MathCAD 15 solve block error

[rosa2679]

Hello,

I am working on creating a model to design an wastewater treatment process. I have set up the mass balances and but keep getting function errors dealing with the flow, which I am hoping to be able to change in further iterations to see how the system will handle it. I have checked and the guesses are within a reasonable range. I have reworked this many times and have been unable to get any answers. I am very new to mathCAD so I could be missing something relatively simple, I appreciate any help you all can give me!

[terryhendicott · Accepted solution]

Hi,

Some of the equations in the solve block are not consistent units.  These need to be fixed first.  I try some of the equations prior to the solve block to test they are unit consistent.  You need to check them all.

[rosa2679]

Thank you all for you help! It was a problem with the units as suggested. I was able to fix the units issue by using L/mg wherever the unit less number was used and it started putting out the numbers I needed.

[LucMeekes]

I see you're using Mathcad 15.

Apart from the fact that your equations must be unit-balanced, as Terry has pointed out:

I remember having read that Mathcad 15 cannot work with units in solve blocks. I hope I'm wrong.

Success!

Luc

[terryhendicott]

Hi,

step one is to get the unit consistency in the equations.

step 2 is to get round the inability of the solve block to use units.

There is a workaround so don't be discouraged from solving the unit inconsistency in the equations first.

For instance i think the factor "Xrh" should be mg/L too?

Cheers

[Werner_E]

I added the mg/L at the definition of X.rh as Terry suggested, but there is still a unit inconsistency in your last equation. I "solved" it in a crude way by adding the unit "day" in the equation itself. This sure is not how it should be and was just done to be able to demonstrate how to use the solve block correctly. You will still have to check your equations.

Mathcad 15 is not able to display or store vectors/matrices with values of mixed dimensions. Your solve vector includes w which is dimensionless while all other variables are of dimension mass per volume. So you cannot show the result using the equal sign. But you can assign the result to a vector consisting of 11 single variables and then singly show the values of those: