Four tips for better temperature measurements

Temperature is arguably the most important and widely used type of measurement made in automotive testing. For example, you must measure and control the temperature inside an environmental chamber to run effective emissions tests. And to gauge the effectiveness of a radiator design, you must be able to accurately measure the temperature of engine coolant. I could come up with dozens more examples.

With that in mind, I asked industry experts for their advice on making better temperature measurements. Here's what they had to say:

Alan Tong, technical director of Pico Technology (Cambridgeshire, UK; www.picotech.com), says that it is important to think about the physical phenomenon whose temperature you are trying to measure. If you are trying to measure the temperature inside an environmental chamber, for example, be aware that the temperature will vary from place to place. If you want to control the temperature inside the chamber, you may have to use several sensors or characterize the temperature distribution inside the chamber before using it for a test.

Tong also notes that it is important to remember that what you are really measuring is the temperature of the sensor, not the temperature of the component or fluid. To get the most accurate reading, the sensor must be placed directly on the component or in the fluid.

The parameters you must consider when selecting a temperature sensor include:

• the range of temperatures you need to measure;
• the accuracy you need;
• the need to replace sensors without recalibrating;
• the instruments to which you will connect the sensors; and
• the physical environment.

You have many choices when it comes to temperature sensors. The most commonly used sensors are thermocouples, resistance temperature detectors (RTDs), and thermistors. Dale Cigoy, a senior application engineer at Keithley Instruments (Cleveland, OH; www.keithley.com), says that all of these give excellent results, but only if you choose the right sensor for your application and use it properly.

Thermocouples . Thermocouples consist of two different types of metals that are soldered or welded together. They are relatively inexpensive, widely available, rugged, and cover a temperature range of –268ºC to 2316ºC (–450ºF to 4200ºF). You must, however, use them with instruments capable of measuring millivolt-level signals with microvolt resolution as they produce only a very small output voltage. Also, their output is non-linear, so the instrument you connect them to must be able to calculate the temperature from the output voltage.

Another disadvantage of using thermocouples is that they require cold junction compensation. When you connect thermocouple wire to your instrument, the wire forms yet another thermocouple at that junction. The voltage produced at the junction depends on the temperature at the junction. If an equal and opposite voltage is not added to the thermocouple voltage, the temperature reading will be incorrect.

Also note that you must use thermocouple wire to connect the sensor to the instrument. If you use copper wire, you will create thermocouples at the junctions of the copper wire and the thermocouple wire, which will create error voltages.

Resistance temperature detectors (RTDs). RTDs are made with metal wires or films whose resistance changes as their temperature changes. The wire or film in an RTD is usually platinum, although RTDs made with other metals, such as nickel, a nickel/iron alloy, and copper are also available. You can use platinum wire RTDs to measure temperatures from –240ºC to 649ºC (–400ºF to 1200ºF).

Like thermocouples, RTDs are nonlinear, and the instrument you use with an RTD must calculate the temperature from the resistance measurement. Also like thermocouples, an RTD's output changes very little with a small change in temperature. The resistance of a standard Pt100 RTD, for example, changes by only 0.385 Ofor a 1ºC temperature change. The instrument you use with an RTD must, therefore, measure small resistances with great accuracy.

One disadvantage of using RTDs is that you must supply excitation to the sensor, and this current can cause self-heating. To prevent self-heating, the excitation current should be less than 1 mA for a Pt100 RTD. Another drawback to using RTDs is that the resistance of the test leads can reduce the accuracy of your measurement. When leads are longer than a few inches, you may have to use a three-wire or four-wire (Kelvin) connection to achieve the needed accuracy.

When choosing a sensor, make sure that you take into account how much the sensor will drift over time. Ryan Wynn, product manager, data-acquisition systems, for National Instruments (Austin, TX; www.ni.com), notes that many automotive temperature measurements are made at very high temperatures for extended periods of time. Under such circumstances, both thermocouples and RTDs will drift, but thermocouples tend to drift more. RTDs are, therefore, the preferred sensor for use in long environmental tests, durability tests, and HALT/HASS tests.

Thermistors. Like an RTD, the resistance of a thermistor changes with temperature. A thermistor's resistance changes more for a given change in temperature than does the resistance of an RTD, but you cannot use a thermistor over as wide a temperature range. Also, thermistors are less accurate than RTDs, they are less interchangeable than RTDs, and their long-term stability is poor. Thermistors are also nonlinear, so the instrumentation used with them must provide compensation.

Because you must bond thermocouples to engines or other charged metallic components to make many automotive temperature measurements, Wynn advises that you isolate the sensor from the data-acquisition system. Bonding a thermocouple to a charged metallic surface can result in noisy, inaccurate measurements because of the high common-mode voltage generated by the charged surface and conducted through the thermocouple.

Isolation increases the accuracy of your temperature measurements by eliminating common-mode voltage. When isolated, the negative input terminal "floats" up to the common-mode voltage level so you measure only the desired differential voltage. Isolation also makes measurements safer, as it prevents potentially damaging current flow and transient voltage spikes from getting into your expensive data-acquisition hardware.

Corey Glassman, automotive marketing manager for Fluke (Everett, WA; www.fluke.com), points out that handheld IR thermometers are very handy for finding problems with heating and air-conditioning systems. An IR thermometer makes accurate measurements in less than a second, and the instrument's built-in laser pointer lets you pinpoint the spot where you want to measure the temperature. Also you can stay a safe distance away from hot engine components (figure).

You can use IR thermometers to:

• measure the temperature of the output of heating and cooling systems;
• scan a radiator or heater core surface to find core restrictions or blockage;
• measure coolant temperature sensors and manifold air temperature sensors to determine if they are operating within the correct tolerance (you can compare the temperature reading to the electrical readings by using a multimeter); and
• monitor thermostat temperature to determine when, and at what temperature, the thermostat is opening.

Glassman notes that when monitoring thermostat temperatures, you should be aware that the actual coolant temperature may be higher due to dissipation.

Clearly, there's more to making accurate temperature measurements than simply hooking up a sensor to an instrument. If you have any doubt about how to connect a sensor or are not sure if your data-acquisition system is sufficient for making temperature measurements, contact an applications engineer at the sensor or instrument company.