Technical issue of Mathcad in calculation with temperature?

[PCES]

Hi everybody,

I am using Mathcad prime 6.0. I have tried to make some calculus of temperature, but it seems that Mathcad has a problem with calculation of temperature. First I tried: 1F+1F=461.67F (it is not 2F!!!), then 2F/1F=1 (it is not 2). 

I don't really know if that is a technical bugs of Mathcad Prime? or it is just a bad copy of Prime in my laptop, or Did I miss something?

[Werner_E · Accepted solution]

Turn your temperatures into Kelvin, then add or divide them and then represent the results again in °F. You will see that Mathcad is perfectly correct in what it does.

The conversion between the various temperature units are not simple multipications by a constant factor like we have in meter and feet, etc.

Then, if you think about it, it may not make much sense to add two temperature values. Rather you intend to add a certain temperature in °F to a given tmeperature. Thats what the unit Delta°F is for:

But admitedly dealing with temperature can be quite a beast - but there is no bug in Mathcad! 🙂

[PCES]

Thanks Werner,

[Hakan1]

Hello 🙂

How do I multiply or divide a temperature? Can someone help me, please?

[LucMeekes]

Today the outside temperature is 0 °C, tomorrow it will be twice as cold. What is the expected temperature tomorrow?

What do you want to double, or cut in two?

You need to understand that Prime's unit °C is 'sort of an absolute temperature': internally it is converted to K (Kelvin). The unit Δ°C on the other hand is a relative temperature, or temperature difference. The value of 1 Δ°C is exactly the same as 1 K. (ISO standards regarding usage of units prescribe that only (!) K should be used for temperature differences.)

When you want to double a temperature in °C, you probably want to double the part of temperature that lies above 0 °C. Hence you would write 0 °C + 2 * 15 Δ°C when you want to double from 15 to 30 °C.

And of course, the better (complying with standards) usage is: 0 °C + 2 * 15 K.

All this still doesn't answer the question at the top, but hopefully it helps you.

Success!
Luc

[Hakan1]

Thank you so much. I have figured it out now. It's working without any problem now. ☺️

[Werner_E]

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@Hakan1 wrote:

Hello 🙂

How do I multiply or divide a temperature? Can someone help me, please?

---

As Luc already indicated, it does not make much sense to ask for the multiple of a temperature unless you exactly state what that should actually be.

Are 60° C three times as hot as 20 °C ?

Are 879.45 K three times as hot as 293.15 K ?

Note the 20 °C = 293.15 K, but 879.45 K = 606.3 °C !

As Luc I suspect that you intend to multiply or divide the DIFFERENCE of a temperature to 0°C. And if thats tru you have to calculate exactly that: Calculate the difference from your temp and 0°C, multiply or divide it and then add 0 °C.

Here is an example which, by using the above assumed intention, multiplies a temperature by 3 and divides it by 5:

Or using your example:

Note that the "unit" °C actually is a function which adds 273.15 to its (dimensionless numeric) argument and then applies the unit Kelvin, while the "unit" Delta°C is simply equivalent (an alias) to the unit K(elvin).

[Hakan1]

Thank you so much. I have figured it out. It's working without any complain ☺️