1-Visitor

Question

Specific Units in the equations

Forum|Forum|14 years ago
December 20, 2011
15 replies
6251 views

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.

Physics

V

ValeryOchkov

24-Ruby IV

Show please your empirical correlations!

F

Fred_Kohlhepp

23-Emerald I

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.

1_units-example-xmcd.zip

24-Ruby IV

Author

1-Visitor

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.

F

Fred_Kohlhepp

23-Emerald I

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.

1_temperatures-xmcd.zip

O

OscarJ.

Author

1-Visitor

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.

1_UnitsEq-xmcd.zip

M

MikeArmstrong

1-Visitor

Have a look at the attached.

Mike

1_UnitsEq_MAA_01-xmcd.zip

O

OscarJ.

Author

1-Visitor

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.

M

MikeArmstrong

1-Visitor

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

O

OscarJ.

Author

1-Visitor

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.

1_UnitsEq-xmcd.zip