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11-01-2017
04:59 AM

11-01-2017
04:59 AM

Watts per meter per Degree C

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.

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7 REPLIES 7

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11-01-2017
05:08 AM

11-01-2017
05:08 AM

Re: Watts per meter per Degree C

Are you using Mathcad or Prime?

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11-01-2017
05:14 AM

11-01-2017
05:14 AM

Re: Watts per meter per Degree C

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.

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11-01-2017
05:32 AM

11-01-2017
05:32 AM

Re: Watts per meter per Degree C

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

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11-01-2017
05:41 AM

11-01-2017
05:41 AM

Re: Watts per meter per Degree C

Luc,

Appreciate the response, I understand the SI prefered value is in K but when the calculation in the standard is

stating °C and information for the values is provided by manufacturers in °C, its very confusing when trying to apply them to the calculations.

My thought was to try W/m*K, not that I'm sure someone writing an international standard would make such an obvious mistake :D.

The only option for a download was Express Prime, the other Math 15 is not mentioned I'll have a look for that.

Appreciate the response, I can now stop baning my head against the table.

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11-01-2017
06:06 AM

11-01-2017
06:06 AM

Re: Watts per meter per Degree C

Which 'International standard' are you referring to?

If you search on this forum for temperature units you may find many discussions on how people have struggled to work with temperature units and Mathcad/Prime.

Another note:

Mathcad (and Prime) will always present evaluation results in the chosen unit system (I guess that's SI for you).

You can override these results by attaching your units at the result.

If you add 1 m + 1 km, the results will be presented as 1001 m, if you attach km to the result, it will change to 1.001 km. This works simply for most units. As said, with temperatures you have to be aware whether you're dealing with temperature, or temperature difference.

Success!

Luc

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11-01-2017
07:23 AM

11-01-2017
07:23 AM

Re: Watts per meter per Degree C

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:

- 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.
- 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.

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11-01-2017
08:39 AM

11-01-2017
08:39 AM

Re: Watts per meter per Degree C

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