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## Problem with a given-find solve block for anaerobic digestion modelling in Mathcad Newbie

## Problem with a given-find solve block for anaerobic digestion modelling in Mathcad

Hello, dear community!

I am trying to find the problem with the resolution of these equations. They correspond to mass balances in CSTRs and a charge balance to determine pH.

Best Regards,

Baltasar

3 REPLIES 3 Ruby III
(in response to aa-8)

## Re: Problem with a given-find solve block for anaerobic digestion modelling in Mathcad

Unfortunately you can't use vector / vector elements as variables to search for that way.

Furthermore a guess value for pH is missing.

You may try something like Find(C1, pH) but then I get the message, that Ka.N is not defined. Not sure why "Trace"  sends me to this equation at the very top which is way outside the solve block. Newbie
(in response to aa-8)

## Re: Problem with a given-find solve block for anaerobic digestion modelling in Mathcad

When I get an error inside a solve block, I copy all of the equations in the solve block and paste them outside of the block and then evaluate them (press equal sign so MathCAD evaluates the equation).  This forces MathCAD to specifically identify the problem with that equation.  For example, in your worksheet you also don't define Q.1 properly because you have a typo in Q1:=100.  You also have a typo of "Q..1" in equation Q.in * C0[4.  And it doesn't look like any of your Ka values have been defined.

So just go through the errors that MathCAD identifies in each one of those equations outside of the solve block and make the corrections inside the solve block.  Once you have all of those worked out we can work on getting the solve block to work properly. Topaz I
(in response to aa-8)

## Re: Problem with a given-find solve block for anaerobic digestion modelling in Mathcad

The following variables (constants?) Are not defined nor initialized: 