How sensitive is a thermocouple ?


    Rep:
    How sensitive is a thermocouple compared to a resistance thermometer ?

    How accurately can either of these measure temperature to ?

    Until now I've only been told that both are 'very sensitive'.

    Also, the mark scheme would not accept thermocouple as a valid answer for Question 2 (b). Why ?

    http://www.xtremepapers.com/papers/C...2_w06_qp_4.pdf

    Rep:
    (Original post by Stonebridge)
    x
    Bump.

    Rep:
    depends on the metals you use for making the thermocouple, it most probably depends on the relative conductivities of the metals in question. regarding this question, i think by resistance thermometer they are referring to thermistor, which has a very high sensitivity over a small range, so more accurate calibration is possible in that range, whereas a thermocouple is unlikely to have such a high sensitivity.


    [can i ask how you are scoring on average in paper 4 past papers?]

    Rep:
    (Original post by bmqib)
    depends on the metals you use for making the thermocouple, it most probably depends on the relative conductivities of the metals in question. regarding this question, i think by resistance thermometer they are referring to thermistor, which has a very high sensitivity over a small range, so more accurate calibration is possible in that range, whereas a thermocouple is unlikely to have such a high sensitivity.


    [can i ask how you are scoring on average in paper 4 past papers?]
    Hmmm, I still think I'd like an elaboration on these points (bold). Isn't a thermocouple just as sensitive ?

    Pretty darn good. A few marks here and there are lost because of stupid mistakes but no systemic errors. Yourself ?

    Rep:
    Metal thermocouple emfs depend on the two metals used.
    Typical values are between about 1mV to 4mV for a 100 deg C temperature difference.
    How useful this is depends on how you then measure this emf.
    For accurate work a potentiometer would need to be used to measure the emf. (Rather than a millivoltmeter.)

    Quote from physics text book:
    "Thermocouples have very small heat capacities and so have very little effect on the temperature of the object or material being measured."
    So as to why the mark scheme doesn't accept a thermocouple as the answer I have absolutely no idea. (Edit: but see my later post re sensitivity of thermocouples)
    Used with a potentiometer it would be fine, as there is no need here to measure rapid changes in temperature.
    The question obviously wants you to realise that the heat capacity of the bulb thermometer being used is affecting the result. To reduce this effect you need a thermometer with a smaller heat capacity.

    Rep:
    (Original post by Stonebridge)
    So as to why the mark scheme doesn't accept a thermocouple as the answer I have absolutely no idea.
    Used with a potentiometer it would be fine, as there is no need here to measure rapid changes in temperature.
    The question obviously wants you to realise that the heat capacity of the bulb thermometer being used is affecting the result. To reduce this effect you need a thermometer with a smaller heat capacity.
    Yeah, glad that I thought the same. Mark scheme is absurd. . .

    Rep:
    (Original post by Stonebridge)
    Metal thermocouple emfs depend on the two metals used.
    Typical values are between about 1mV to 4mV for a 100 deg C temperature difference.
    How useful this is depends on how you then measure this emf.
    For accurate work a potentiometer would need to be used to measure the emf. (Rather than a millivoltmeter.)

    Quote from physics text book:

    So as to why the mark scheme doesn't accept a thermocouple as the answer I have absolutely no idea.
    Used with a potentiometer it would be fine, as there is no need here to measure rapid changes in temperature.
    The question obviously wants you to realise that the heat capacity of the bulb thermometer being used is affecting the result. To reduce this effect you need a thermometer with a smaller heat capacity.
    I think it's because thermistors are more sensitive over narrow-range temperatures, as well as of course having low heat capacity, the steady temperature measured was correct to one decimal place. I can't seem to find how accurately a thermocouple can measure to, my book just says it can be sensitive if appropriate metals are chose, is there a general trend in how accurate it can measure?

    Rep:
    Yes, it looks like it's also a matter of sensitivity.
    On checking this out it looks like thermocouples are at best able to measure to +- 0.5 deg C. in this range.
    I thought better was possible.
    If the temperature in the question needs to be measured to 0.1 then that would rule out using one.
    It only leaves a thermistor as an option.

    Rep:
    (Original post by Stonebridge)
    Yes, it looks like it's also a matter of sensitivity.
    On checking this out it looks like thermocouples are at best able to measure to +- 0.5 deg C. in this range.
    I thought better was possible.
    If the temperature in the question needs to be measured to 0.1 then that would rule out using one.
    It only leaves a thermistor as an option.
    Wow ! Only ±0.5 ? I've always been lead to believe thermocouples were high precision, high accuracy instruments.... That's no better then a mercury in glass thermometer !

    Yes, in this question there was a temp change of around 0.4 or 1.4 degrees.... I can't remember which one. Definitely rules out a thermocouple.

    Thanks !!

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    Info here
    http://en.wikipedia.org/wiki/Thermoc...ple_comparison

    Rep:
    (Original post by Ari Ben Canaan)
    Hmmm, I still think I'd like an elaboration on these points (bold). Isn't a thermocouple just as sensitive ?

    Pretty darn good. A few marks here and there are lost because of stupid mistakes but no systemic errors. Yourself ?
    I'm averaging around 85-90+ in mocks, which is great considering you need like 60-65 to get an A in Paper 4 usually.

    Just verifying, that is true right, about grade threshold? Worst case scenario

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