In part 1 of our 3 part series, “Sensors for Temperature Measurement and their Application”, we introduced various kinds of sensors  and discussed the linear and exponential relationships that temperature has in the operation of the electronic components.  In part 2 we’ll cover three specific sensor types: the resistor thermometer, thermocouple and diode transistor.  In part 3 of our 3 part series we’ll finish up and discuss infrared or radiation, flu0rescent detector, and liquid crystal.

1 — Resistance Thermometer

With these sensors, the resistance of the sensing element changes with temperature. The sensors come in two primary forms: thermistors (lightly doped semiconductors) and metal resistors. Equations 3 and 4 represent the relationships between resistance and temperature for these two sensors, respectively:

Figure 1 shows a surface-mounted RTD (resistance temperature detector) that can be installed onto a surface for temperature measurement:

Figure 1: Surface mounted RTD (photo courtesy of RDF Corporation)

 The following must be considered when using these types of sensors:

1. The sensor (resistor) must be in intimate contact with the test specimen — solder or careful epoxy is recommended.
2. The sensor must be placed in an isothermal region — constant temperature over the sensor.
3. The resistor power dissipation (if in voltage mode) must be minimized to not impact the problem.
4. This sensor is suitable for part-level measurement as it can be embedded directly on the die.

2 — Thermocouples (TC)         

These sensors are far and away the most commonly used devices in the field. Wide flexibility and broad availability enable their use for a variety of temperature measurements. TCs work on the principle that bringing together two wires of different elements or alloys produces a voltage as a result of temperature. Equation 5 provides the governing principle for TCs:

Table 2 shows some of the typical TC types that are used in electronics thermal measurement.

Table 2: Thermocouple Types and Their Respective Voltage Outputs [2]

Of the TC types shown above, E, J, K and T are the most commonly used. Many thermocouple meters on the market can use all of these sensors interchangeably. That’s because the voltage output of these TCs is in the same range; hence, the internal electronics can be designed to accommodate each of them.

There are some unique features about each sensor type that one needs to know. For example:

• E-type —  Though accurate, has a limited range.
• J-type —  Should not be used in a humid environment, since the iron component of the TC will oxidize, resulting in erroneous output.
• K-type —  Though widely used, the voltage output can be negatively impacted if the wire kinks.
• T-type —  Can be an effective heat transfer medium, because of its copper component, either as a fin or a conductor.

It is also important to note that thermocouples measure temperature at the point where the two wires are connected. The smaller the junction, the more precise the temperature that is read. A large TC junction will result in the temperature being averaged over its entire area. Multiple junctions, as shown in Figure 2, will have the same impact. In Figure 2, the multi-junction created as a result of twisting the wires prior to spot-welding the ends (the TC below), creates a significantly larger junction. Whether measuring surface or fluid temperatures, the number reported by this TC will report an average temperature over a 2-3mm junction length.

Thermocouple errors can be attributed to the following areas:

• Poor junction connection
• Galvanic action
• Thermal shunting
• Electrical noise
• Installation problem due to tester

 Figure 2: Single and Multi-junction Thermocouple Sensors [3]

Of the errors listed above, electrical noise is uniquely problematic, especially in today’s high frequency equipment. A TC can be used in a 4-wire format to resolve the electronic noise that may affect the reported temperature. Using a 4-wire thermocouple, as shown in Figure 3, we can measure temperature and electrical noise.

Let us consider a J-type thermocouple formed of Iron and Constantan. All four wires are spot-welded together to form the TC junction. The temperature can be read across any of the Iron and Constantan combinations (?), and the electronic noise can be read across either the two Irons or the two Constantans. Because two similar metals cannot create the Seebeck effect (convert thermal differentials to electric voltage), whatever signal is measured on these wires is the electronic noise in the measurement domain.

 Figure 3: Four-wire Thermocouple System for the Measurement of Electronic Noise and Temperature

Measuring surface temperature is always a challenging process. The following steps will help to increase the accuracy of such measurements:

• Keep installation size as small as possible.
• To reduce conduction errors, bring thermocouple wires away from the              junction, along an isotherm for at least 20 wire diameters.
• Locate the measuring junction as close to the surface as possible.
• To avoid changes in convective or radiative heat transfer, design the installation so that it causes minimum disturbance to any fluid flow or the least possible change in the emissivity of the surface.
• Reduce the thermal resistance between the measuring junction and surface to as low a value as possible.

3 — Diode or Transistor

Diodes and transistors are parts whose electrical properties are a function of temperature. Diodes are broadly used for temperature measurement, either as embedded sensors in functional devices or as a thermal test chips. Figure 4 shows one such thermal test chip for device-level simulation.

Figure 4.  Thermal Test Chip [3]

The following depicts the general considerations for usage of semiconductor materials for temperature measurement:

• Every semiconductor device has at least one electrical parameter that is a              function of temperature.
• Thermal test chips use the thermally sensitive parameter of semiconductor devices to measure chip junction temperature.
• Separate heating and sensing elements are usually used to avoid for electrical switching.
• Thermal calibration of the sensing device is necessary.
• Thermal test chips provide an effective means of measuring chip junction temperature in an actual package configuration.
• Use of materials is subject to availability/suitability for the intended package application.

We’ll conclude our series with part 3, addressing infrared thermography, optical probes and liquid crystal thermography

References:
1. Klinger, D., Nakada, Y., Menendez, M., AT&T Reliability Manual,
Van Nostrand Reinhold, 1990.
2. Azar, K., Thermal Measurement in Electronics Cooling, CRC Press,
1997.
3. Advanced Thermal Solutions, Inc., Tutorial Series, “Principles of
Temperature Measurement”.
4. thermVIEW™ System, product of Advanced Thermal Solutions, Inc.
5. White, F., Viscous Fluid Flow, McGraw-Hill, 3rd Ed., 2005.

If  you are in need of sensors for thermal measurement, click now to ATS’s sensor family.  Tired of using thermocouples that are finicky and breakable? Try ATS’s spot sensor.  It’s durable and cost effective.  Learn more by clicking to ATS Spot Sensor.   You can also email or call us with your questions on temperature measurement and one of our engineers will be happy to help you.  Email us at ats-hq@qats.com or call  us at 781-769-2800.