Physics
How Exactly Do We Measure Temperature?
Believe it or not, the temperature is the most widely measured variable on the planet. From taking your temperature when you’re sick to making sure your home stays nice and cool during a heat wave, measuring temperature is critical to our everyday lives! But, how exactly do we do it?
Well, first of all, what exactly is temperature? Sure, you could define it as how hot or cold something is (sort of), but what exactly does that mean? What actually makes an object hot or cold?
The short and sweet answer is, that temperature is the measure of how much “thermal energy” an object possesses. The more thermal energy an object has, the higher the temperature, and therefore the hotter it is.
When something is cold, it simply means that it lacks thermal energy. When your morning cup of coffee cools on your kitchen counter as you get ready in the morning, what’s really happening is that it is losing thermal energy.
Well, not exactly. The thermal energy is actually being transferred to the kitchen counter, being used to convert the liquid water in the coffee to steam vapor, and otherwise decreasing the amount of thermal energy it has.
Remember, energy, even heat energy, can neither be created nor destroyed. It is transferred and released — but never lost.
So, how the hell do we measure this “thermal energy”?
Such a Cute “Couple”
One of the most basic instruments for measuring temperature is called a “thermocouple”. In fact, a thermocouple can even communicate with other instruments around it…
Alright, so what the hell is a thermocouple, and how does it work?
The image above shows what a typical thermocouple looks like in its most simplified form. Very rarely will the wires of the thermocouple be exposed to the process, but that’s not important for now.
The wires are made of “dissimilar metals”. That means metals which are widely different in their molecular characteristics. Two metals, like the above picture, are considered to be more dissimilar when they have a larger difference in their electrode potential.
There are several different “types” of thermocouples and they have very specific metals used for various temperature ranges.
You’ll notice that the two exposed wires are not touching. However, they actually are joined further down the wire. In fact, that junction is precisely what makes the magic happen.
A very wild thing happens between these two dissimilar metals. They naturally produce a very small voltage, in the mV (millivolt) range. What’s even more wild is that that voltage is proportional to the temperature of the two pieces of metal.
As the temperature of the two pieces of metal increases, they produce a higher voltage at the junction point of the thermocouple.
Now, there are very specific tables used for calculating which mV signal represents which physical temperature, but those have already been figured out and you can always check them out on your own if you’re so inclined.
The point is, that mV signal is sent to some sort of instrument, like a remote display, that can convert it into a usable temperature measurement for whoever needs to know the temperature!
There are some limitations of thermocouples, of course, and there are other instruments we can use to measure thermal energy in those situations.
Join the Resistance!
One of the advantages to a thermocouple is that it requires zero power to actually measure temperature. Of course, if you wanted a digital indicator there would need to be some power needed, but it wouldn’t be needed for the actual thermocouple.
The other popular way to measure temperature does require some power, however. Not much, but some!
Think of what happens to a piece of metal as it heats up. The metal will begin to expand. Or, if you were to cool a piece of metal, it would contract.
That is called thermal expansion, and different metals and alloys will have different rates of thermal expansion. For example, standard 6061 aluminum will expand at a rate of .005" per degree per square inch of material.
While that doesn’t seem like a lot, it can actually make a significant difference in very large pieces of metal, like the wings and body of an airplane.
The point is, since we have already catalogued these thermal expansion rates for various metals, it’s easier to use this property to measure “temperature”.
So… what does any of this have to do with measuring temperature?
Well, we use this thermal expansion to our benefit with an instrument called an “RTD” or “Resistance Temperature Detector”. As the name suggest, it uses resistance to communicate temperature. Thermocouples use voltage, and RTD’s use resistance.
But, how does it “communicate” using resistance instead of a voltage?
The trick is, it sort of uses both!
You’ll recall that I said this instrument requires some power to work properly. We typically call this, “excitation voltage”. Excitation voltage typically runs in the 5 to 10 volt range.
So, let’s say you apply 5 volts to a chunk of metal of a specific size. In fact, the RTD itself is going to essentially be the circuit.
Let’s assume we’re looking at a typical 100 ohm copper RTD. You apply your voltage to the RTD, the voltage meets 100 ohms of resistance , and that requires a specific amount of current to complete the circuit.
That means we have a known variable. A reference point.
However, what happens if you heat that piece of copper up by a few hundred degrees? Well, that piece of copper is going to expand quite nicely. And what do you think happens to the resistance when the piece of metal expands?
Well, since more current is allowed to pass through a larger piece of metal, the resistance actually decreases as the temperature of the metal increases. That change in resistance will result in changes in current, and that is precisely how an RTD is used to measure temperature!
Much like the thermocouple, an RTD can be connected to a remote device that will convert the electrical signals from the RTD into a useable temperature reading.
In fact, most devices used to read out the temperature can also provide the necessary excitation voltage!
The above image shows the signal connections for a device used to monitor and control a temperature by using an RTD. You’ll notice that pin number one is labeled as, “EXC” for “excitation”.
Pin 2 can be considered the positive terminal, and pin 3 is the negative terminal, though it is labeled, “COM” for “common” in the above diagram.
The point is, an RTD and a thermocouple are indeed very similar in how they function, except that an RTD needs a power source while a thermocouple does not.
Both of those methods for measuring temperature have been around for quite a while, and they aren’t too advanced. But, their simplicity is its value. It’s a reliable technology that has been used for decades.
