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ESHTraceHeating1-Visitor
1-Visitor
November 1, 2017
Question

Watts per meter per Degree C

  • November 1, 2017
  • 3 replies
  • 5076 views

Forgive my ignorance I'm very new to this system and currently trialling it.

 

We are looking at some calculations from International Standards and struggling with the MathCad recognition of the units.

 

When I set the unit value up as Watts/m*°C its constantly highlights the °C in red and tells me it should be a scalar or in a matrix, which I'm still trying to understand the link to this?

 

I can get this box to go away if I replace it with the symbol ∆°C, which then allows the calculation to function.

 

The only problem then is the result from the equation should be in W/m*°C but the calculation keeps forcing a K, )which I'm guessing is Kelvin) to the front to the units which then causes problems with another calculation as the units become undefined even though they are a result of an equation?

 

We end up with K*W/m*°C, and the answer should simply be W/m*°C?

 

Not the worlds best mathematician but struggling to understand why the programme forces a change to a recognised International unit of measurement?

 

Any help in understanding this would be appreciated, I'm hoping I'm missing something very obvious.

 

 

3 replies

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LucMeekes23-Emerald IV
23-Emerald IV
November 1, 2017
Are you using Mathcad or Prime?
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ESHTraceHeating1-VisitorAuthor
1-Visitor
November 1, 2017

Very new to this so not 100% sure what I'm looking at.

 

I downloaded the trial of Mathcad with the full functionality version for 30 days option.

 

On the top of the page it says PTC Mathcad Express Prime 4.0

 

In the options it is Release Prime 4.0 64-bit, Datecode M010 (2017.06.01.001) Locale en-US.

 

If that identifies it.

 

L
LucMeekes23-Emerald IV
23-Emerald IV
November 1, 2017

You're using Prime then. Note you are also entitled to use Mathcad 15 for the same time period, except that after the trial that application reduces to 0 functionality, whereas with Prime you can use it in a stripped form (No programming, no symbolics and no advanced mathematical functions to name a few) indefinitely.

 

Anyway. The SI preferred unit for temperature difference is the Kelvin, K.

Mathcad (and Prime also) is unit aware. Most units have the advantage that they can be converted to other units by multiplication with a constant. m and in are both units of length, and 1 in= 0.0254 m.

Withe temperature units it's not so easy. 1 °C cannot be converted to 1 K by multiplication, except when used for a temperature difference, and that's where you use DeltaC.

So while other units can be easiliy converted, to convert a certain temperature from °C to K the unit conversion is defined as a function in Prime.

 

But I guess in your case you're better off using the K so expressing the value in W / m K.

 

Hope this helps. It may help if you post your worksheet (you may have to zip it first; due to a bug in this forum you might not be able to attach the file unzipped).

 

Success!
Luc

F
Fred_Kohlhepp23-Emerald I
23-Emerald I
November 1, 2017

Mathcad is 'unit aware" as you've seen--it will balance units in an expression.

 

It works in a default set of units, usually MKS, although you may set the default to others.

 

With temperature scales things get complicated, mostly because of attempts to respond to complaints.  These complaints tended to focus in two areas:

  1.   The original units were the absolute scales, Kelvin and Rankine.  People wanted Centigrade and Fahrenheit, so those were put in (although conversions are still challenging.)  So when you type "C" Mathcad assumes you want centigrade scale, and zero is freezing water.
  2. The second problem is where you are, when temperature is really a difference between two.  watts per meter per degree is really watts per meter per degree difference.  Which is why the switch to DC worked.

 

Just one of the areas where wants and needs caused additional confusion.

 

The basic punchline is that you can have whatever units you want.  If Mathcad puts the answer up in meters and you wanted inches, you go to the "meters" and edit it to inches--Mathcad will alter the numeric value to match.  If you change the units to what you want and Mathcad adds one or more units then your equation is not "unit balanced," and Mathcad has added units to balance it.

V
24-Ruby IV
ValeryOchkov24-Ruby IV
24-Ruby IV
November 1, 2017

I have one gift from NIST - one paper reference book with properties of water and steam.

I have asked people in NIST why in the book the unit of entropy is Btu / lb / °R but the unit of thermal conductivity is Btu / hr /ft / °F (see please pics)

The answer was so:

It is much easier to change the US Constitution than to force American heat engineers to use Rankin's degree, rather than the Fahrenheit's degree in the heat conductivity unit!
The correct unit of thermal conductivity is (SI) W*m/m^2/K but we use the simplified form W/m/K
R.pngF.png
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