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12-20-2011
12:11 PM

12-20-2011
12:11 PM

Specific Units in the equations

Good morning,

Does anyone know if I can specify the units on equations?

For example, when I want use some empirical correlations; the expression uses a formula that only works with the correct set of units. In MathCad, when I use the formula, the program uses the units of the Base System, but if only for that expression I want to use, for example, °F instead of K, it is possible?

Regards,

Oscar J.

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15 REPLIES 15

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12-20-2011
12:46 PM

12-20-2011
12:46 PM

Re: Specific Units in the equations

Show please your empirical correlations!

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12-20-2011
12:56 PM

12-20-2011
12:56 PM

Re: Specific Units in the equations

Oscar Delgado wrote:

Good morning,

Does anyone know if I can specify the units on equations?

For example, when I want use some empirical correlations; the expression uses a formula that only works with the correct set of units. In MathCad, when I use the formula, the program uses the units of the Base System, but if only for that expression I want to use, for example, °F instead of K, it is possible?

Regards,

Oscar J.

Check out the example attached, just a starting point.

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12-20-2011
01:11 PM

12-20-2011
01:11 PM

Re: Specific Units in the equations

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12-20-2011
02:37 PM

12-20-2011
02:37 PM

Re: Specific Units in the equations

Good morning,

I'm working in this formula:

Flash=0.69*Temp-181

Flash,Temp: °F

When I use the formula in MathCad I have first to add D°F to seconf term to balance the units but the answer its not the same when you use SI (actual base system in my model) :

Temp=547°F

Flash=259°F (if the Temp in the equation is in °F)

Flash=116°F (MathCad result)

Usually when I have this kind of problems I multiply the constant by some factor in order to adjust the expression to the base system or I use adimensional values and then I multyply, but I can't do this when I'm dealing with temperatures (not tempratures changes).

Regards,

Oscar J.

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12-20-2011
03:14 PM

12-20-2011
03:14 PM

Re: Specific Units in the equations

Oscar Delgado wrote:

Good morning,

I'm working in this formula:

Flash=0.69*Temp-181

Flash,Temp: °F

When I use the formula in MathCad I have first to add D°F to seconf term to balance the units but the answer its not the same when you use SI (actual base system in my model) :

Temp=547°F

Flash=259°F (if the Temp in the equation is in °F)

Flash=116°F (MathCad result)

Usually when I have this kind of problems I multiply the constant by some factor in order to adjust the expression to the base system or I use adimensional values and then I multyply, but I can't do this when I'm dealing with temperatures (not tempratures changes).

Regards,

Oscar J.

Please post a worksheet. I can't replicate your results.

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12-20-2011
03:52 PM

12-20-2011
03:52 PM

Re: Specific Units in the equations

Hello,

Excuse me,a typo:

Flash=0.69*Temp-118.

But in the file attached you can see the problem. When I use the units, the result is different. So I can't use it to make other calculations (or I should use no units in the other expressions)

Regards,

Oscar J.

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12-20-2011
05:50 PM

12-20-2011
05:50 PM

Re: Specific Units in the equations

Have a look at the attached.

Mike

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12-21-2011
08:15 AM

12-21-2011
08:15 AM

Re: Specific Units in the equations

Hello,

The file that you hve attached didn't work with temperature (°F) but Farad (F). The problem in the expression is with the temperature because you can't make the conversion only multiplying by some factor.

Regards,

Oscar J.

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12-22-2011
03:39 AM

12-22-2011
03:39 AM

Re: Specific Units in the equations

To me it looks like the equation is empirical, so I would tend to work through it unit less.

Mike

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12-22-2011
04:25 AM

12-22-2011
04:25 AM

Re: Specific Units in the equations

Each empirical and physical formula must have units:

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12-22-2011
07:32 AM

12-22-2011
07:32 AM

Re: Specific Units in the equations

I have found that many empirical equations are unit dependant and the best way to input them in Mathcad is unitless, well most of the time.

I have worked around the issue, but more often than not, I just input them unitless.

Mike

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12-20-2011
06:13 PM

12-20-2011
06:13 PM

Re: Specific Units in the equations

The Fahrenheit scale gets converted by Mathcad into an absolute temperature, or converts an absolute temperature back to Fahrenheit. With care it works.

See attached.

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12-20-2011
05:51 PM

12-20-2011
05:51 PM

Re: Specific Units in the equations

As Valery indicates you can divide the variable by the correct unit. I have many issues when dealing with the American standards when doing Above Ground Storage tank design.

Mike

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12-21-2011
08:34 AM

12-21-2011
08:34 AM

Re: Specific Units in the equations

Thank you all for the answers.

I get an expression that gives the right result. But it could be interesting to have the posibility to choose the units for the evaluation of one specific equation.

What do you think?

Regards,

Oscar J.

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12-21-2011
10:10 AM

12-21-2011
10:10 AM

Re: Specific Units in the equations

You can either work with units and understand them, or you can trick them.