Temperature Measurement with a Thermocouple or RTD

Two of the most common temperature transducers are the thermocouple and RTD.> Click on the links below to learn more about temperature-related topics and to find RTD and thermocouple-specific application examples:

• Configure a DAQ System for Temperature Measurements
• Find Thermocouple and RTD Data Acquisition Application Examples
• Find a Thermocouple or RTD

Hardware Solutions for Temperature Measurements

National Instruments offers modular, customizable solutions for temperature measurement in a variety of applications. The table below consists of several systems to measure thermocouples or RTDs in different applications. Click on an application type to view system components, prices, and other information. All hardware comes with free software for configuration based data logging and is also compatible with a variety of programming environments including LabVIEW, C++, Visual Studio .NET, and VisualBasic 6.0.

What Is Temperature?

Qualitatively, the temperature of an object determines the sensation of warmth or coldness felt by touching it. More specifically, temperature is a measure of the average kinetic energy of the particles in a sample of matter, expressed in units of degrees on a standard scale.

For more information on temperature visit the temperature tutorial, or return to the top of the page .

What Is a Thermocouple?

One of the most frequently used temperature sensors is the thermocouple. Thermocouples are very rugged and inexpensive and can operate over a wide temperature range. A thermocouple is created whenever two dissimilar metals touch and the contact point produces a small open-circuit voltage as a function of temperature. This thermoelectric voltage is known as the Seebeck voltage, named after Thomas Seebeck, who discovered it in 1821. The voltage is nonlinear with respect to temperature. However, for small changes in temperature, the voltage is approximately linear, or
where DV is the change in voltage, S is the Seebeck coefficient, and DT is the change in temperature.

S varies with changes in temperature, however, causing the output voltages of thermocouples to be nonlinear over their operating ranges. Several types of thermocouples are available; these thermocouples are designated by capital letters that indicate their composition according to American National Standards Institute (ANSI) conventions. For example, a J-type thermocouple has one iron conductor and one constantan (a copper-nickel alloy) conductor.

For more information on thermocouples visit the thermocouple tutorial, or return to the top of the page .

Temperature Measurement with a Thermocouple

To measure a thermocouple Seebeck voltage, you cannot simply connect the thermocouple to a voltmeter or other measurement system, because connecting the thermocouple wires to the measurement system creates additional thermoelectric circuits.



Consider the circuit illustrated in Figure 1, in which a J-type thermocouple is in a candle flame the temperature of which you want to measure. The two thermocouple wires are connected to the copper leads of a DAQ board. Notice that the circuit contains three dissimilar metal junctions – J1, J2, and J3. J1, the thermocouple junction, generates a Seebeck voltage proportional to the temperature of the candle flame. J2 and J3 each have their own Seebeck coefficient and generate their own thermoelectric voltage proportional to the temperature at the DAQ terminals. To determine the voltage contribution from J1, you need to know the temperatures of junctions J2 and J3 as well as the voltage-to-temperature relationships for these junctions. You can then subtract the contributions of the parasitic junctions at J2 and J3 from the measured voltage at junction J1.

Thermocouples require some form of temperature reference to compensate for these unwanted parasitic “cold” junctions. The most common method of cold-junction compensation is to measure the temperature at the reference junction with a direct-reading temperature sensor and subtract the parasitic junction voltage contributions. This process is called cold-junction compensation. You can simplify computing cold-junction compensation by taking advantage of some thermocouple characteristics.

For more information on thermocouples visit the thermocouple tutorial, or return to the top of the page .

What Is an RTD?

Resistance temperature detectors (RTDs) operate on the principle of changes in the electrical resistance of pure metals and are characterized by a linear positive change in resistance with temperature. Typical elements used for RTDs include nickel (Ni) and copper (Cu), but platinum (Pt) is by far the most common because of its wide temperature range, accuracy, and stability.

RTDs are popular because of their excellent stability, and exhibit the most linear signal with respect to temperature of any electronic temperature sensor. They are generally more expensive than alternatives, however, because of the careful construction and use of platinum. RTDs are also characterized by a slow response time and low sensitivity, and because they require current excitation, they can be prone to self-heating.

RTDs are commonly categorized by their nominal resistance at 0 ^oC. Typical nominal resistance values for platinum thin-film RTDs include 100 W and 1000 W. The relationship between resistance and temperature is very linear and follows the equation

For < 0^oC R_T = R₀ [ 1 + aT + bT² + cT³ (T - 100) ]
For > 0^oC R_T = R₀ [ 1 + aT + bT² ]

Where R_T = resistance at temperature T
R₀ = nominal resistance
a, b, and c are constants used to scale the RTD

The most common RTD is the platinum thin-film with an a of 0.385%/^oC and is specified per DIN EN 60751. The a value depends on the grade of platinum used, and also commonly include 0.3911%/^oC and 0.3926%/^oC. The a value defines the sensitivity of the metallic element, but is normally used to distinguish between resistance/temperature curves of various RTDs.

For more information on RTDs visit the RTD Tutorial, or return to the top of the page

Temperature Measurement with an RTD

Because RTDs are resistive devices, you must supply them with an excitation current and then read the voltage across their terminals. If extra heat cannot be dissipated, I²R heating caused by the excitation current can raise the temperature of the sensing element above that of the ambient temperature. Self-heating will actually change the resistance of the RTD, causing error in the measurement. The effects of self-heating can be minimized by supplying lower excitation current.

RTD and thermistor output signals are typically in the millivolt range, making them susceptible to noise. Lowpass filters are commonly available in RTD and thermistor data acquisition systems and can effectively eliminate high-frequency noise in RTD and thermistor measurements. For instance, lowpass filters are useful for removing the 60 Hz power line noise that is prevalent in most laboratory and plant settings.

You can also significantly improve the noise performance of your system by amplifying the low-level RTD and thermistor voltages near the signal source. Because RTD and thermistor output voltage levels are very low, you should choose a gain which optimizes the input limits of the analog-to-digital converter (ADC).

For more information on RTDs visit the RTD Tutorial, or return to the top of the page
Related Links:
Temperature Measurement Resource Page

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