Any of y'all ever use a YSI DO meter? They have both replaceable membrane and twist of 'tips' with membranes. Models, from memory are about 200 for the economy version and maybe 800 for the main model. I used them extensively for water quality monitoring in a former life and have always thought they would be useful. Only drawback is they are probe based and require 'dipping' into the sample but can be sanitized like anything else. They were very accurate. I may ask to 'borrow' one from an old colleague and test it myself.
Chris Burcher, Wolf Hills