#GoogleAsk – Answering 9 questions about temperature measurement
We all ask Google to answer our questions, right? And of course it has its own pros and cons. We get plenty of feeds, articles, and technical papers to widen our spectrum of knowledge, which is an amazing thing. On the flip side, have you seen how many results come up for the weird searches?
Well, we found an efficient way to peek at what people ask Google and help them find quick and direct answers in the field of automation. (We plan to expand our horizons, but until then, y’all on your own).
So let’s unveil the common questions regarding temperature measurement!
1 – What is temperature measurement used for?
Before we get technical about temperature measurement though, let’s get a few things straight. People sometimes confuse the words “heat” and “temperature” and think they mean the same thing. SO not true, neither in what they mean or how they apply.
Heat refers to a measurement of energy. All matter has molecules, and those molecules move. Now, the faster the molecules move, the more heat you have in that matter. Hence, the heat of an object means the total energy of all the molecular motion in it. Best of all, you can measure it.
Temperature refers to a measure of the average heat of the molecules in a substance. Molecules move with a range of energies (called an energy spectrum) and interact with each other, which changes their energies. Thus averaging the thermal energy of all the molecules together produces the basic unit of measurement we call temperature.
For instance, consider pasteurization, the process of sterilizing a beverage by heating and cooling it through several cycles. We use this process to reduce the number of germs present in the beverage that can make consumers sick. But you have to use specific temperatures for it to work. Milk, for example, must remain between 71 and 4 degrees Celsius to maintain its proteins while still eliminating germs.
We use temperature measurement for a number of other processes which require more or less heat than found in their environments.
2 – Who invented temperature measurement?
The year 1593 was a good one for physicists, because Galileo Galilei invented the first thermoscope, a device that indicates temperature differences. Two decades later, around 1612, Galileo’s friend Santorio invented the first thermometer.
The next novelty came into existence 42 years later in 1654, when Ferdinand II, Grand Duke of Tuscany, produced the first liquid-in-glass meter. This meter marked the first use of fluids for measuring temperature.
Ten years later in 1664, Robert Hooke proposed that we call the freezing point of water absolute zero. And the beginning of the 18th century became a milestone for Ole Roemer, who established two fixed points, Hooke’s zero point and the boiling point of water in 1702.
In 1714, Daniel Gabriel Fahrenheit invented the first mercury-in-glass thermometer. And in case you couldn’t guess, he also invented the first standard temperature scale.
In 1730, Rene Antoine Ferchault de Reaumur created another scale, naming the freezing point of water as zero degrees and the boiling point as 80 degrees. We don’t use this scale, but thanks for playing, Rene.
Then a decade later in 1742, Anders Celsius, a Swedish scientist, set the freezing point as 100 degrees and the boiling point as zero. And Jean Pierre Christin switched these points to make the scale we use today.
Finally, in 1848 Sir William Thomson, also known as Lord Kelvin, created the absolute scale, one of the last milestones in the era of temperature measurement.
3 – How does temperature measurement work?
We can use temperature measurement with a wide variety of devices, and each has a specific purpose, process, and output. So I’ve listed the most popular with a few details here.
Thermocouple: A thermocouple has two different metals joined to form a loop. The potential difference between the two metals created across the junction results in a force directly proportional to the heat affecting the sensor. So with a reference table, you can see the relationship between the voltage and the temperature.
Resistance temperature detector (RTD): As the name suggests, these sensors detect changes in temperature by changes in the resistance of the wires within. We’ll talk more about this one in a bit.
Infrared sensor: Every object emits energy that you can measure through thermal frequency distribution. This device also can sense certain characteristics by detecting radiation under the visible range of 700 nanometers to 1 millimeter in wavelength.
4 – What is an RTD?
As mentioned in the last answer, RTD means resistance temperature detector. This device detects changes in temperature by changes in resistance of the wire within. The control system constantly supplies current through the sensor, so the sensor can detect any change in resistance and report it.
