# BDD & PD: VaporPressureCalculation

This diagram shows the Block `VaporPressureCalculation` with a supporting ConstraintBlock `VaporPressureCalculationConstraint`:
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The Constraint equation is:
``{pressure=vapor*(gasConst*((roomTemp+celciusOff)/(molecularW*volume)))}``
If you compare that with the famous `P⋅V = n⋅R⋅T` ideal gas law, where n is the number of moles and `R = 8.314 J/(K⋅mol)` you get:
``pressure*volume=(vapor/molecularW)*gasConst*(roomTemp+celciusOff)``

This implies `vapor/molecularW` is just equivalent to the number of moles `n`. But this makes no sense if "vapor" is elsewhere a rate mL/s, it only makes sense if "vapor" is a mass (in g).

Aha, maybe they've used a sneaky not-quite-accurate trick in the spec equation:

As it happens, 1 mL of water at most temperatures and pressures does not have a mass of exactly 1g. But even if the "vapor" were taken as g/s then the dimensional analysis suggests there is a major problem, you'd end up with the output being a pressure rate:

Also:
If you assume the `volume` is litres (L) the dimensional analysis is off by a factor of 1000.

So it seems this assumption used in this trail is correct:

And we are indeed dealing with a commercial humidifier for a large building, not a "humidified room":

Notes
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