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Interesting problem ... basic engineering solution

gkenyon
So, today's engineering problem goes like this.


Someone in our family is dying their hair. The chemical (probably a Chromate-based hair dye) must be mixed with 180 ml of water at between 40 and 50 °C - if the temperature is too low, mixing is very difficult, too high and the compound breaks down, and perhaps you might not get the hair colour you need.


Simples ... except we have no thermometer.


How was this quick and easy to resolve?


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    I'm not sure that would work with the volumes you stated, as I'm pretty sure the ice is not at 0 °C when it melts. So I think I'd be going for about 45 °C.


    The real answer is a little easier, but still uses the rule for mixing volumes of liquid at different temperatures.


    And the first step is the Engineering bit. Assume tap water is 10 °C +/- 5 °C (not a bad guess, given here it's been 10 °C or so for a few days and nights now).


    Aim for 45 °C by ratio of volume (just in case our assumption is a couple of degrees out either way - another bit of Engineering):
    • 110 ml of cold tap water, and

    • 70 ml of water straight out of the kettle, that we know is 100 °C to about a degree or so

Reply
  • 0
    I'm not sure that would work with the volumes you stated, as I'm pretty sure the ice is not at 0 °C when it melts. So I think I'd be going for about 45 °C.


    The real answer is a little easier, but still uses the rule for mixing volumes of liquid at different temperatures.


    And the first step is the Engineering bit. Assume tap water is 10 °C +/- 5 °C (not a bad guess, given here it's been 10 °C or so for a few days and nights now).


    Aim for 45 °C by ratio of volume (just in case our assumption is a couple of degrees out either way - another bit of Engineering):
    • 110 ml of cold tap water, and

    • 70 ml of water straight out of the kettle, that we know is 100 °C to about a degree or so

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