Triple Point Water: Why Used for SI Thermodynamic Temp

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Discussion Overview

The discussion revolves around the rationale for using the triple point of water to define the SI base unit of thermodynamic temperature. Participants explore the implications of temperature and pressure in relation to the triple point compared to the ice point and boiling point of water.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions why the triple point is used for defining the SI unit when it requires both temperature and pressure to be defined, suggesting that the ice point and boiling point could suffice under defined conditions.
  • Another participant argues that the ice point and boiling point are pressure-dependent, while the triple point remains constant, making it a more reliable reference.
  • It is noted that at higher pressures, the boiling point increases and the freezing point decreases, indicating that these points are not uniquely defined, unlike the triple point temperature.
  • A later reply reiterates the initial question about the necessity of defining both temperature and pressure for the triple point, suggesting that the triple point itself defines both parameters.
  • One participant emphasizes the importance of reproducibility in measurements, explaining a method to achieve the triple point and suggesting that it allows for accurate calibration of temperature probes.
  • There is a mention that, under normal atmospheric conditions, the difference in temperature readings between methods is minimal, with a specific temperature of 273.15 K noted.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and reliability of using the triple point versus the ice point and boiling point for defining thermodynamic temperature. No consensus is reached regarding the superiority of one method over the other.

Contextual Notes

Participants highlight the dependence of the ice point and boiling point on pressure, which introduces uncertainty in their use as reference points compared to the triple point. The discussion does not resolve the implications of these dependencies.

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

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