this month, we'll take a little break from calculations to have a look at one of the more common tasks of the thermal engineer. most of you readers probably have had some experience with doing temperature measurements. if you are like most of us, you have made some mistakes along the way. this article is about some of those mistakes, for the benefit of our less experienced colleagues. we will focus on thermocouples. feel free to add your experience to the discussion forum.

let's start with a quick overview of how thermocouples work. (for more information, go to the technical reference section of omega engineering's website, http://www.omega.com/temperature/z/zsection.asp.)

essentially, a thermocouple is a voltage generator that is sensitive to temperature. two dissimilar metals generate the voltage between two junctions by the seebeck effect. a fancy voltmeter measures the voltage and converts it to temperature if you tell it the correct metal combination (thermocouple "type"). sounds simple enough.

the big challenge is making sure that the thermocouple junctions are at the correct temperatures: the reference junction, which you can think of as being at the voltmeter, should be at a known temperature. usually the meter "knows" this (i.e. measures it internally) and corrects it to the standard reference of 0 °c.

the other junction is the one you install on the surface of interest. the big challenge lies in making sure this junction is the same temperature as the object of interest. and the worst of it is that if you make a mistake here, you are most likely to get a reading that is lower than the real temperature - fooling you with what usually ends up being an optimistic result.

there is no little alarm bell going off to alert you that there is anything wrong with the installation.

have a look at this example. the thermocouple is the twisted wire with the frayed insulation. it is supposed to be measuring the temperature of the rectangular glass object near the lower end of the photo. this picture brings to mind several comments besides the thermal knowledge of the installer (and others of a less charitable nature).

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• where is the junction that is doing the temperature measurement? it is where the wires touch each other closest to the meter - somewhere in between the last visible twist and the end of the insulation, unless the insulation has frayed through enough to allow the wires to touch somewhere we can't see.

• generally speaking, a thermocouple's thermal characteristics shouldn't influence those of the object being measured. that is, the wire gauge should be much smaller than the object. while it might work okay for this particular example where steady-state data was needed, i would have used much thinner wire - half as thick or smaller. there are two aspects to this: thermal mass and thermocouple-as-fin. the thermal mass is important if you are doing transient measurements (for example, furnace and oven profiling, or on-off switching).

the thermocouple-as-fin phenomenon works by moving heat across the thermal contact resistance between the junction and the object of interest. look at the schematic view at right. convective cooling across the lead wire will draw heat out of the junction (shown as a spherical bead), just like a heat sink fin would do.

the junction draws heat from the surface, so heat is moving across the interface between the junction and the surface. while this may not change the temperature of the object in every case, it will make the temperature of the junction be different from that of the surface. this can result in a reading that is artificially low - and you'll never know it. i have seen errors as much as 15 °c!

there are countless other examples we could investigate. but let's look at some general guidelines for good thermocoupling technique. actually, most of these guidelines will apply to the installation of any thermal sensor.

physics

the sensors and equipment should not disturb the original physics of what you are trying to investigate. this is a good general rule with several practical implications.

• the sensor should not disturb any of the interface geometries in your system. for example, a heat sink thermocouple should not make a space between heat sink and heat source that didn't exist before. machine relief areas for the wires to run out to the meter if necessary. recess the bead into one of the surfaces first.

• the sensors should be as small as possible, given the handling conditions they will have to endure. they should not affect the thermal mass of the system.

• if thermal radiation is present in your system, use radiation shielding on the wire, or match the emissivity of the shield to the emissivity of the object. the thermocouple-as-fin effect can work in reverse, too.

bead temperature = object temperature

this sounds obvious, but in practice it is more difficult to achieve than you might expect.

• the thermal contact between the sensor and the object should be as good as possible. a flat sensor, minimum thickness of thermal grease or glue at the interface, and compression all can work well, depending on the situation.

• know where the measuring junction really is. avoid unnecessary wire twists, pinching or melting somewhere else along the lead.

• use fine gauge leads with insulators that will stand up to your test conditions. teflon insulation melts!

• install the sensor securely. you don't want it lifting off the surface during the test. use a method that will stand up to the measurement conditions. some adhesives become brittle with age; some soften and lose adhesion in elevated temperature. most adhesives won't stick in the presence of thermal grease.

thermocouple-as-fin

the lead wire should not move heat across the thermal contact resistance between the sensor and the object of interest.

• dress the wire out of the way; keep convective cooling on it to a minimum.

• at least 10 (insulated) wire diameters should be in good thermal contact with the surface, and as isothermal as possible. that way, the wire won't sink heat from the sensor. if the wire has high thermal conductivity, for example type t, use 100 wire diameters. follow isotherms in the object of interest, instead of routing the wire perpendicularly to the isotherms.

miscellaneous

if the object of interest is electrically "live", insulate the sensor from it. accept the small thermal penalty of the insulation (kapton tape works well) and take pains to avoid the thermocouple-as-fin effect to minimize the penalty.

• correctly identify the thermocouple type to the meter. be aware that the color codes vary by country.

• if electromagnetic frequencies are an issue in your system, get advice from an electrical engineer colleague about grounding, ground loops, and such. i never could make sense out of that stuff!

no doubt you will be able to add your own guidelines to this list; it is by no means complete! there is no way a newsletter article can hope to cover everything you need to know for successful temperature measurement.

for more in-depth guidance, there are several technical seminars available on experimental techniques offered through various organizations -- coolingzone, asme (www.asme.org) and other technical societies, and several consulting and software companies.

about cathy biber

dr. catharina biber is senior thermal engineer at infocus corporation where she works with product design teams to solve optical and electronic cooling issues in advanced digital data/video projection systems. she particularly enjoys collaborating with cross-functional team members to address all the aesthetic, manufacturability and regulatory aspects of design needed for a successful product.   previously, she was a technical staff member at wakefield engineering, inc., where she was involved in the design, analysis, and optimization of high performance heat sinks. she has taught seminars on electronics cooling and basic thermal analysis throughout the u.s. and in europe.