Fix apparent typo in JavaDoc of Quantity

[vorburger]

@keilw

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This change is 

[keilw]

@vorburger Kelvin is wrong here.
The numeric value is indeed a little incorrect,
It should be 2°C + 1°C = 276.15°C.

If you could fix that I'd be happy to merge the PR.

[vorburger]

should be 2°C + 1°C = 276.15°C

Wait, what? Is that a typo, or intentional? It doesn't make sense to me, like this... adding two quantities (2+1) of the same °C unit, becomes a 276.15°C, really?

[keilw]

Yes, that's the whole point of Quantity.Scale.

Check out the Indriya (RI) TemperatureTest
or
QuantitiesTest by Seshat, another compatible implementation.

As well as TemperatureDemo for

• Indriya
• Seshat

Ask @desruisseaux for the motivation.

[desruisseaux]

Hello @vorburger

adding two quantities (2+1) of the same °C unit, becomes a 276.15°C, really?

Yes, really at least in some cases for the particular case of °C. Actually it depends on whether 1°C is an absolute temperature or an increment. This is why there is a Quantity.Scale enumeration with two values: ABSOLUTE and RELATIVE.

If 1°C is an increment (Scale.RELATIVE), then 2°C + 1°C is equal to 3°C. But if 1°C is a temperature measurement (Scale.ABSOLUTE), then the calculation must be done in K. This is required by thermodynamic laws. So 2°C + 1°C is equal to 275.15 K + 274.15 K = 549.30 K = 276.15°C. The reason is that we are computing the amount of energy in a system.

So if you want 2°C + 1°C = 3°C, you need to tell the library that 1°C is an increment rather than an absolute measurement. This ambiguity exists for °C and not for other standard units (ignoring °F) because °C has a zero which is shifted compared to the base SI unit. For non-shifted units, the distinction between absolute and relative makes no difference, because the shift is zero.

[desruisseaux]

A few resources are below. We have to be careful with the answers of this kind of forum as they may sometime be questionable, but the high-ranked answers have good chances to be good resources:

• How do you add temperatures?
• Addition of Temperature Units

[keilw]

@vorburger Any chance you could fix this PR? As it's just a single digit, we could also close it and address the viable part of it separately.