Tag Archives: thermocouples

Rethinking Thermocouples: Creating Micro-Scale High Precision Sensors

In the Qpedia Issue 99 article, “Minimizing Thermocouple Errors in Electronics Thermal Characterization,” we examined how the use of thermocouples can alter the measurements that are made, whether it is because the thermocouple itself acts as a fin that dissipates heat, or any of a number of other reasons [1]. We saw that even use of a very fine 36-gauge thermocouple wire can introduce measurement error of more than 5%.

(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)

The influence of the sensor on the measurement can be even more challenging as the item under test becomes smaller and more sensitive to ambient influences. As an example, a thin film made of carbon nanotubes presents an extreme challenge, as the film is only 100 nm thick, or about 1/1000 of the diameter of an average human hair. At this scale, the material has practically no thermal mass, so the temperature of the film can easily be affected by most temperature transducers. In order to create a sensor small enough to measure temperatures on the thin film, Shrestha et al. chose to use a glass micropipette as a base [2]. A sensor was then fabricated by creating a junction of dissimilar metals, using the same thermoelectric principles as a thermocouple.

Thermocouple sensors had been made using micropipettes previously, but past examples used materials that were much more expensive, and the construction was very complex. The platinum and gold materials were chosen because of their use in a biological environment, but as a thermocouple junction, they had a low voltage output in response to temperature differences [3,4]. The sensors that Shrestha et al. created were meant to address these issues.

Glass micropipettes can be made with tips so small that they are commonly used to inject substances into individual living cells. The tip diameters that Shrestha et al. created ranged from 2 to 30 µm. The micropipettes were made using a micropipette puller which heated a 1.5mm glass tube and then pulled the tube apart, creating a finely tapered section (see Figure 1a). The micropipettes were then filled with a tin based soldering alloy (Figure 1b), and a micropipette beveler was used to sharpen and achieve the finished tip shape (Figure 1c).

Fig. 1 – Creating a Micropipette Thermocouple. [2]

The next step in the construction was to coat the outside of the micropipette with a nickel film using a sputtering process (Figure 1d). On the end of the tip where the tin was exposed, the deposition of nickel created the working thermocouple junction. The deposition conditions were varied to create different film thicknesses, which we will examine later. Finally, lead wires made of tin and nickel were attached to the respective materials in the sensor (Figure 1e).

The sensors were calibrated in a water bath at temperatures from 21 to 40°C, while the cold junction was maintained at a constant 24.5°C. The voltages generated by the sensors at each temperature were recorded using a Nano voltmeter, as the measurement scale was in the range of 100µV. The calibration curves for some of the sensors are shown in Figures 2 and 3 below, where a linear response is clearly demonstrated. It can also be seen that the thicker layer of nickel significantly increased the sensitivity of the sensor, more than doubling its voltage output for a given temperature.


Fig. 2 – Voltage response of a Micropipette Thermocouple with most-nickel deposition. [2]

Fig. 3 – Voltage response of a Micropipette Thermocouple with least-nickel deposition. [2]

Shrestha et al. [2] saw sensitivities as high as 8.9 µV/°C, compared to the average of 2.1 µV/°C for the platinum-gold thermocouples made by Kakuta et al. [4]. Fish et al. [3] achieved about 7 µV/°C from their Pt-Au thermocouples, but as a point of reference, a standard J-type thermocouple has a sensitivity of about 40 µV/°C [4]. With all of the thin film-based thermocouples, the output signal is low, and measurement noise can become a problem if the environment is not well controlled. Some of the researchers found it necessary to use preamplifiers to boost the output voltage of the sensors, which complicates setup and can introduce noise.

It should also be noted that because of the sizes of all of these sensors and the test subjects, the micropipette sensors need to be positioned with micromanipulators, a basic version of which is shown in Figure 4 below.


Fig. 4 – Sutter MM-33 Micromanipulator. [6]

Clearly these are not quite like the standard thermocouples that we use for standard thermal measurements, and the setup and fabrication of the sensors is more involved. The payoff is sensors that have an extremely fast response time to temperature changes, only a few microseconds in air [3], as well as very little thermal influence on the samples they are testing, which is critical at the small scales that Shrestha et al. examined. In addition, Shrestha et al. determined that the measurement accuracy of their sensors was 0.01°C, which compares very favorably to a standard J-type thermocouple. Even with the “special limits of error” grade wire, J-type accuracy is only about 1ºC. [5]

None of the three researchers quantified the possible heat transfer through the sensor, but they were confident that it was negligible even in the context of their micrometer-scale measurements. With sensor diameters from 2 to 30 µm, the cross-sectional area is so small that the thermal resistance via conduction is very high.

