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I just started my class on fluid mechanics. There's a problem that requires absolute viscosity $u$ of water at 25C. I looked up my table in the book and I only have it for water at 20C and 30C. I figured we had to manually calculate it in this case.

The school notes only have Andrade's equation $u = D \times e^{\ B/T}$

So I've got the absolute temperature which is 25C. There is no information of what D and B are(Defined as constants).

So how do I figure out the absolute viscosity at any temperature? I've looked up a table online with the value for water at 25C and it gives me the right answer, is it okay to interpolate the values of my table of 20C and 30C to find the value for 25C?

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    $\begingroup$ What about finding D and B constants from several data points given in the table in your book? $\log u = \log D + B \times \frac{1}{T}$ $\endgroup$ – Deer Hunter May 13 '13 at 12:40
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One good resource for these kinds of questions which involves looking up some kind of quantity is wolfram alpha. As you can see you can get the answer for any temperature. As far as interpolating the values, it is fine if your interpolation error is small. That depends on the nature of the data you are trying to interpolate (is it linear/nonlinear) and the distance between the data points. In the case you mention there is a lot of room between 20C and 30C so I am thinking an interpolation would incur a large error (although it is hard to say for sure without looking).

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step 1: Linearize the given model

step 2: substitute viscosity value at a known temperature $T_1$

step 3: substitute viscosity value at another temperature $T_2$

step 4: solve for B and D by any correct method.

step 5: substitute the values of B and D in the given model and use it to calculate viscosity at any temperature between $T_1$ and $T_2$

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  • $\begingroup$ Please observe the preview of the rendered output for the typed markdown before posting your answer. The newline formatting didn't show up. My edit added another newline character to the markdown for each desired new line. $\endgroup$ – user191954 Aug 12 '18 at 9:47

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