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Temperature is a measurement of the average kinetic energy of the substance -- so if we have a thermometer calibrated for water, could it be used for measuring the temperature of air? My inclination is to think no, because the amount of kinetic energy that is being pulled from the thermometer (e.g. 1 degree C water) is going to be much greater in water than in air, but I'm not sure if I am missing something.

Pallen
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4 Answers4

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Yes. At equilibrium everything has the same temperature. Particles on average have the same kinetic energy. That is, particles with above average kinetic energy collide with particles that are on average less energetic. The more energetic particles tend to lose energy and less energetic particles tend to gain. This works even if different kinds of particles are mixed together.

mmesser314
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Not necessarily. The answer by mmesser314 is not wrong, but it makes the unfounded assumption that your thermometer is passive, like a mercury thermometer: That it contains no energy source. This assumption fails if your thermometer is electronic. Most electronic thermo sensors dissipate (a low amount of) heat. That means that the heat conductivity of the medium being measured becomes important: The less conductive, the higher (and therefore off) the temperature reading, because it requires a higher temperature difference between the medium and the sensor to bring the heat generation by the sensor and the dissipation by the medium into equilibrium. This temperature difference may have been calibrated away for one medium (e.g. water), in that case the thermometer needs to be recalibrated for a different medium.

Having said that, sensors usually are designed to generate little heat flow, so if your accuracy requirements (you did not specify any) are not too strict, the existing calibration for water may be good enough for you in air.

Peter Rottengatter
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Nope, there is a difference. I'm assuming that you are using an ordinary glass thermometer with some kind of fluid that expands.

  • In air the whole thermometer is in the air.

  • For immersion in a liquid the thermometer is typically used so that the observed level of the thermometer's fluid, which indicates the temperature, is outside the liquid whose temperature is being measured. Since part of the thermometer is in air and part in liquid, (assuming the air and the liquid are not at the same temperature) there is a difference in the expansion of the glass. In order to best correct for this variation you need a thermometer with a huge bulb volume compared to the volume of fluid in the bore of the thermometer.

So called immersion thermometers have a ring marking the level at which the thermometer is supposed to be immersed in the liquid. Such thermometers work well enough because you don't get 18 significant figures in the temperature reading.

MaxW
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There is no difference - in theory. I say in theory because different ways of measuring temperature have their own ideosycracies that could cause differences, but those will be minimal, and others have discussed.

The important point here - and the one the other answers seem to have missed - is the Zeroth Law of Thermodynamics.

https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/zeroth-law-thermal-equilibrium/

If two things are in thermal equilibrium they are at the same temperature.

If your thermometer is reading 10 degrees, it is in thermal equilibrium with a medium at 10 degrees, whatever that medium is. That is really how we define temperature.

Thermal equilibrium means the thermometer is pulling energy out of the medium just as fast as it is losing energy to the medium. And if the water is in contact with the air, they too are in thermal equilibrium (evaporation complicates the situation, so just ignore that). The fact that the air molecules are moving very fast and the water molecules much more sluggish does not matter. They both sit there happily exchanging equal amounts of energy so neither is warming and neither is cooling. Thermal equilibrium.

Hence, the two media - and the thermometer which ever it is in - are at the same temperature because that is how temperature is defined.

F2Andy
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