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Solve Block Problems - Variable Undefined?

jaypee_6680
1-Newbie

Solve Block Problems - Variable Undefined?

Hi there!

Wondering if anyone can help me out. I'm new to mathcad and having difficulty with solve blocks.

As far as I can see I'm entering all data correctly but keep getting an error variable undefined when I come to us the "find" function.

I'll attatch a couple of questions I cant get to work to see if anyone has any ideas.

Any pointers on this would be much appreciated.

Kind Regards,

John

1 ACCEPTED SOLUTION

Accepted Solutions
RichardJ
19-Tanzanite
(To:jaypee_6680)

In the first equation you can't add the two terms in the denominator because kp and kins have different units.

Error messages from Mathcad solve blocks are essentially worthless, because there is only one: "This variable is undefined". You get this message regardless of what the actual error is. To get a real message select one of the equations and hit "=". If the equation has no problems it will evaluate to something (usually 0, since the two sides are not equal), otherwise you will get a more informative message.

View solution in original post

9 REPLIES 9

I think you need to look at your units. You have defined H and HW with units of kg/kg.

The units throughout your worksheet do not seem constant.

Mike

RichardJ
19-Tanzanite
(To:jaypee_6680)

In question 10 you have m raised to the power of 3*s^-1 (in two places). In question 11 your units do not match in equations 1 and 4.

Thank you very much! I couldn't see the wood for the trees! I was convinced it was a mathcad problem rather than an operator problem!

I have one more that I still cant get to work however. I've been looking at it for hours at I'm getting nowhere. I guess it's just practice though.

Any help with that would be much appreciated also. The trace error function thinks there is something wrong with one of my given functions? It should contain a temperature mulitplier apparently!

Kind Regards,

John

RichardJ
19-Tanzanite
(To:jaypee_6680)

In the first equation you can't add the two terms in the denominator because kp and kins have different units.

Error messages from Mathcad solve blocks are essentially worthless, because there is only one: "This variable is undefined". You get this message regardless of what the actual error is. To get a real message select one of the equations and hit "=". If the equation has no problems it will evaluate to something (usually 0, since the two sides are not equal), otherwise you will get a more informative message.

View solution in original post

Thanks very much again! I can't believe I didn't notice. Another operator error! I knew I had to be pretty close.

Thanks for the advice with the error messages, much appreciated.

Kind Regards,

John

Thanks for the advice with the error messages, much appreciated.

Error messages can be an issue in Mathcad, especially with solve blocks, but I feel the error messages are sufficient when dealing with wrongly defined units.

Mike

Sorry to keep bothering you guys with solve block errors. I have a new one which I cannot seem to solve. I've been on this one for days trying not to look like an idiot posting on here again but here goes!

Thanks again,

John

RichardJ
19-Tanzanite
(To:jaypee_6680)

Should the second equation be log(P2/mmHg) rather than log(P1/mmHg)?

With Richard's change (P2 for P1), and dividing by units so the entire Find string has no units) the solve block works!

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