Temperature’s importance comes from its impact on so many environmental and processing situations. We, or more accurately the equipment we employ, require accurate temperature measurements to heat and cool our homes, to operate our car’s engine, to cook food, to process industrial materials, to monitor a patient’s vital signs, and the list goes on and on. Back in Chapter 1, we saw an example of such a device, the mercury bulb thermometer, in which the absolute temperature is indicated by the level of the liquid mercury inside the thermometer. Unfortunately, the fragility of that type of sensor (usually a glass tube enclosing the column of mercury) and the toxicity of its sensing element (mercury is highly poisonous) limit the mercury thermometer’s usage to certain well-controlled environments. The sensor system we develop here employs a sensing element that is far better suited to a wide variety of environments and applications: the thermocouple. Before we delve too deeply into the details of thermocouples, let’s first look at the variety of temperature-sensing elements available to us.
Types of Temperature Sensors
Over the years, scientists have developed a host of specialized sensing elements that respond to absolute temperature or to changes in temperature by varying some physical property. In this book, we’re interested primarily in sensing elements that we can monitor using electronics, for the very simple reason that doing so allows us
to easily convert the monitored physical property into a measurement that we can manipulate using the dsPIC DSC. That’s not to say that other approaches (such as visually monitoring the mercury bulb thermometer) are invalid; it merely means that those nonelectrical techniques are unsuitable for our purposes because of the monitoring platform. Nails and screws can both be used to fasten two boards together, but one is far more likely to use a nail if the tool at hand is a hammer. Currently, there are five widely used types of sensing elements whose outputs can be monitored electrically. Advances in technology are sure to build upon this list, but most electrically based temperature sensors are one of the following types:
1. thermocouples,
2. resistance temperature detectors (RTDs),
3. thermistors,
4. silicon sensors, or
5. infrared sensors.
Although we’ll discuss each of these sensor types in the following sections, the application we develop in this chapter uses the thermocouple exclusively because thermocouples are widely used, well understood, accurate (when utilized properly) and relatively inexpensive (which is one reason they’re so widely used).
Thermocouples are two-wire sensing elements that make use of the Seebeck effect to measure the temperature of the junction of the two wires. The Seebeck effect, discovered by the scientist Thomas Seebeck in 1821,2 creates a voltage across the junction of any two dissimilar metals that correlates to the temperature of the junction. Although this voltage is quite small, on the order of several microvolts per degree of temperature, it’s possible to create systems that are accurate over a wide temperature range provided that proper analog and digital signal processing techniques are used. The caveat “that proper analog and digital signal processing techniques are used” is a major consideration. The very small voltages produced by thermocouples (on the order of millivolts) require that designs employ good grounding and shielding techniques to avoid introducing unacceptable levels of noise in the measured voltage.
In addition, because the traces on a printed circuit board are made of a metal that differs from those of the thermocouple, the very circuitry that we use to measure the original thermocouple voltage introduces an additional Seebeck junction whose output varies with temperature!
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