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How to Measure Thermocouples
Dec 23, 2017

How to Measure Thermocouples

Temperature and Thermocouple Overview


Temperature is the average kinetic energy of the particles in the object sample measurement, the 

standard unit is "degree." Temperatures can be measured in different ways, and the cost and 

accuracy of the measurements can vary. Thermocouples are one of the most common temperature sensors

, because thermocouples are relatively inexpensive and accurate, and have a relatively wide 

measurement range.

Whenever two different metal contacts, the contact point assembly produces a lower no-load voltage

 as a function of temperature, which is the thermoelectric effect. The temperature difference is

 the Seebeck voltage, to 1821 found the phenomenon of physicist Thomas Seebeck named. This voltage

 is non-linear with respect to temperature, but can be approximated as being linear for small 

changes in temperature, or:

Where ΔV is the voltage change, S is the Seebeck coefficient, and ΔT is the temperature change.


Thermocouples are available in a variety of types and are specified in larger letters according to

 the American National Standards Institute (ANSI) convention. For example, a J-type thermocouple 

consists of an iron conductor and a copper-nickel alloy conductor. Other types of thermocouples 

include B, E, K, N, R, S, and T.

How to Measure Thermocouples


background knowledge


For a better understanding of how thermocouple measurements are made, it is important to understand

 how thermocouples work. The first part of this document will explain the basic principles of 

thermocouples, the later part of the thermocouple will explain how to achieve the connection 

between the instrument and how the temperature measurement.


The thermocouple Seebeck voltage, if connected directly to the measuring system, is connected to

 the measuring system and generates an additional thermocouple. This can not be measured simply by

 connecting it to a voltmeter or other measuring system.


Figure 1. J-type thermocouple


As shown in Figure 1, the circuit using the J-type thermocouple on the candle temperature 

measurement. Two thermocouple lines are connected to the copper terminals of the data acquisition

 device. Note that there are three metal connections in this circuit - J1, J2 and J3. J1 is the

 thermocouple measurement point, producing a Seebeck voltage proportional to the temperature of the

 candle. In addition, J2 and J3 each have their own Seebeck coefficient, and in the data 

acquisition terminal will produce a temperature proportional to the temperature difference voltage

, known as the cold-side voltage. In order to determine the voltage component of J1, it is

 necessary to know the temperature of J2 and J3 contacts and know the relationship between the

 contact voltage and the temperature. Thus, the voltage of the J1 junction can be obtained by 

subtracting the J2 and J3 parasitic junction voltage components from the measured voltage.


Thermocouples require a specific temperature reference to compensate for the cold side of the

 error generated. The most commonly prescribed method is to use a temperature sensor that can be 

directly read to obtain the reference junction temperature, minus the parasitic terminal voltage 

component. This process, known as cold junction compensation, can simplify the calculation of cold 

junction compensation using the characteristics of some thermocouples.


By using the thermocouple law of the metal transition layer and other assumptions, we can see that

 the measurement of the voltage data acquisition system depends only on the type of thermocouple,

 the voltage at the measurement terminal, and the cold junction temperature. The measured voltage

 is independent of the voltage components of the measuring leads and the cold terminals J2, J3.


Figure 2. Metallic transition layer thermocouple law


Consider the circuit in Figure 3. This circuit is similar to the circuit described in Figure 1

 above, except that a short piece of copper-nickel alloy wire is inserted before the J3 junction.

 All contacts are at the same temperature. Assuming that the J3 and J4 junction temperatures are 

the same, the metal transition layer thermocouple law states that the circuit in Figure 3 is 

electrically the same as the circuit in Figure 1. Therefore, any of the results of the circuit of

 Fig. 3 can be applied to the circuit shown in Fig.


Figure 3. Inserting an additional conductor in an isothermal environment


Figure 3, J2 and J4 contacts belong to the same type (copper-nickel alloy); because the two are in

 an isothermal environment, J2 and J4 is the same temperature. Because of the direction of the 

current in the circuit, J4 produces a Seebeck positive voltage, J2 produces a Seebeck negative 

voltage. Therefore, the contact offset the influence of each other, measuring the total voltage is

 zero. J1 and J3 contacts are iron-copper-nickel alloy contacts. But their temperatures may be 

different, as they may not be in an isothermal environment. Because they are in different 

temperature environments, J1 and J3 contacts can produce Seebeck voltage, but the size is different

. In order to compensate for the cold junction J3, the temperature is measured and the applied 

voltage is subtracted from the thermocouple measurement.


