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MEASURING TEMPERATURES USING THERMISTORS

by Jonathan Valvano

Start ý Alternative Transducers ý Resistance vs. Temperature Calibration ý Dissipation Constant ý Low-Cost Embedded Temperature Measurement ý High-Precision Temperature Measurement ý Sources and PDF

HIGH-PRECISION TEMPERATURE MEASUREMENT

Another approach to interface this transducer to the microcomputer is to use an astable multivibrator (see Figure 6).

Figure 6ýThis circuit converts thermistor resistance to period. As the thermistor resistance varies from 198.3 to 551.2 kilohms, the period of the square wave varies from 22,542 to 61,671 ýs. The square wave is connected to a 6812 input capture pin.

In this system, I used a thermistor with a higher resistance (see Table 5). The period of a 555 timer is about 0.693 ý C_T ý (R_A + 2R_T). Given a fixed R_A and C_T, the period of the output signal (P_OUT) is a linear function of R_T.

Microcontrollers have a rich set of mechanisms to measure frequency, pulse width, or period. To change the slope and offset of the conversion between R_T and P_OUT, the fixed resistor and capacitor can be adjusted. Even though the period does not include zero, the precision of this measurement is over 32,000 alternatives, or more than 15 bits. Because of the uncertainties in the 555 timer and the capacitor, the accuracy of this system will not be as good as the first example, but it can resolve temperature changes of 0.001ýC. The software shown in Listing 3 measures period with a resolution of 1 ýs and calculates temperature using a table lookup scheme similar to the previous example.

WHATýS LEFT?

Iýve provided you with a brief overview of thermistor-based temperature measurement. Measuring high-speed temperature transients will require special transducer design and instrumentation circuits. I have not discussed the choice of sampling rate, nor shown you the software to implement periodic real-time sampling. Because 60 Hz is a typical problem that most data acquisition systems must deal with, you can include either a digital low-pass or a digital notch filter to remove this unwanted noise. For a more detailed treatment of these issues, I refer you to the references provided.

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