Here are some photos of the items in question, Sir @greenbuggy, in repayment for your very helpful answer!
To clarify, @cyclopath, the smaller TX model Hubbell on-demand water heater’s inlet flow sensor (3/8" male NPT, I think, shrinking the inlet diameter down from 3/4" sharkbite fittings and copper pipes) states a rating of only 158°F, which does not pair well with TruSteel’s AutoVap operating temperature of 190°F… and that’s the one that died after perhaps 1.5 weeks of continuous use (when evaporating and when draining or stopping evaporating in the AutoVap).
To further obfuscate the issue a bit, the rating on the newer HX model Hubbell’s inlet flow sensor (full 1" compression fittings from 1" sharkbite and pipes) is stated at 85°C… which is 185°F… which means the smaller sensor’s label might just have a typo, swapping the 8 and 5.
We are running the heated water with a Taco 1/8HP continuous duty floor-heating pump on over 100 feet of 1" pipe and insulated heater hose. On the restricted TX, the flow was about 2.5 GPM, and on the HX, the flow is about 8.5 GPM.
Previous on-demand water heaters we used for large BHO system boilers only needed set around 90 to 110°F to get the resin concentrating job done. They were “Eco” brand or models, I think. We ran them on a much shorter loop, using a fast-flowing plastic hydroponics pump.
My solution, for now, is setting the water heater to 185°F… so the outflow hovers around 192°F, and the inflow returns at 172°F (vacuum evaporating) and 182°F (no vacuum or evaporating), max.