Using the VJx (Ty) symbol to represent the voltage generated by the Jx junction at Ty temperature,

 the general thermocouple problem is simplified as follows:

VMEAS = VJ1(TTC ) + VJ3(Tref )     (2)


Where, VMEAS data acquisition system that the measured voltage value, TTC J1 junction thermocouple 

temperature, Tref said reference side of the temperature.


Note that in the formula (2), VJx (Ty) represents the voltage generated in a Ty temperature 

environment with respect to a certain reference temperature. Equation 2 holds as long as VJ1 and 

VJ3 are temperature functions related to the same reference temperature. For example, a NIST 

thermocouple reference table as described above is generated with the reference end held at 0 

degrees Celsius.


Because J3 and J1 are of the same type, but produce relative voltages, VJ3 (Tref) = -VJ1 (Tref).

 Also, because VJ1 is the voltage generated by the thermocouple type test, the voltage can be 

renamed to VTC. Therefore, 2-type can be rewritten as follows:

VMEAS = VTC (TTC ) - VTC (Tref )   (3)


Therefore, by measuring VMEAS and Tref know the relationship between the thermocouple voltage and

 the temperature, you can determine the temperature of the thermocouple measurement side.


There are two technologies to achieve cold junction compensation - hardware compensation and 

software compensation. Both techniques require the use of a direct-read sensor to obtain the 

reference-side temperature. The direct-reading sensor has an input that is only determined by the 

temperature of the measuring point. Semiconductor sensors, thermistors and RTDs are commonly used 

to measure the temperature of the reference side of the instrument.


Using hardware compensation, a variable voltage source can be inserted into the circuit to remove 

the parasitic thermoelectric voltage. The variable voltage source generates a compensation voltage

 according to the ambient temperature, which is added to the correction voltage to cancel the 

unwanted temperature difference signal. When these parasitic signals are removed, the only signal

 measured by the data acquisition system is the voltage measured from the thermocouple measurement

. In the case of hardware compensation, the temperature of the data acquisition system terminal is

 uncorrelated because the parasitic thermocouple voltage in the data acquisition system has been 

canceled. The main disadvantage of hardware compensation is that each thermocouple must have a 

separate compensation circuit can be added to correct the compensation voltage, which will greatly

 increase the cost of the circuit. Under normal circumstances, the hardware compensation is not on

 the accuracy and software compensation.


Or you can choose to use the software for cold junction compensation. After using the 

direct-reading sensor to measure the reference-side temperature, the software can append a suitable

 voltage value to the measured voltage to eliminate the influence of the cold-side voltage. Recall

 (3) that the measured voltage VMEAS is equal to (thermocouple) measuring the voltage difference

 between the termination point and cold junction.


The thermocouple output voltage is highly non-linear. The Seebeck coefficient varies due to three

 or more factors in the operating temperature range of some thermocouples. Therefore, you must use

 a polynomial to model the voltage versus temperature curve in a thermocouple or use the look-up 

table method.


Connect the thermocouple to the instrument

This section uses the NI cDAQ-9172 backplane and the NI 9211 C-series thermocouple module as an

 example. A similar procedure is applicable for connecting thermocouples to different instruments

 (see Figure 4).


Required equipment:


DAQ-9172 8-slot high-speed USB backplane for NI CompactDAQ

NI 9211 four-channel, 14Sa / s, 24-bit, ∓ 80 millivolt thermocouple input module

J type thermocouple


Figure 4. NI CompactDAQ system


The NI 9211 features a 10-pole, detachable screw terminal connector that provides connections for

 up to four thermocouple input channels. Each channel has a connection point for the positive 

thermocouple, TC +, and a connection to the negative terminal, TC-. The NI 9211 also has a common 

wiring point, COM. Normally this port is internally connected to the reference ground of the module

. Figure 5 shows the wiring assignments for each channel, and Figure 6 shows the wiring diagram.

Figure 5. Terminal assignment

Figure 6. Connection diagram


View your measurements: NI LabVIEW


Now that the thermocouple is connected to the test equipment, you can use LabVIEW graphical

 programming software to transfer the data to the computer for visualization and analysis.


Figure 7 shows an example of displaying the measured temperature data in the LabVIEW programming

environment.


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