And for every day purposes - where one or two degrees error is acceptable - it is that easy.

However, for scientific purposes - the measurement of temperature is actually very difficult.

Later, in science classes, we used the metric scale where water freezes at 0°C (degrees Centigrade) and boils at 100°C.

While researching various subjects related to atmospheric physics, I learned that water freezes at 0.01°C (degrees Celsius) and boils at 99.974°C.

To the casual observer, this makes no difference - water still boils when it boils and the temperature depends more on the atmospheric pressure than on some definition. (In the 3 examples above, the atmospheric pressure must be 1013.25 hPa - by definition.) The following table shows the expected boiling points at various pressures (using the IAPWS formulation [as provided via Vömel 2011] to compute relative humidity).

Pressure (hPa)	Boiling Point
	°C	°F
1050.00	100.98	213.76	Typical high
1013.25	99.974	211.95	Sea level
980.00	99.04	210.28	Typical low
828.00	94.41	201.95	Denver, CO 5,470 ft (1.667 km)

Note:

Computations of this type can be made with my water vapor programs.

Without getting too deep into the physics, (See BIPM - Thermodynamic and practical temperature scales for details.)

• 1948 - degree Centigrade was officially replaced with degree Celsius
• 1954 - A new temperature scale was based on the triple point of water (273.16 °K) and absolute zero
• The International Practical Temperature Scale of 1968 (IPTS-68)
  • Changed degree Kelvin to kelvin and replaced °K with K
  • The scale above 273.15 K used a different calibration method than the scale below
  • Recalibrated a thermocouple to define the scale from 630.74°C to 1064.43°C
• International Temperature Scale of 1990 (ITS-90)
  • Defined a new standard water to use for calibration - Vienna Standard Mean Ocean Water (VSMOW)
  • Changed the calibration point from 273.15 K to 273.16 K
  • Changed the calibration method above the triple point to use the melting points of a number of pure metals
  • No longer uses a thermocouple to define the scale from 630.74°C to 1064.43°C

Below the triple point, the temperature scale is defined by 2 points - zero K and the triple point using water of a specific composition (VSMOW) and a specific pressure. Above the triple point, a different calibration method is used. For (IPTS-68), the boiling point of water at 100°C (and 1 atm) was used. ITS-90 changed the calibration to use the melting points of a number of pure metals. As a result, the boiling point of water became a measured value instead of a defining value.

Stated another way, using atmospheric pressure to define a temperature scale is a bad idea.

The latest standard is adequate for normal usage since most thermometers are only accurate to about 0.1°F and, before the digital age, read to only the nearest half degree. But there is still an issue - basically, temperature is measured on 4 different scales, each with its own calibration and interpolation methods. As a result, when extending the ITS-90 zero to 273.16 K scale, the boiling point of water is about 373.1339 K (99.9839°C). However, when using the scale defined by pure metals, the boiling point of water is 373.124 K (99.974°C). ref While this difference is small, it effects the equations used in many branches of science.

The International Temperature Scale of 1990 (ITS-90)

Note:

The primary source, Preston-Thomas H., The International Temperature Scale of 1990 (ITS-90), is paywalled at $33 per peak!

URL: http:// mc-computing.com / Science_Facts / Temperature_Standards.html