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B Weird sound coming from thermos flask

  1. Jan 12, 2018 #1

    Wrichik Basu

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    I had a thermos flask that I had never used since I had bought it. Today, I decided to take it out and use it for storing hot water, so that I don't have to heat water every time I feel thirsty.

    An interesting incident occurred. I heated water to near about boiling temperature, and filled the thermos flask with it. Then I tightened the lid. And then I started to hear this sound from the flask.

    I realised that the sound is coming only if I put in hot water, not cold water. It's a weird sound. It is stopping if the flask is kept undisturbed for some time, but starting again if I move the flask.

    Can anyone account for this sound?

    The room temperature is about 15°C.
  2. jcsd
  3. Jan 12, 2018 #2


    Staff: Mentor

    maybe expanded air is leaking out?
  4. Jan 12, 2018 #3


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    The air inside the flask is either heating up or cooling down, which causes a pressure difference with respect to the outside atmosphere and makes air flow with a whistling sound.
  5. Jan 12, 2018 #4


    Staff: Mentor

    I agree. Air expanding and leaking out.
  6. Jan 12, 2018 #5

    Wrichik Basu

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    Just now I thought about dipping the flask in a bucket of water. You people are right - I could see bubbles in the water in the bucket.

    Thanks for the explanation!
  7. Jan 12, 2018 #6


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    Your noise must be air leaking in or out. If the stopper is well wetted when screwed in, the seating will be better and leaks are less likely.
    When you fill a flask with hot water (and you should always leave some air in the top) the thing to do is to shake it up a bit, with the stopper loosely inserted. This will then warm up the air inside and the excess volume (Charles' Law - lookitup) will vent to the outside. If you don't do this, the air inside will have nowhere to go. Its pressure will increase and the stopper will always be under positive pressure. If the stopper is not a perfect seal and the flask is not upright, liquid or air can be forced out. However, if the air is pre-warmed, the pressure can only drop inside and a small amount of air may be forced in from outside as the internal pressure drops - no problem.
    Also, all round containers are stronger with excess pressure outside than with excess pressure inside.
    I have an Espresso coffee maker (old fashioned with a pressurised boiler and a hand espresso pump). Now the seals are getting a bit past it, there is some escape of steam which is inaudible during operation but suddenly, as the temperature drops, water condenses around the leak(s) and it starts to sing - apparently for no reason. It was very disturbing, half way through breakfast until I sussed it out.
    I am sure that a number of cases of 'hauntings' could be explained by this sort of thing.
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