Celsius degree (difference)
Temperature differenceexact by definition
The Celsius degree used as a temperature difference. Because a Celsius degree and a kelvin are the same size, a difference of one Celsius degree is a difference of exactly one kelvin, and the conversion factor is 1 with no offset. The factor is exact by definition.
Watch out: The 273.15 offset that separates Celsius from kelvin plays no part here. It cancels in any subtraction, which is why a specific heat quoted in J/(kg·K) can be used directly with a temperature rise measured in Celsius degrees, with no conversion at all.
| 1 C° | 1 C° |
Written C° rather than °C in careful engineering writing, with the degree sign trailing, precisely to signal that it is an interval and not a reading. The convention is not universal but it is useful, and this catalog follows it.
About the celsius degree (difference)
The Celsius degree as an interval is the workhorse unit of metric thermal engineering, and its convenience is that it is numerically identical to the kelvin. A heating load of \(Q = \dot{m} c_p \Delta T\) with \(c_p\) in J/(kg·K) takes a \(\Delta T\) in Celsius degrees without a moment's thought, because the two units are the same size.
The trap is only ever on the imperial side. A rise of 10 C° is a rise of 18 F°, not 50 F°, because the freezing-point offset cancels out of any difference. Ten degrees of change, times 9/5, is eighteen degrees of change. Meanwhile a reading of 10 °C really is 50 °F. Same two scales, two completely different conversions, because they are answering two different questions.