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bbomst�rk1-Visitor
1-Visitor
March 1, 2016
Solved

Samme numbers, different results???

  • March 1, 2016
  • 5 replies
  • 4860 views

Can anyone please tell explain to me why these two calculations give two different results???

Screen Shot 2016-03-01 at 18.27.13.png

Best answer by MJG

Search the forum for "temperature" and you'll get a lot of info.

Mathcad's base unit for temperature is Kelvin.  Since 0°K does not equal 0°C, you end up with issues that need addressed.

If you clarify the meaning behind each of the temperatures in your equation, we'd be happy to help you sort it out.  If you replace all the units with Δ°C, then you get the result you're expecting.  However, that might not be the best way to represent whatever you're trying to calculate.

5 replies

V
24-Ruby IV
ValeryOchkov24-Ruby IV
24-Ruby IV
March 1, 2016

300C=(300+273.15)K etc

B
bbomst�rk1-VisitorAuthor
1-Visitor
March 1, 2016

Could you elaborate on that? Can't see what Kelvin has to do with this!?

R
RichardJ19-Tanzanite
19-Tanzanite
March 1, 2016

Kelvin has everything to do with it. Most units convert with just a scaling factor, but temperature units do not. There's also an additive component. If you write 300C+1C what you usually mean is increase the temperature of 300C by 1C. That is, 300C+1deltaC, where 1deltaC=1K, as opposed to 1C=274.15K. There is no other way to handle temperature units in unit aware software, because the software cannot know what you mean by 300C+1C. Add 1K, or add 274.15K? The user has to distinguish between the two possibilities, because only the user knows what they want.

M
MJG1-VisitorAnswer
1-Visitor
March 1, 2016

Search the forum for "temperature" and you'll get a lot of info.

Mathcad's base unit for temperature is Kelvin.  Since 0°K does not equal 0°C, you end up with issues that need addressed.

If you clarify the meaning behind each of the temperatures in your equation, we'd be happy to help you sort it out.  If you replace all the units with Δ°C, then you get the result you're expecting.  However, that might not be the best way to represent whatever you're trying to calculate.

B
bbomst�rk1-VisitorAuthor
1-Visitor
March 1, 2016

It's a part of this formula where the fraction above is the one in the ln(...). So that specific fraction should just come out as a number without any unit. How can I do that without deleting the temperature units

Screen Shot 2016-03-01 at 19.02.27.png

B
bbomst�rk1-VisitorAuthor
1-Visitor
March 1, 2016

You're right,  Δ°C does work and though it's not the best solution it might be the only one

V
24-Ruby IV
ValeryOchkov24-Ruby IV
24-Ruby IV
March 1, 2016

More interesting

V
24-Ruby IV
ValeryOchkov24-Ruby IV
24-Ruby IV
March 1, 2016

Mathcad 15

R
RichardJ19-Tanzanite
19-Tanzanite
March 1, 2016

You can't add temperatures in C (or F) like that.

300C=573.15K and 234.5C=507.65K. Therefore 300C+234.5C=1080.8K=807.65C.

When you add them mentally and get the result 534.5C what you are actually doing is removing the units to get just the numeric values, adding the numeric values, and then putting the units back on. Look up temperature units in the help.

V
24-Ruby IV
ValeryOchkov24-Ruby IV
24-Ruby IV
March 1, 2016

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