There is a multitude of process parameters nowadays that need to be measured in the industrial environment (temperature, pressure, humidity, force etc.). Out of these, undoubtedly the most common one is temperature, as it influences most manufacturing parameters. It is no wonder then that many solutions have been developed over time to measure it. There are a few general categories any industrial temperature sensor will fall into: thermocouples, RTDs (Resistance Temperature Detectors), thermistors and integrated silicon sensors. There is no “best sensor” rather they all have pros and cons which need to be individually evaluated for each application.
The RTDs are the most expensive, but they also provide best accuracy and best resolution for the measurement. This, however, only if appropriate analogue circuitry will be used (which of course will add cost to the already high price of the sensor itself). The appropriate analogue circuitry constitutes the subject of this article. RTDs are regarded as the best quality temperature sensors (when it is worth paying for them). They provide accurate and stable measurements over time, and, most important, they provide a linear resistance-temperature characteristic. In the figure 1 is shown the resistance-temperature characteristic of the most common RTD, the PT100, which gives 100Ohms at a temperature of 0 Celsius degrees.
2. Features and design
RTDs also represent a continuously expanding technology, better materials being researched and used, further improving the characteristics of the sensors. The purpose of the analog circuitry buffering the sensor is to transform the resistance variation of the sensor in a variation of voltage which can easily be converted in digital values by an ADC. Although there are several methods to do this, the most common one is to build an analogue precision constant current source that will force a known and constant current through the RTD. The variation of the voltage will linearly depend on the variation of the sensor resistance, and thus on the temperature.Great care should be taken care for the excitation current to be as low as possible. A good practice is to keep the excitation current below 1mA, but the drawback of such a small current is that it translates the temperature variation to a quite narrow voltage interval (figure 2). If this is the case, a higher resolution ADC will be required in order to obtain a satisfactory resolution of the final result (of the measured temperature). For, instance, the resistance variation of PT100 between 0 and 100 Celsius is 38.5 Ω, and a 1mA constant current source would translate this in a 38.5mA voltage interval (between 100mV and 138.5mV) – hardly a wide interval for the plain 10-bit ADC usually provided on chip by the microcontrollers. If using the proper devices tough, excellent accuracy and resolution can be obtain from an RTD. The table of figure 1 only shows the resistance-temperature variation between 0 and 200 Celsius. The RTD is quite linear in that interval, but even if wider ranges are required, simple first order up to third order mathematic formulas may be used to estimate the temperature based on the measured resistance. Even if this introduces some strain on the software algorithms, it must be weighed if this is acceptable against a maximum +/-4.3 Celsius at the highest end of the measurement range (800 Celsius). The graph of figure 3 indicates the measurement error (with appropriate analogue circuitry) against the measured temperature (note than in the most common measurement interval, between 0 and 100 Celsius, the measurement error is minimum).
For more detail: How to Measure RTD over long distances