This approach may be excessive for most electronics thermal management purposes, but it does illustrate that even something as familiar as a thermocouple can be approached in many different ways. The sizes of these sensors allow them to make temperature measurements with pinpoint precision, and to identify very small heat sources and heat flow paths. One such application could be localizing hot spots on a CPU die, for example. Such small sensors could generate a temperature map by traversing the surface of a die.

They could also be used to measure any small component or heat sink where a standard thermocouple would draw too much heat away. As Shrestha et al. show with their temperature measurements on a 100 nm thick film [2], the applications can vary widely, and are limited only by our preconceptions of what thermocouples are capable of.

1. Advanced Thermal Solutions, Inc. (ATS), “Inaccuracies in Thermocouple Measurements,” Qpedia, Issue 99.
2. Shrestha, R., Choi, T. Y., Chang, W. S., “Measurement of Thermal Conductivity of Thin and Thick Films by Steady State Heat Conduction,” University of North Texas, Denton, TX, 2012.
3. Fish, G., Bouevitch, O., Kokotov, S., Lieberman, K., Palanker, D., Turovets, I., Lewis, A., “Ultrafast Response Micropipette-Based Submicrometer Thermocouple,” The Hebrew University of Jerusalem, Jerusalem, Israel, 1994.
4. Kakuta, N., Suzuki, T., Saito, T., Nishimura, H., Mabuchi, K., “Measurement of Microscale Bio-Thermal Responses by Means of a Micro-Thermocouple Probe,” University of Tokyo, Tokyo, Japan, 2001.
5. “Thermocouple – Thermocouple types – J,” http://www.thermometricscorp.com/thermocouple.html, [Dec. 16, 2015].
6. “Sutter Instrument Company – MICROMANIPULATION,” http://www.sutter.com/MICROMANIPULATION/mm33.html, [Dec. 22, 2015].

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

An Instrument for Measuring Air Velocity, Pressure and Temperature in Electronics Enclosures

For engineer-level thermal management studies, the iQ-200 instrument from Advanced Thermal Solutions, Inc, ATS, can simultaneously measure air velocity, air pressure and the temperature of components and surrounding air at multiple locations inside electronic systems. This enables users to obtain full and accurate profiles of components, heat sinks, PCBs and other electronics hardware to enable more effective thermal management.

Developed by Advanced Thermal Solutions, Inc., ATS, the iQ-200 system simultaneously captures data from up to 12 J-type thermocouples, 16 air/velocity sensors, and four pressure sensors.

The thermocouples provide surface area temperature measurements on heat spreaders, component packages, housing hardware, and elsewhere to track heat flow or detect hot spots. Temperature data is tracked from -40 to 750°C. The sensors (available separately) measure both air temperature and velocity at multiple points allowing a detailed analysis of airflow.

Candlestick Sensor from ATS

Thin, low profile ATS candlestick sensors can be easily positioned throughout a system under test and measure airflow from -10 to +6°C. Air velocity is measured from natural convection up to 6 m/s (1200 ft/min). The iQ-200 can be factory modified to measure airflow to 50 m/s (10,000 ft/min) and air temperature up to 85°C. Four differential transducers capture pressure drop data along circuit cards, assemblies and orifice plates. Standard pressure measurement capabilities range from 0- 1,034 Pa (0 – 0.15 psi).

The ATS iQ-200 system comes preloaded with user-friendly iSTAGE application software which effectively manages incoming data from the various sensor devices, and allows rich graphic presentation on monitors and captured on videos or documents. The iQ-200 connects via USB to any conventional PC for convenient data management, storage and sharing.

More information on the iQ-200 system from ATS can be found on Qats.com (http://www.qats.com/products/Temperature-and-Velocity-Measurement/Instruments/iQ-200/2632.aspx), or by calling 781-769-2800.

How To Make a Thermocouple Video from ATS

ATS’s Latest Video, “How To Make a Thermocouple” has just been published.  If you’ve ever needed to make a thermocouple in your lab or shop, this is the video for you. Let Greg from our engineering team show you how it’s done.

And for a more robust alternative to a thermocouple, consider ATS Spot Sensor,  you can learn more about our spot sensor at this link or get a quote at this link.

Great Viewing: ADI’s New White Board Series on Thermocouples

In our Twitter feed we saw that ADI has a new 8-video series on thermo- couples. It’s a great set of videos on this topic. Well presented and informative, we’d recommend our reader’s put them in their bookmarks to check out. Well worth your time.

You can reach the full series by visiting ADI’s site here:  ADI’s Thermocouple 101 Video Series

ATS has also covered thermocouples here on our blog. Click to this link to read our two part series, “Thermocouples for Thermal Analysis: What they are and How they Work“.

