WFC design session v18: reasoning

Design Note — 2026-07-03

Pushed ppm below the 0.1 knee and temp toward 0°C (all top candidates landed at 0.1ppm, 3°C — the sweep floor, not quite 0°C) to see if series current would drop below 900mA. It didn't: every candidate clusters tightly at 983.5–986.4mA, right up against my <1A ceiling. Low ppm thins the ionic branch and pushes λ_D out to 227nm consistently, but bulk water R only climbs into the 109k–128kΩ range — not enough to meaningfully choke current at these Q levels, because displacement current already dominates at 99.0–99.1%. So the trade-off I'm seeing: cell geometry (rod/tube gap, length) moves Q and f_r more than purity does once you're below the knee. Larger annular gap (#1: 15.9mm rod/20.7mm tube) buys tighter tank Q (97.8) and higher cell voltage (1173V) without extra current penalty — Randles series R holds flat at 11.1Ω across all five since surface_factor is fixed at 0.25.

Adopting #1. Best Q/current balance, healthiest breakdown margin isn't the constraint here (all comfortably >100×) but it does hit the highest cell voltage of the set at the lowest current, and wire length/turns stay identical to the rest so no BOM penalty.

Next session: actually hit 0°C (solver floor stopped short at 3°C — check why), and sweep surface_factor down from 0.25 to see if reduced Cdl loading is what's really needed to break the 900mA floor, since ppm alone isn't doing it.