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Topic: Galileo Thermometer: density compound / chromatic colour change?  (Read 255 times)

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Offline kiowes

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Galileo Thermometer: density compound / chromatic colour change?
« on: December 29, 2019, 05:56:18 PM »
Receiving a Galileo Thermometer this Christmas I was disappointed that the glass vials differentiating densities only began to function around 18ºC/64ºF. As I am living in an area of which is closer to zero degrees celsius at this time of the year, but capable of spanning upward of thirty (where the GT tops out) throughout, I would like to know if there are commercially available GT that sink to lower temperatures so as to be more useful for my conditions as I simply cannot find any?

Explanations online seem to focus less importance of the compound used inside the sealed vials to how the overall system works, so I would like to know what it is and if it is a simple case of having less of it to achieve a wider range of temperature readings, or if different, or a combination of, other differing compounds could be used to the same effect? Although I read that it's the tags themselves that are the weights, is there a way around this?

I would further be interested in the pigmentation of the compound: could be made colourless to the human eye until fluoresced under exposure to the ultraviolet (~400nm) light wavelength, or chromatic changes based on the temperature itself - are there any examples of which could prove this concept please?

Offline Enthalpy

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Re: Galileo Thermometer: density compound / chromatic colour change?
« Reply #1 on: December 30, 2019, 01:36:55 PM »
Hi kiowes,

What is the liquid in the column? If it's water, bad luck. First, you risk to break the glass below 0°C, and second, water has its minimum volume at 4°C, which means that around 4°C, the density change very little, so does the buoyancy, and you have no reading.

The answer would take a different liquid that doesn't risk to freeze and does expand over the whole desired temperature range. But if you change the liquid in an existing thermometer:
  • You must calibrate the plungers again.
  • You risk to pollute the liquid.

From 17°C to 18°C, water's density changes by 0.15kg/m3. Little pollutant detunes your thermometer. Any operation must be done cleanly, and a different liquid must be pure for reproducibility.

The coloured liquid in the plungers matters little, because gas tops it, so this liquid influences the plungers' volume little. Glass at the plungers expands little with temperature, less than water does, and it's stiff enough to impose a constant volume, so the enclosed liquid gives only a mass.

So you have some freedom to chose the liquid in the plungers, and a fluorescent one is possible, sure. Varied colours and combinations are easy.

Colour changing with the temperature: I've seen such an indicator on a fridge. It needs no other light than what the eye needs to see. I ignore the details.

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