In the annals of the Metric System, aside from the kelvin (K), there two units of temperature that are most commonly used throughout the world: the degree Celsius (°C) and the degree Centigrade. (The degree Centigrade, as far as I know, doesn’t really have its own unit symbol, so I propose °Ce for this role.)

Most people tend to think these are the same thing, and for most practical purposes they are; but when you get down to the nitty-gritty, heavily detailed world of thermometry, one begins to realize that these two scales, though similar, are not the same at all. They are, as it were, more like twins than clones, having originated from a single source yet still fundamentally different.

The difference between them is that whilst 0 degrees Centigrade and 0 degrees Celsius are both equal to the Ice Point of water (273.15 K, in absolute terms), a value of 100 degrees Centigrade is not the same as that of 100 degrees Celsius. As it stands, 100 degrees Centigrade is equal to the Steam Point of water, approximately 373.13 K, while 100 degrees Celsius is ever-so-slightly higher at 373.15 K.

The value of the Steam Point has a rich and variable history, largely because what it is exactly depends entirely on how sensitive the equipment used to measure it is. Throughout the years, this equipment and their sensitivity have improved, and thus so has the value measured.

Here is a brief timeline showing how the two scales have changed and related to each other over the years, expressed in degrees Celsius:

Year Ice Point of Water Lower Bound (°Ce) Lower Bound (°C) Steam Point of Water Upper Bound (°Ce) Upper Bound (°C)
1973 0 °C 0 °C 0 °C 99.970 °C 99.970 °C 100 °C
1990 0 °C 0 °C 0 °C 99.975 °C 99.975 °C 100 °C
2014 0 °C 0 °C 0 °C 99.974 °C 99.974 °C 100 °C
2016 0 °C 0 °C 0 °C 99.984 °C 99.984 °C 100 °C

Here’s the same information, but expressed in kelvins:

Year Ice Point of Water Lower Bound (°Ce) Lower Bound (°C) Steam Point of Water Upper Bound (°Ce) Upper Bound (°C)
1973 273.150 K 273.150 K 273.150 K 373.120 K 373.120 K 373.150 K
1990 273.150 K 273.150 K 273.150 K 373.125 K 373.125 K 373.150 K
2014 273.150 K 273.150 K 273.150 K 373.124 K 373.124 K 373.150 K
2016 273.150 K 273.150 K 273.150 K 373.134 K 373.134 K 373.150 K

So, there you have it, folks! For all the world to see, it should be clear now that the Centigrade scale is not at all the same one as the Celsius scale, despite what we’ve always been told. For most everyday purposes, of course, it makes no difference whether you treat the two as equal, but just know that this does not mean that they are truly the same at a fundamental level. They are only ever the same until you get down to the level of thousandths.

 


Sources for Table Data:


Image Credit:

[Untitled], by Pēteris Caune.
Released under a CC BY-NC 2.0 Generic license.
Image has not been modified.
This use does not imply endorsement of the above content.

Note: I’d like to apologize to any mobile users viewing this post. I know the 2nd table below goes off the screen, but try as I might I couldn’t figure out how to get the table small enough without reducing the values’ magnitudes to two digits, which would have made some of the differing values ambiguous.
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