RTDs have positive and negative thermal coefficients of resistivity. That means the resistance increases or decreases with the increase in temperature according to Ohm’s law. The type of material used to make the wire can affect the coefficient of resistivity, the range, and the linearity.
RTDs hold their linearity over a wide operating range and maintain stability at high temps. However, shock and vibration may reduce the accuracy of the RTD.
5 – What is a thermocouple?
Thermocouples detect temperature using the thermoelectric electromotive force (thermal emf) that results in a junction of two metals when current passes through it.
The three basic laws of thermal emf by Seebeck, Peltier, and Thomson will help in understanding the mechanics.
Seebeck’s law states that the exent of the emf will depend on the temperature differences between two junctions and the material used for the two wires.
Peltier’s law states that when an electric current crosses a junction between two different metals, one junction heats up and the other evolves the heat.
Thomson’s law states that a conductor subjected to a temperature gradient will create a corresponding voltage gradient.
Thermocouples use a number of metals like copper-constantan and platinum-rhodium. These devices can cover a wide range of temperatures, going as high as 2700 degrees Celsius. However, if you need good accuracy, then you must have cold junction compensation. More on that later.
6 – What are the types of thermocouples?
Every thermocouple type has a specific range and accuracy.
32 to 3100
-454 to 1600
±1.7 to ±0.5
-346 to 1400
±2.2 to ±0.75
-454 to 2300
±2.2 to ±0.75
-454 to 2300
±2.2 to ±0.75
-58 to 2700
±1.5 to ±0.25
-58 to 270
±1.5 to ±0.25
-454 to 700
±1.0 to ±0.75
7 – How do I calibrate a thermocouple?
Since they all differ, let’s look at the options.
You can use this method if your lab has the thermocouple and voltmeter at the same ambient temperature and you use a compensating cable.
Install your sensor in the calibration furnace and set the furnace to increase and decrease the temperature. The voltmeter measures the potential difference that exists in millivolts, and a reference table will translate the millivolts into degrees.
A cold junction will calibrate your sensor for higher accuracy. The greater the difference between the hot junction and cold junctions, the more precise your millivolt values from an accurate voltmeter.
Comparative calibration with an external reference junction
Here we have a second thermocouple connected to an external reference junction. This helps you compare your sensor output with the reference sensor. Both these start at zero degrees Celsius.
In this setup, you shouldn’t connect the sensor directly to the calibrator, since it will compensate for the ambient temperature. However, you can apply these calibrators with the calibration furnace to increase and decrease the temperature, making data collection quick and automatic.
8 – What is thermocouple compensation?
The word “compensation” in the context of thermocouples has multiple references but predominantly two. The first refers to a compensation cable and how it differs from an extension cable. The second refers to cold-junction compensation for higher accuracy.
We use compensation to avoid thermal emf in the connection terminals. A change in the terminals can change the output values entirely. However, the transmitter can adjust for this difference.
You can also use the electrical bridge method or thermoelectric refrigeration method. For more information on these methods, feel free to post a #WishIknew!
9 – Why should I use a thermocouple with a transmitter?
We’ve reviewed a lot today, haven’t we? Our temperature measurement journey ends with the use of a thermocouple with a transmitter. First of all, the transmitter will cut down external noise from signal attenuation. Secondly, it can translate the sensor data to a standard format. Also, using a digital transmitter will deliver more data than just temperature.
Compensation cables can save money on installation for short distances. The transmitter usually compensates to give you precise output values. It also has a remote input/output (IO) in the field, multiplexers, and other tools.
This article aimed to provide clarity on the nine common questions frequently asked about temperature measurement in Google. I’m glad Google only had nine. A three or four digit number would’ve created a potential difference between my ears, turning my head into a thermocouple!
Anyway, I’ll keep digging up common Google questions in automation and get back to answering your queries soon. See you later!