Thursday, December 16, 2010

Transducers for making Temperature Measurements with a DMM

Four types of transducers are commonly used for making temperature measurements with a DMM: resistance temperature detectors (RTDs), thermistors, IC sensors and thermocouples. Each type has advantages and disadvantages. The follow is a quick summary of each.

Use thermistors for better sensitivity
A thermistor is a temperature transducer who's resistance changes as a function of temperature. They consist of semiconductor materials and provide excellent sensitivity, but their temperature range is limited, commonly from -80°C to 150°C. Thermistors have highly nonlinear temperature-resistance relationships; therefore, their conversion algorithms are complex. Modern DMMs make thermistor measurements by storing a table of thermistor resistance values and the temperatures they map to. Agilent DMMs like the 34401A, 34410A, and 34411A use the standard Hart-Steinhart approximation to provide accurate conversions, with a typical resolution of .08°C. Check out Agilent’s E2308A thermistor temperature probe

Use RTDs for more accuracy
Resistance temperature detectors (RTDs) work a lot like thermsistors in the sense that they are resistive temperature transducers. They provide high accuracy over a range of roughly -200 to 500°C. There is very little conversion complexity for an RTD since it is so intrinsically linear. Modern DMMs such as the Agilent 34410A provide measurement for the IEC751standard RTD, which has a sensitivity of .0385%/°C.

IC temperature sensors are an easy to use linear solution
Many vendors provide probes that produce a voltage proportional to temperature in degrees C or F. The probes typically use an IC temperature sensor such as the National Semiconductor LM135 series. A temperature IC can cover temperatures from -50°C to +150°C. You can easily compute the temperature from the probe output shown on the DMM display. For example, 270 mV is 27°C.

Thermocouples offer a wide measurement range and ruggedness
Thermocouples can measure the broadest range of temperature, from -210°C to 1100°C, and their rugged construction makes them ideal for harsh environments. Thermocouple temperature measurements are based off the Seebeck Voltage. Take two dissimilar metals and connect one end of each metal together and a small voltage will be present at the open end of the metals, this is the Seebeck voltage. This voltage is a function of the temperature at the junction of the two metals. This relationship between the temperature and the voltage produced by the two dissimilar metals is how thermocouple temperature measurements are made. If you directly connect a thermocouple wire to a DMM connector (which is metal) you create more thermocouple junctions which will distort the original intended measurement junction. Because of this a reference junction is needed between the DMM and the thermocouple cabling. This adds complexity to the measurement. There are DAQ instruments out there with a built-in reference junction like the Agilent 34970A or 34972A. For more technical details on thermocouple temperature measurements reference junctions check out the application note link below. 

By far the most common way I make temperature measurements is with thermocouple because of its ruggedness and its range. For instance I can use the same thermocouple wire to measure the temperature in an environmental chamber, to measure the ambient room temperature, or to measure the heat on a processor chip without having to worry about does its range cover what I am measuring or being delicate when I attach it to a surface. Below is a table that runs through each transducer’s pros and cons (click on to enlarge). 


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