Fix: Missing Temperature / Temperature Operator

[0xferit]

• Fixed missing temperature / temperature operator

• Fixed obsolete call to .DegreeCelciusDelta to .DegreeCelcius in the statement return LapseRate.FromDegreesCelciusPerKilometer(left.DegreesCelsius / right.Kilometers);

[angularsen]

Are you sure you don't mean to divide by a temperature delta?

This is tricky and not super intuitive to me, but I have realized that different temperature units have different scales and arithmetic on those are tricky since we talk about degrees Celsius both as a point on the scale as well as a range on the scale, although you can't add two points together since they may have different zero points so you add a delta to a point.

[0xferit]

Hmm, so you didn't implement Temperature / Temperature on purpose?

What else can we do? Would Temperature / TemperatureDelta be correct?

We are trying to obtain reduced temperature by diving actual temperature to critical temperature. By doing that, I noticed that / operator is missing.

@gratestas knows better than me, perhaps he can explain better.

[gratestas]

Hi,
actually it should not be a problem dividing two values of the same quantity with each other as far as both are considered to be on the same scale. And this goes the same way as solid pattern for all arithmetical operations applied for two different points on the phase diagram of any substance for any unit base. Naturally, no one would feel a need to divide two points of a quantity before getting units normalized.
Concerning our case, we need to use a reduced state of a substance so that we can proceed on "standardized"/dimensionless plane, where the analysis of the most substances, if not all, can be treated more easily. In thermodynamics, a reduced state is a normalization of a point with respect to critical point of a substance. And it is very useful way to analyze real behavior of a substance with respect to its ideal state.

So i think @ferittuncer does not have wrong sense.

@angularsen maybe we misunderstood you statement. Maybe you should explain us a little bit more what you meant so that we can talk about the same thing

[angularsen]

Here is some context on why TemperatureDelta was added in the first place:
#218

Dividing 20 Celsius with 5 Celsius is as simple as 20/5=4, since they are the exact same scale. Dividing 20 Celsius with 5 Fahrenheit requires some conversion because they have different zero points (0 C != 0 F) as well as different slopes (increasing with 1C is different than increasing with 1F).

actually it should not be a problem dividing two values of the same quantity with each other as far as both are considered to be on the same scale.

I think you nailed the key issue here. When you have a Temperature in UnitsNet, that represents a temperature that is not yet tied to a particular unit or scale - not until you convert it to Celsius or Fahrenheit.

Internally a Temperature is represented with Kelvins and arithmetic is performed using that scale, however Kelvin has a different zero point than both Celsius and Fahrenheit This is why arithmetic is tricky with temperature, because adding or dividing two temperatures will be done on Kelvin scale even if you are really working with Celsius or Fahrenheit - so it will not match your expectations.

This is why we added TemperatureDelta to support arithmetic that is agnostic of the scale. A Temperature represents a measurement point and TemperatureDelta represents a range or interval. You add a delta to a temperature, and you multiply or divide a temperature by a delta.

So back to your example.

Naturally, no one would feel a need to divide two points of a quantity before getting units normalized.

I agree dividing two points in the same scale is fine, but doesn't that mean you should be doing something like this?

Temperature temp, criticalTemp; // Values retrieved from somewhere
Temperature normalizedTemp = Temperature.FromDegreesCelsius(temp.DegreesCelsius / criticalTemp.DegreesCelsius);

Does this help or am I maybe still not understanding what you want to achieve?

[0xferit]

I'm satisfied with the explanation. Thank you.

I'm closing this pull request.

[gratestas]

I got your point. And I am okay with that too.

thanks again