Measuring temperature – that’s quite simple, right?

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Measuring temperature – that’s quite simple, right?

Yes, at first glance … but measuring temperature correctly is a bit more tricky.

I believe all of us have experienced that different thermometers do not show exactly the same value, two thermometers outside the kitchen window or two thermometers in the refrigerator. It can of course depend on the measurement accuracy or calibration of the thermometer, but before we suspect that, we should always check the position of the temperature sensor. The location of the thermometer is absolutely crucial for the result. In addition, like my good friend the temperature expert usually says, the only thing you can be sure of is that it’s impossible to measure the “right” temperature!

I will try to explain: We published a blog post in March 2016 which gave a little insight into the background to temperature and how to measure temperature. But in short, temperature is a measure of a body’s energy content. Physically, it is a measure of how fast the atoms of the substance move. When it gets warmer, the atoms move faster and when it gets colder, the movement of atoms decreases. Finally they do not move at all, ie the absolute zero -273, 15 ° C. Temperature is thus a measure of the kinetic energy of the atoms.

Heat exchange is also important for us to understand. Nature strives to level out temperature differences between two bodies or substances that come into contact with each other. Then heat is always transferred from a warm body to a colder. For example, when you put hot coffee into a mug, the mug itself becomes so hot that you can barely hold it with your hands. This heat exchange can only happen in three ways, such as conduction (eg heat transfer via a metal rod), convection (eg circulation in gases and liquids) or radiation (eg infrared heater or grill function in an oven). All materials are not good at leading the heat, examples of good heat conductors are metals. Think of how you stir in the coffee cup, if you use a silver spoon you can barely hold it without burning your fingers, but if you take a plastic spoon there is no problem at all (except that you can have opinions about whether the coffee tastes as well when using plastic). Plastic is generally a bad heat conductor and is therefore rather an insulator.

So, how do we measure temperature with temperature sensors? My good friend claims that it is impossible to measure the “right” temperature … what he means is that a temperature sensor always measures the temperature of the sensor (or probe), not the temperature of the object. It is important to remember that we can try to get as close as possible to the temperature of the object but our measurement will always detect the sensor temperature.
There are a variety of factors that will play roles and that will give rise to possible measurement errors:

  • Sensor mass; lower mass means faster measurement, but on the other hand, the measured value also will be affected by the environment more quickly i.e. when you open the door to the fridge.
  • Heat conductivity in the sensor’s sensing body; If you are to measure a surface temperature, you may not have to choose a plastic housing sensor.
  • Thermal conductivity of the measurement object; For example, liquids lead heat better than air.
  • The interface between sensors and measuring objects; i.e. Is there good contact / heat conduction between the sensor and the object (eg a surface). Note, however, that at surface temperature measurement, we want to minimize the conductivity to the environment, or when we measure the temperature inside a refrigerator, we want to avoid contact surface with the wall inside the refrigerator.
  • Any convection in the measurement object; ie how the heat varies and moves within or within the object. Think refrigerator, cold storage room, incubator etc.

As mentioned before, it is very important to understand why things can go wrong when making measurements. Once you have understood why a measurement error occurs, you have good opportunities to take action that reduces the error. Having said that, we need to be quite aware of what we actually want to measure before we make the placement of  the temperature sensors.

Most important, we must be aware that we ALWAYS measure the sensor’s temperature!

By | 2018-01-17T06:59:54+00:00 January 19th, 2018|1. Boomerang - Lab, 2. CoolGuard - Kitchen, 5. Tips & Tricks|Comments Off on Measuring temperature – that’s quite simple, right?

About the Author:

Per Hammargren och är försäljnings & marknadschef på ICU Scandinavia. Han är en av grundarna av ICU Scandinavia och har en bakgrund inom IT.