P&F advocated open systems for calorimetry, which opens one up to evaporation. If the water evaporates away (or is electrolyzed away) then the thermal mass reduces, and a watt of heat would produce more temperature rise at the end of the experiment than at the beginning. This may not apply to Lewis’s system, but then again it might. It certainly applies to some of the P&F systems, because they advocated open systems.
still waiting to see all those promised units working and producing.....what’s the current time frame???? (other than before the twelfth of never)