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01-31-2016
06:50 PM

01-31-2016
06:50 PM

Unit Problem

Hey All

Im having to endure the uphill battle of using a new program especially on windows making it even harder given I'm a mac user

Please see the picture below

What went wrong here?

The answer very clearly should be 2.5 C/w

And this thing is freaking out giving me whatever the heck that is.

Im sure I'm just missing something simple

Thanks in advance.

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3 REPLIES 3

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02-01-2016
09:59 AM

02-01-2016
09:59 AM

Re: Unit Problem

Well, it's not very helpful but it's correct - it's displayed the units using the most basic elements (mass, length, time, temperature).

°C = K, provided you're talking about relative values (true for a gradient such as this).

W = J / s

J = N.m (therefore W = N.m/s)

N = kg.m/s^2 (therefore W = kg.m/s^2 . m/s = kg.m^2 / s^3)

Therefore you have K / [kg.m^2 / s^3] which rearranges to K.s^3 / kg.m^2.

As a MathCAD newbie, I have no idea how to make it display the units in a more helpful form, however!

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02-01-2016
10:16 AM

02-01-2016
10:16 AM

Re: Unit Problem

You can require Mathcad to use your units. Select and delete the units applied and type or insert the ones you want. (Try it simply first, type "1 ft =". Mathcad should respond: . Not delete the m and type "in", Mathcad should respond: . Mathcad will do the unit conversions for you. Note that it will also force unit balance-- if your equation doesn't unit balance, you can see some strange results:

Which is a really good check on your equations!

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02-01-2016
10:27 AM

02-01-2016
10:27 AM

Re: Unit Problem

You have learned from the previous answers that Mathcad per default will display results in standard basic units of the chosen unit system and that you can change the units of your results by simply overwriting the displayed units with the units you would like to see. But now for the real tricky part - temperature units:

You thought that 150 °C - 25 °C would yield 125 °C, right?

Give it a try and be surprised by the result -148.15 °C !

Temperatures are somewhat trick unless you solely work in Kelvin. Don't blame Mathcad for that - the reason is, that the conversion from standard unit Kelvin to Celsius is not a simple multiplication as with most other unit conversion but that we have a scale shift here.

Mathcad makes its calculations concerning temperature in Kelvin. 10°C=283.15 K and 0°C = 273.15 K. So 10^C - 0°C = 283.15 K - 273.15 K=10 K = -263.15 °C - ouch! We have to distinguish between temperature and difference of temperatures!

You may have noticed that Prime offers a "unit" Delta_ degree_Celsius, too, and this could be the solution to your problem.

This subject can be quite confusing at first sight, I know.

You may find a lot of threads in this forum covering that subsject and maybe the following blogs can help to clarify

The Trickiness of Temperature Units | Product Lifecycle Report

Temperature Units | Product Lifecycle Report

Haven't compared them word by word, but I guess its the same article, published twice.

You may also use you favorite search engine to find a lot more articles covering that "problem". Even if they cover Mathcad 15 and older versions, the information still applies.

http://duckduckgo.com/?q=Mathcad+temperature+unit

Regards

Werner