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7-Bedrock

## Units are not returning correct answer

I've made a MathCad template and the answer is correct when I don't include units, but when I include units the answer is incorrect. Also the units that get returned are not what they are supposed to be.

Thanks!

ACCEPTED SOLUTION

Accepted Solutions
24-Ruby IV
(To:Johnruehlz)

@Johnruehlz wrote:

Can you explain how you go the answer to change the units? Sorry VERY new to this.

11 REPLIES 11
23-Emerald I
(To:Johnruehlz)

Without seeing what you're doing, we can only guess.

If you have an emperical equation with a "constant" in it (old engineering Handbooks were great at this) there's a chance that that constant is really a unit conversion factor.  Since Mathcad is "unit aware," it will translate units for you, so to calculate power (in watts, kilowatts, or horsepower, or any other unit of your choice) you multiply torque times angular velocity, and Mathcad takes care of the unit conversion for you.  (No P = T w/5252 because T is ft lbf and w is rpm.)

The other possibilities include:

• The units are right, but they look wierd.  Try replacing what Mathcad defaults to with the unit you expected.  If it's equivalent all the strange notation will disappear.  If it's not equivalent, Mathcad will add units to make the units balance.  If you expected horsepower and type "hp" and get "hp sec," then you are computing work instead of power.
• You have a unit declaration wrong.  Did you want force, but typed "lb" instead of "lbf"?
• Your Handbook equation cannot unit balance-- it's an empirically derived expression with no physical reality.

If you zip your file, you can attach it.  That gives the rest of the world a chance. . .

7-Bedrock
(To:Fred_Kohlhepp)

I tried to include before but, couldn't figure how. Here it is

23-Emerald I
(To:Johnruehlz)

I think your units are okay, but you're calculating energy/area.  Why a factor of 3?

Attached

7-Bedrock
(To:Fred_Kohlhepp)

Factor of 3 is because the area of arc is not uniform since it is in an enclosure.

24-Ruby IV
(To:Johnruehlz)

@Johnruehlz wrote:

I tried to include before but, couldn't figure how. Here it is

Sorry! May be I do not understand some one!

7-Bedrock
(To:ValeryOchkov)

The units should be cal/cm^2. I don't understand either why it is giving the wrong answer with units and the right answer without.

The correct answer should be 21.096 cal/cm^2

23-Emerald I
(To:Johnruehlz)

And that's the answer you get!  See amended attached!

7-Bedrock
(To:Fred_Kohlhepp)

Can you explain how you go the answer to change the units? Sorry VERY new to this.

24-Ruby IV
(To:Johnruehlz)

@Johnruehlz wrote:

Can you explain how you go the answer to change the units? Sorry VERY new to this.

23-Emerald I
(To:ValeryOchkov)

Thanks Val!!

7-Bedrock
(To:ValeryOchkov)

Huh. I must of been doing something wrong I tryed that a million times but, this time it worked. Thanks both of you for the help!

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