Convert Units for Thermodynamics Calculations?

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Discussion Overview

The discussion revolves around the necessity and implications of converting units in thermodynamics calculations, particularly when dealing with non-SI units such as Fahrenheit, BTU, and others. Participants explore the requirements for using the ideal gas equation and the importance of absolute temperature in calculations.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether it is necessary to convert all units to SI, suggesting that converting only temperature to Kelvin might suffice.
  • Another participant emphasizes that temperature conversion is more complex due to the offset in temperature scales, recommending the use of Kelvin for safety in calculations.
  • Some participants argue that using Rankine, the absolute scale for Fahrenheit, is a valid alternative for temperature in the ideal gas equation.
  • There is a suggestion that functioning in both SI and US Customary units can be practical, as unnecessary conversions may lead to complications.
  • One participant notes that absolute temperature is required, but clarifies that "absolute temperature" and "Kelvin" are not synonymous, indicating that Rankine is also acceptable.
  • Concerns are raised about the limitations of using Celsius in certain equations, reinforcing the idea that SI units may be safer for calculations.

Areas of Agreement / Disagreement

Participants express differing views on whether all units must be converted to SI for thermodynamic calculations. While some advocate for using absolute temperature in any form, others highlight the practicality of using mixed units. The discussion remains unresolved regarding the necessity of unit conversion.

Contextual Notes

Participants acknowledge the complexity of unit conversion, particularly for temperature, and the potential for confusion when switching between systems. There are also references to specific equations and their requirements, but no consensus is reached on the best approach.

KFC
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I am reading something about thermodynamics and everything are not given in SI units. In the material, they use Fahrenheit for temperature, BTU for heat, lb for mass and in^2 for area. Now I need to calculate something with the data given in those units, well, I need to apply equation for ideal case, to assure the result of my calculation makes sense, must I convert all unit to SI? I know ideal gas equation defined in Kelvin unit. Can I only convert the temperature to Kelvin while keeping others units unchanged ?
 
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You can google all the conversions you need. I suggest using whatever units you are comfortable with - that's what I do. I switch back and forth, depending on the situation.
 
Using temperature is a bit more tricky than other quantities, since for other quantities there is usually a well-defined ratio for different for units, but for different units of temperature there is a ratio with an offset. To be safe, it's usually best to do all temperature calculations in Kelvin.
 
Remember google does automatic conversation of units - just type the question into the search box

enter "9.81m/s^2 in furlongs/fortnight^2"
gives the answer "9.81 (m / (s^2)) = 7.13499487 × 10^10 furlongs / (fortnight^2)"
 
Thanks all your reply. I know how to do the units conversion, my question is just want to know if it is a must to convert temperature to kelvin before calculation? Since I was told that ideal gas equation only work in absolute temperature.
 
You can use Rankine (the Fahrenheit absolute scale) and the value of R will be different but otherwise the equation is the same.
 
Use unit converter, I use one below
http://www.convertcenter.com
 
Last edited by a moderator:
KFC said:
Thanks all your reply. I know how to do the units conversion, my question is just want to know if it is a must to convert temperature to kelvin before calculation? Since I was told that ideal gas equation only work in absolute temperature.

mgb_phys said:
You can use Rankine (the Fahrenheit absolute scale) and the value of R will be different but otherwise the equation is the same.

Correct. Temperature must be in absolute units. Both Kelvin and Rankine units would work. Fahrenheit and Celsius will not.

"absolute temperature" and "Kelvin" are not synonymous.
 
We live in a mixed world, and it is much smarter to learn to function with both SI and US Customary units. Those who think that every thing must be converted to SI units before any calculations can be made will make a lot of pointless conversions, and then will find that it is necessary to convert the final results back in many cases. Until such time as the US adopts the SI system, it is foolish not to learn to function in both systems.
 
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Dr.D said:
We live in a mixed world, and it is much smarter to learn to function with both SI and US Customary units. Those who think that every thing must be converted to SI units before any calculations can be made will make a lot of pointless conversions, and then will find that it is necessary to convert the final results back in many cases. Until such time as the US adopts the SI system, it is foolish not to learn to function in both systems.

Thanks for reply. Well, I totally understand that. The reason I am asking this is because in some case you don't have freedom to use any unit you want. For example, in the state equation of ideal gas, we cannot plugin the termperature in Celsius. So I am just thinking it is safe to use SI units in calculation.
 

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