GOTCHA: Mathematica v13.2+: Operations on "DegreesFahrenheit" °F and "DegreesCelsius" °C are now performed using Kelvins (K). CASE: Naive percentage operation gives answer relative to Kelvins. Use "DegreesCelsiusDifference"/"DegreesFahrenheitDifference"!

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Naively using "DegreesCelsius" instead of "DegreesCelsiusDifference" as the Quantity unit:


PercentForm[ 
  Quantity[90.0, "DegreesCelsius"]/
  Quantity[100.0, "DegreesCelsius"] 
]  

97.32%

The result is the percentage relative to Kelvins.

Correctly using "DegreesCelsiusDifference":


PercentForm[
 Quantity[90.0, "DegreesCelsiusDifference"]/
 Quantity[100.0, "DegreesCelsiusDifference"] 
] 

90%

The result is the percentage relative to 0 °C.

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 Mathematica: v13.2+: Division by temperature units will produce a quantity equivalent to the temperature converted to Kelvin before division, with results canonically given in Kelvin.
 Mathematica: v13.2+: Division of one temperature by another will result in a numeric ratio given by the value of both temperatures in Kelvin.
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https://reference.wolfram.com/language/tutorial/TemperatureUnits.html