I’m sorry but temperature, volume and pressure are correlated and the statement “Whether the boiler expands or not doesn't matter to the relationship between temperature and pressure.“ is not correct.

In theory you could design a boiler of variable volume which maintains constant pressure, so that when the temperature rises the boiler expands and the pressure doesn’t increase.

I appreciate that all coffee machines have a relatively rigid boiler so the (approximately) linear relationships between pressure and temperature is normally implicitly assumed, but from a general physics perspective the statement “The relationship between temperature and pressure in a boiler is essentially linear.“ is true only if the boiler is rigid (fixed). So Mike’s statement was factually correct.

In theory you could design a boiler of variable volume which maintains constant pressure, so that when the temperature rises the boiler expands and the pressure doesn’t increase.

I appreciate that all coffee machines have a relatively rigid boiler so the (approximately) linear relationships between pressure and temperature is normally implicitly assumed, but from a general physics perspective the statement “The relationship between temperature and pressure in a boiler is essentially linear.“ is true only if the boiler is rigid (fixed). So Mike’s statement was factually correct.

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