Here’s one of the first of ADI’s thermocouple series to check out!

Thermocouples for Thermal Analysis: what they are and how they work (part 2 of 2)

In part 1, we covered the basics behind what thermocouples are and how they work. In part 2, we’ll cover how thermocouples can be made and know when to select the right thermocouple for your project.

Thermocouples can be made of any two dissimilar metal wires, and their emf voltage depends on the composition of the chosen metals. However, what makes thermocouples so popular is that the materials used to construct them are restricted and their output emf’s have been standardized. Certain materials and combinations are better than the others, and some have basically become the standard for given temperature ranges. Table 1 lists some of the available thermocouples in the U.S. market.

Thermocouple Type Material Composition Temperature Range Uncertainity Color Code
T Cu vs. Constantan -250 to 350°C Greater of 1°C or 0.75% Blue-Red
K Chromel vs. Alumel -200 to 1250°C Greater of 2.2°C or 0.75% Yellow-Red
J Iron vs Constantan 0 to 750°C Greater of 2.2°C or 0.75% White-Red
R Platinum vs. Platinum-13% Rodium 0 to 1450°C Greater of 1.5°C or 0.25% None Established
S Platinum vs. Platinum-10% Rodium 0 to 1450°C Greater of 1.5°C or 0.25% None Established
C Tungsten 5% Rhenium vs. Tungsten 26% Rhenium 0 to 2320°C Greater of 4.5 °C to 425°C, 1% to 2320°C None Established
E Chromel vs. Constantan -200 to 900°C Greater of 1.7°C or 0.5% Purple-Red

Table 1: Different types of thermocouples

Selecting the right type of thermocouple for an application depends on many factors. These include sensitivity, temperature range, corrosion resistance, linearity of output voltage, and cost. For example, types R and S are relatively expensive and are not sensitive. However, they perform well at high temperatures up to 1768°C and are resistant to a number of corrosives. Type C thermocouples are suitable for higher temperature applications, but they are relatively expensive and corrode easily in an oxidizing environment. A Type T thermocouple is inexpensive and very sensitive, but will corrode at temperatures above 400°C. Type K is very popular for general use, relatively inexpensive, reasonably corrosion-resistant, and can be used at high temperatures, up to 1372°C. K-type thermocouples also have provide relatively linear output as compared to the other types [2].

The actual magnitude of the thermocouple emf is very small, and is in the order of few millivolts. At a given temperature, Type E has the highest output emf among common types, but this voltage is still measured in millivolts. The sensitivity of thermocouples is also relatively low. For instance, the voltage change per degree Fahrenheit from 38 to 93°C is only 36 microvolts. As a result, thermocouples require accurate and sensitive measuring devices and cannot be used for temperature changes of less than about 0.1°C. Traditionally, expensive voltage balancing potentiometers were used to measure emf. Today, a high quality digital voltmeter is sufficient [3].

The National Institute of Standards and Technology (NIST) has developed standard calibration curves for determining temperature based on the measured emf voltage. These data represent the output emf of thermocouples when an ice cold junction is used, and are incorporated in the memory of most DAQ systems.  Unfortunately, the temperature-voltage relationship of thermocouples is nonlinear and curve-fitted using polynomial functions. Obviously, the higher the order of the polynomial function, the higher the accuracy of temperature reading. The polynomial function should only be used inside the temperature range of the thermocouple type and should not be extrapolated. To save computational time, a lower order polynomial fit can be used for a smaller temperature range.

Thermocouple wires come in variety of sizes. Usually, the higher the temperature, the heavier should be the wire. As the size increases, however, the time response to temperature change increases. Therefore, some compromise between response and life may be required.

Thermocouples can be connected electrically in series or in parallel. When connected in series, the combination is usually called a thermopile, (whereas there is no particular name for thermocouples connected in parallel). A wiring schematic of a thermopile combination is shown in Figure 3.

Thermocouples example diagram

Figure 3 – Series-connected thermocouples forming a thermopile

The total output from n thermocouples will be equal to sum of the individual emf’s. The main purpose of using a thermopile rather than a single thermocouple is to obtain a more sensitive element. Parallel connection of thermocouples is used for averaging.



  1. Omega Temperature Handbook, Omega Engineering, Inc. 2nd edition.
  2. Wheeler, A. and Ganji, A., Introduction to Engineering Experimentation, Prentice Hall Inc., pp. 240-246, 1996.
  3. Beckwith, T., Marangoni, R., and Lienhard, J., Mechanical Measurements, Fifth Edition, Addison Wesley Longman, pp. 676-685, 1993.