Triple Point Water: Why Used for SI Thermodynamic Temp

[gunparashar]

why triple point of water is used to define SI base unit of thermodynamic temperature even though for triple point we need to define both temperature (. 01) and pressure(partial pressure = 611.73 pa). this thing we can do with ice point and boiling point which is same when temperature and pressure is defined.

 

[ModusPwnd]

The ice point and boiling point change depending on your pressure. The triple point does not.

 

[dauto]

The boiling point and freezing point of water depend on pressure. At higher pressure the boiling point moves up (higher temperatures) while the freezing point moves down. So boiling point and freezing point are not uniquely defined while the triplr point temperature is unique.

 

[256bits]

why triple point of water is used to define SI base unit of thermodynamic temperature even though for triple point we need to define both temperature (. 01) and pressure(partial pressure = 611.73 pa). this thing we can do with ice point and boiling point which is same when temperature and pressure is defined.

i think it is the other way around, isn't it. The triple point defines the temperature and pressure.

Reproducibility is the key.
Take a container of water and evacuate all the air so there is just liquid water and gas. Cool the container until some ice forms. You now have the triple point at the solid, liquid, gas interface and any temperature probe can be calibrated for 273.16 K.

With you method, one has to know the pressure beforehand for the ice point, as said above that can change with pressure. If the pressure is off, so is the temperature.

Luckily, at normal atmospheric temperature there is little difference. I believe it is 273.15K.

 

[PJC01]

The triple point of water is used to define the SI base unit of thermodynamic temperature because it is a fundamental and easily reproducible physical state of water. At the triple point, water exists in a state of equilibrium between its solid, liquid, and gas phases, at a specific temperature and pressure. This makes it a reliable reference point for defining temperature, as it eliminates any ambiguity that may arise from using other reference points such as the ice point or boiling point, which can vary slightly depending on external factors.

Additionally, the use of the triple point of water allows for a more precise and accurate measurement of temperature, as it eliminates the need for extrapolation or interpolation of data from the ice point and boiling point. This is important in scientific research and experimentation, where precise and consistent measurements are crucial.

While it is true that defining the triple point of water requires the measurement of both temperature and pressure, this can easily be achieved using modern instrumentation and techniques. In fact, the international standard for temperature, the International Temperature Scale of 1990 (ITS-90), uses the triple point of water as one of its fixed points, along with the melting point of gallium and the freezing point of zinc.

In conclusion, the use of the triple point of water to define the SI base unit of thermodynamic temperature is based on its fundamental and reproducible nature, as well as its ability to provide precise and accurate measurements. This makes it a reliable and universally accepted reference point in scientific research and measurement.