changes, he is changed with temperature . Although all contact Thermometer , but their different temperature range , thermocouple used in high temperature environments, such as platinum and rhodium platinum and rhodium 30 --- 6 (B -type ) measurement range of 300 degrees ~ ~ 1600 degree of short-term measurable 1800 degrees . S-type measuring a 20 ~ ~ 1300 ( Short 1600), K -type measuring a 50 ~ ~ 1000 , short-term 1200). XK type a 50 ~ ~ 600 (800), E -type a 40 ~ ~ 800 ( 900 ) . There J-type , T type. Such instruments are generally used for more than 500 degrees of high temperature, low temperature thermoelectric power output when it is , when the electric potential of hours , the interference measure and the second table and the requirements are high, otherwise the measurements are not allowed , and, in the lower temperature region , the cold junction temperature and ambient temperature changes caused by the relative error becomes very prominent and difficult to obtain full compensation . Then at low temperature, is generally used as a heat resistance temperature range from 200 to 500 , and even lower temperatures measured ( as measured by carbon resistor temperature of about 1K ) . Now normal use platinum RTD Pt100, ( also Pt50, 100 , and 50 represents the thermal resistance of the resistance at 0 � . indexing using the old BA1, BA2 to that , BA1 at 0 degrees to 46 ohms resistance , is also useful in the industrial copper resistance , dividing number is CU50 and CU100, but the temperature range is small, in a 50 ~ ~ 150, and in some special occasions there is resistance indium , manganese resistance, etc. ) .
Second, the two sensors detect the temperature range is not the same, the general resistance temperature detector 0-150 degrees ( of course, negative temperature can be detected ) , the thermocouple can detect the temperature range of 0-1000 ( or higher ), so that the former the low-temperature test , which is the temperature detected .
Third , from the material of resistance, which is a metal material with changes in temperature-sensitive metal thermocouple bimetallic materials , both of two different metals , due to temperature changes in the two wires of different metals potential difference generated across .
Fourth , the work of field judgment thermocouple
Thermocouple with positive and negative, compensation wire has positive and negative points , first ensure connect, configure correctly. During operation . Common short circuit , open circuit, poor contact ( with a multimeter can be judged ) and metamorphism ( depending on the surface color to identify ) . Inspection , to make the thermocouple and the secondary table separately , with the tools on the table shorted secondary compensation line, the table indicates room temperature and then shorted thermocouple terminals, table indicates where the ambient temperature of the thermocouple ( not compensate Line fault ) , then multimeter mv file roughly estimate thermocouple thermoelectric power ( such as normal , check the process ) . Thermal resistance short circuit and open circuit with a multimeter can be judged , in operation, the suspect short, just remove one end of the resistor thread see display instruments, such as the maximum thermal resistance short circuit back to zero, wire short circuit, to ensure the normal connection and configuration , the table values indicate low or unstable , the possibility of flooded protective tube display max.
