Ok - if no gaps - what is the amount of D2O lost to evaporation, please?
I do not understand what you mean by "no gaps." But anyway, evaporation is a function of temperature, and the vapor pressure of air and water. See Dalton's law. It is the term labeled "L" in the calorimetry equation shown here:
See "enthalpy content of the gas stream." Dalton's law includes P partial pressure, and P* atmospheric pressure.
Other people use less elaborate versions of that equation, but all the ones I have seen include a term for evaporation.
Evaporation is measured in calibrations with resistance heating and electrolysis. With electrolysis you also lose water from electrochemistry. The amounts can be separated by the equation. Fleischmann discussed that in detail. In the above paper, F&P explain:
" . . . at a cell current of 0.5A and atmospheric pressure of 1 bar, the cooling due to evaporation of D2O reaches 10% of that due to radiation at typically 95-98°C for Dewar cells of the design shown in Fig 1 . . ."