Mathcad error-This value has units:unitless, but must have unit: leght

[danodco]

Hi,

Attached is an equation in mathcad which seems to show an error. It is an equation from the DNV-RP-B401 code "Cathodic protection Design" - table 10-7 - Recommended Anode Resistance Formulae. The error is "This value has units: unitless, but must have unit: lenght." I have checked all the constants and they are all defined earlier with their respective units. Could someone help?

Thanks in advance.

 

[IRstuff]

Don't you think it would be more useful to other people to have the actual sheet instead of a picture of the sheet? There are lots of possibilities, and without the actual sheet it's rather pointless.

TTFN
faq731-376

 

[jghrist]

If r has units length, then ALI has to have units length also so that the units cancel before taking a logarithm. Trouble with this is, if ALI has units length, then you can't add it to the now unitless (r/2ALI)². I suspect that the equation is an empirical one that requires everything to have the particular units that were used in its development. If so, the solution would be to either not use units and assign the variables with the proper values or to divide everything by the proper units in the equation.

 

[PeterSleet]

I think you've mis-typed the equation, the ALI before the error should be 1 (unitless) ALI and r should have units of length. You also appear to have added an extra variable at the start of the equation that isn't in my copy of the DNV doc.

 

[zdas04]

In general terms, if ALI has units (which it appears to), then if "r" has units of other than ft^1.5, you can't add the two terms together. MathCad will not let you add ft to 1/ft because it doesn't make sense.

For empirical equations (which this seems to be) I always do something like:

(ALI/ft+(r/ft / ALI/ft)^2)

and then it is just numbers added to numbers, it also lets me put the values in using any units that I want (MathCad has no trouble dividing mm by ft and canceling units).

David