cancel
Showing results for
Search instead for
Did you mean:
cancel
Showing results for
Search instead for
Did you mean:

The PTC Community email address has changed to community-mailer@ptc.com. Learn more.

8-Gravel

## Re: Units reverting into base units

As per the original post

The more I tend to know how to use temperature units more complex it gets for me. Any Suggestions for below behavior

6 REPLIES 6
24-Ruby V
(To:TR_9781963)

I am not sure what the puzzle in your pic should be ??

You don't explain what your first expression should have to do with the question and you don't show the values of Q, but they seem to be approximately 12 and 14 kW.

I guess that for temperature per power DeltaF° would be the correct unit to use which actually simply is 5/9 K.

8-Gravel
(To:Werner_E)

Q is same as above picture 2707 and 330 but with unit.

24-Ruby V
(To:TR_9781963)

Looks like you mixed up your variable Qn with Q

I don't understand why you are expecting the same numeric values as with the unitless calculation.

When you use DeltaF° and W as input units you can't expect that the result in K/W being the same numbers.

8-Gravel
(To:Werner_E)

Is there any way I can use defF and see correct result as K/W?

24-Ruby V
(To:TR_9781963)

Sure you can the results also display in K/W but the values of course must differ because 1 Delta°F is not the same as 1 K.

Is the formula you use an empiric formula from former times meant to be used without units? If thats the case you have to use it without units and add the desired unit K later.

8-Gravel
(To:TR_9781963)

OK, first of all here's how temperature units work in MathCAD Prime:

If you are dealing with absolute temperature, you're using degrees C, degrees F, or K. They each have a different origin, unlike every other unit.

If you are dealing with a change in temperature, you're using Delta-degrees-C, Delta-degrees F, or K. The advantage of using Kelvin is that zero Kelvin is absolute zero, the temperature at which matter stops radiating. Celsius is better than Fahrenheit for a lot of reasons, but F is still used for things like weather in the USA, or hot tubs in Canada. At least 1 delta-degree-C is the same as 1 K.

So, depending on the context you should use a slightly different unit, unless you're working in Kelvins. If you're using Kelvins, a temperature difference of 0 K can be interpreted the same as an absolute temperature of 0 K. This is different than F or C, in which case an absolute temperature of 0 F is 255.372 K and 0 C is 273.15 K, so you must use separate units for temperature difference versus absolute temperature.

You can also define your own units, which may make it a bit faster for you instead of typing the degrees symbol. See my example below:

If this helps, hit the kudo button below.

Announcements