However, one of the major disadvantages to the technology, especially in today’s world, is that the signals coming from either instrument are relatively weak and susceptible to electrical noise. This means that the signals cannot be transmitted too far without first amplifying them.
That is where our next instrument comes in to save the day!
Temperature Transmission, Complete
This next instrument isn’t used to physically measure any temperature, but rather they are used to “amplify” the signal from a measuring element so that it is powerful enough to transmit over a long distance (typically a few hundred feet, or less).
Below is what a typical “Temperature Transmitter” looks like.
You’ll first notice how much more complex the device appears to be. While that is true, the actual measurement element is going to be the same tried and true technology we previously looked at.
The temperature transmitter above has two roles to fill. It is used to display the temperature reading for an end user, and it is also transmitting that temperature reading to another piece of equipment like a central control system.
We do that by “transducing” the mV signals from a thermocouple, for example, into a much stronger and useable signal. Of course, the signal is a standard used in the industry, and it’s the most common signal used in process instruments!
We use what’s called a “4–20 mA” (four to twenty milliamps) signal in about 90% of applications. That signal, in this scenario, represents the rage of possible temperatures that can be measured in the process.
Let’s say you were measuring a pot of water, and you want to see how long it takes to go from 1 degree Celsius to the boiling temperature, 100 degrees. Yes, I know, a watched pot never boils, but just stick with me for a minute.
When the water is at one degree, the transmitter will be sending out a 4 mA signal since that’s the bottom of the measured range. When the water reaches 100 degrees, and begins to boil, then the transmitter will output a 20 mA signal since that is the upper range of the measurement being made.
All temperatures in between will be represented by some 4–20 mA signal which is directly proportional to the specified measurement range of the instrument.
Of course, a lot goes into selecting and configuring a temperature transmitter correctly, but that’s the very basic operation of how it works. More times than not, if someone is measuring temperature, a 4–20 mA signal is going to be involved in some form or another.
Are There Any Temperature Instruments from THIS Century?
All of the devices we’ve seen so far are relatively old technology. While the temperature transmitter can be considered modern technology, there must be some newer and more exciting way to measure temperature, right?
Remember that whole COVID thing that still hasn’t gone away?
Right… but do you remember having to get your temperature taken upon entering virtually every building with a contactless device?
Those devices are actually not meant to accurately measure body temperature but that is completely beside the point. They are typically used to measure the surface temperature of something without having to come into contact with the surface for safety reasons. When you’re measuring a giant piece of hot metal, being off by five degrees or so isn’t a huge deal.
But measuring five degrees off when measuring a human’s temperature?
Getting back to the point, there are various types of contactless measurement devices that are used for all sorts of reasons. We won’t go into great detail about how they work in this article, but it’s still worth taking a quick peek.
In fact, both the technologies we’ll go over use infrared and electronics to measure temperature!
Infrared Thermometers actually come in different forms. However, they all use infrared to measure temperature. No, it’s not that the measurement device is outputting infrared radiation and comparing it to something.
Rather, it is collecting the infrared energy emitted from an object that has a temperature. Typically you’ll see a laser beam when you point the IR thermometer at a surface, but that is simply for aiming purposes. That light doesn’t assist in the measurement at all.
It’s almost like having infrared vision and seeing the world as a heat gradient map. But, instead of looking at the object with an eye, the device has a sensor which collects that IR information from the object and translates that into electrical signals, similar to those we’ve previously discussed.
Speaking of infrared vision, would you believe that there is actually a technology which uses vision to measure temperature?
Yes, there are actually portable infrared cameras which can not only display the temperature gradient, but it can also measure and display the temperature of multiple surfaces at once.
Or, some of these types of vision sensors are fixed into place and are programmed to look at a specific area within its field of view. For example, let’s say you extrude plastic bottles and need to make sure they are cool enough to go onto the next part of the bottling process.
Rather than having to check every single one of them individually with some sort of temperature probe or thermometer, the vision camera can capture the temperature of each bottle quickly enough that it can be mounted right on the conveyor belt and check each one as they go by.
To take it a step further, those devices can also be wired to a controller of some sort that actuates an arm which “rejects” bottles that fall outside of the desired temperature range to avoid complications further down the process.
In my opinion, it’s actually quite an impressive technology, and it’s only going to get faster, more accurate, and more useful as time goes on. Just imagine if you were able to buy some sort of sunglasses that has infrared vision built-in that can be switched on or off.
Sure, you can get something of the sort now, but not at a reasonable price.
The point is, I get excited about technology like this and its potential for future use!
So… How Do We Measure Temperature?
The more modern technologies are rather sophisticated, especially the vision systems, but at the heart of it, measuring temperature is actually quite simple!
A chunk, or chunks, of metal are heated up by whatever we want to measure, and it naturally has electrical properties we can easily and cheaply manipulate.
And that’s a good thing! As I mentioned, temperature is the most widely measured variable on the planet. Even indirectly sometimes, temperature is being measured, controlled, and maintained.
One thing we didn’t get into here is actually maintaining a certain temperature. Aside from measuring it, the actual control of temperature is really where this technology is best used. If you would find that interesting to read about, let me know and I’d be happy to write it!
That said, those are the basics of how temperature is measured here on Earth in every day life. Well, every day factory life, that is.
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What do YOU think?
Are these kind of articles of interest? Would you rather read about some of my zany, scientifically-iffy theories?
Thanks for reading!
