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Homework Help: Help understanding error progression.

  1. Jul 26, 2011 #1
    1. The problem statement, all variables and given/known data
    We have made a standard tank in our lab. I need to calculate the uncertainty in the concentrations of the compounds we added to the tank.

    2. Relevant equations
    My professor indicated that I would need to use error progression to calculate an accurate uncertainty for the tank. I need some help understanding how this is done. Searching online he may have meant uncertainty propagation.

    For the tank we calculated the final concentrations this way.

    1) volumetric injections of compounds.
    2) pressurized tank with nitrogen gas.
    3) calculated volume of tank by filling with nitrogen to a measured pressure and calculating the volume using ideal gas law
    4) weighed tank before and after addition of compounds and gas, so we know the weight of the nitrogen.
    5) calculate concentration by converting to mols and dividing mols of compound by mols of nitrogen in the tank.

    So the measurements we have are

    injection volumes ex: 180 uL +- 2.5
    pressure reading 1.01 bar +-0.01
    Weight of N2 7.0 g +- 0.1

    3. The attempt at a solution
    I understand what uncertainty is, I have calculated the uncertainty in the final concentrations, but only using the uncertainty in the volumes of compound we added to the tank like +-2.5 microliters.

    Is there a function in excel to calculate the uncertainty using error progression? That is what I used to calculate the tank concentrations.
    Last edited: Jul 26, 2011
  2. jcsd
  3. Jul 27, 2011 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    I'm not following in detail how you calculated everything, but it seems it involves the basic arithmetic operations (addition, subtraction, multiplication, division). The rules usually used are:

    For addition and subtraction, add the errors of the quantities. (This would apply to subtracting the before-and-after tank weights, for example, to get the nitrogen weight.)

    For multiplication and division, add the percentage (or fractional) errors of the quantities.
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