Practical Sensors: The Many Ways We Measure Heat Electronically
By Al Williams
Measuring temperature turns out to be a fundamental function for a huge number of devices. You furnace’s programmable thermostat and digital clocks are obvious examples. If you just needed to know if a certain temperature is exceeded, you could use a bimetalic coil and a microswitch (or a mercury switch as was the method with old thermostats). But these days we want precision over a range of readings, so there are thermocouples that generate a small voltage, RTDs that change resistance with temperature, thermistors that also change resistance with temperature, infrared sensors, and vibrating wire sensors. The bandgap voltage of a semiconductor junction varies with temperature and that’s predictable and measurable, too. There are probably other methods too, some of which are probably pretty creative.
You can often think of creative ways to do any measurement. There’s an old joke about the smart-alec student in physics class. The question was how do you find the height of a building using a barometer. One answer was to drop the barometer from the top of the building and time how long it takes to hit the ground. Another answer — doubtlessly an engineering student — wanted to find the building engineer and offer to give them the barometer in exchange for the height of the building. By the same token, you could find the temperature by monitoring a standard thermometer with a camera or even a level sensor which is a topic for another post.
The point is, there are plenty of ways to measure anything, but in every case, you are converting what you want to know (temperature) into something you know how to measure like voltage, current, or physical position. Let’s take a look at how some of the most interesting temperature sensors accomplish this.
Thermocouples take advantage of something called the Seebeck effect. When two dissimilar metals form a junction and experience a temperature gradient, an electric potential forms. The key is that it is a gradient in temperature that makes the device work. Thermocouples have a hot junction and a cold junction. If you want to measure temperature, you need a reference junction. As an aside, the effect works in reverse — the Peltier effect — where passing current through a pair of junctions makes one side hot and the other one cold.
In the old days, you’d plunge the cold junction into a bucket of ice. Today, it is more likely that you’ll use another method to get the temperature of the cold junction and then compensate. There are chips that will do that for you, of course.
Why not just use the other method to start with? Thermocouples have several advantages. For one thing, you can measure up to a few thousand degrees with one. Since they are just two pieces of wire, they are robust and reliable. In hot or harsh environments, they are easy to manage and, usually, at the cold end you have a little nicer environment.
The downside is that the temperature reading is not linear. You’ll see different types of thermocouples and each type uses two different wire materials. The type tells you what calibration curve to use and, of course, you select the metal for the application you need. For example, a type J uses iron as one of the two wires and a type T uses copper.
The only other big consideration is how you run wires to the thermocouple. Since the device operates on a junction between two different types of wires, you have to be careful how you connect other wires to the device. Want to know more? [Bil Herd] did a deep dive into how to build a thermocouple amplifier.
Thermocouples that measure infrared from a distance are known as thermopiles. These are common in non-contact thermometers and passive IR (PIR) sensors. A PIR sensor detects the difference in temperature between two sensors and infers that something has changed in the field of view.
There are several different types of material that can exhibit temperature changes with resistance. The biggest factor is if the device has a positive or negative temperature coefficient. In other words, does the resistance go up or down in response to a change in temperature?
Most of these devices are also non-linear, but they are also inexpensive and easy to use. You can measure the resistance using any method you like. A common technique is to use the resistor as part of a voltage divider or bridge and measure the output voltage. However, if you’d rather not tie up an analog input, you can connect the resistor to a capacitor and measure the time it takes to charge.
Thermistors are slightly different in construction from resistance temperature detectors, or RTDs. Usually, thermistors have less hysteresis and self-heating problems than the metal-based (often platinum) RTDs. However, in either case, you’ll have to measure resistance and fit it to a curve to get the real temperature.
Reading thermistors is a very common operation and there are a lot of tricks people have developed over the years. You can also spend math processing to get better curve fits, or do simple math and get less accuracy.
Semiconductor Junctions and Accidental Sensors
The bandgap voltage of semiconductor material varies predictably with temperature. If you ever get deep into solid state design, you’ll see the T term in the diode equation and all its manifestations. It is no surprise, then, that a lot of ICs use this property for sensing temperature.
Some chips are made to be temperature sensors. For example, the common LM34 and LM35 chips exploit this property with some additional circuits to provide a nice 10mV per degree (the LM34 measures Farhenheit and the LM35 measures Celcius). That makes them very easy to use.
Some chips, like the CPU in your PC, use the same method to measure internal temperature for reporting and thermal management. However, there are other ways non-temperature sensor ICs can measure temperature.
It turns out, almost all of our circuits are sensitive to temperature in some way. Measuring the internal clock of a CPU against an external reference can show temperature-induced changes.
There are a multitude of other ways to measure temperature. For example, a vibrating wire sensor uses what amounts to a guitar string. The measurement involves exciting the string and detecting the frequency of vibration. As the supporting structure shrinks and expands with temperature changes, the tone of the string changes.
You can get an approximate temperature in degrees Fahrenheit by counting the number of chirps crickets make. Count the number of chirps in 15 seconds and then add 37. It wouldn’t surprise me if someone’s done that in some obscure instrument. [Kevin] in Terra Haute says the number is 40 in the video below, and not 37, but I guess it isn’t an exact science.
Of course, an increasingly common way to measure temperature is to use some form of smart sensor. A module or IC can use any of the methods we’ve talked about, convert it to engineering units, and send the data over something like an I2C bus. This is a level abstraction, but you still ought to understand the underlying benefits and limitations involved with the sensor you want to use.
While there may be more, there aren’t any other common techniques for measuring temperature. But there are still lots of sensors left to talk about in future articles.
February 19, 2021 at 07:01PM
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