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  1. #1
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    Mar 2018
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    Question ASHRAE Fundamentals - error between tables and calcs?

    Hi all, first post and I have tried to see if anyone else has asked this before but not found it.

    I知 trying to use ASHRAE Fundamentals formulae to calculate some of the moist air data in the Fundamentals tables (2 & 3) but I知 finding an error in calculating Ws (of around 0.4%. I知 using equation 23 [Ws = 0.621945 pws / (p pws)] and calculating pws to within 0.02% of the table values, as well as using p = 101325 Pa (standard atmospheric pressure at sea level, used in the tables).

    Example, using pws from the Fundamentals Table 3 for 0C = 611.15 Pa and you get
    Code:
    Ws = 0.621945 x 611.15 / (101325 - 611.15)  = 0.0037741
    Except that in Table 2 Ws for 0C = 0.0037900.

    The difference may seem tiny in absolute terms but it's around 0.4%, which I consider significant, given that I'm using their equations and input data and they're using so many significant figures. Does it matter for my application? I think so - I'm designing a sensor suite and if I allow 0.4% in the system error budget here it's going to render the accuracy of the remaining components irrelevant. I could transpose Tables 2 and 3 into a look-up table and interpolate between the data points but at this point, I'm asking which is closest to right? The table or the formula?

    You'll have guessed by the SI units that I'm not in the US (UK) but I'm using the SI version of Fundamentals, so there shouldn't be a drama. If you have any wisdom about how I could track the source of this error down, I壇 be very grateful to hear it.

  2. #2
    Join Date
    Feb 2004
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    I have seen a correction sheet included with the book. Problem is the ones I've seen are loose and could be lost. Don't know if this relates. In books like this errors can occur.
    I should have played the g'tar on the MTV. MK

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  4. #3
    Join Date
    Mar 2018
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    Thread Starter
    That's something worth considering - thanks for the suggestion. I'm using the 2009 PDF version if anyone has a newer one to compare e.g. the Ws at 0C to (although it's perhaps wishful thinking that an SI one will be readily available to many of you - here's hoping for some international assistance!).

    Nevertheless, the standard atmospheric pressure is easy to check against other sources and so is the coefficient (which does vary across editions but only in a way which has negligible effect on the error, say 0.42% to 0.44%). I have tried several temperature entries and the error is consistent, so it would have to affect the whole column of Ws calculations.

  5. #4
    Join Date
    Apr 2017
    Location
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    when u buy the book suite there is a series of correction sheets... but im pretty sure these corrections are just detailing when and where corrections were made, not what figures were changed to.

    IDK for sure, but maby you could find the correction sheets for the most recent edition and see if there were any changed to that figure without buying the newest editions?

    this sort of thig is very common btw. in the steel design manual there are always errors... some of them go for a decade or more according to my old structures prof.

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