One flat piece of acrylic has a hole in it. The membrane goes across the hole, over the O-ring, and then between the two pieces of acrylic. That leaves a disc consisting of membrane, piece of sponge soaked with KH reference water, acrylic, with the O-ring sealing in the KH reference water. I wasn't too happy with this design, and I'm not all that sure the tyvek membrane doesn't leak a bit. Personally, I am giving up the quest for faster response, preferring the simplicity of not using the membrane.I like this design as I have all the supplies to do this one but I can't figure out where the reading is coming from... Is it just the sandwiched Tyvek membrane spaced from the O ring that is allowing the exchange in reading ? Or am I missing something as to where membrane is exposed to the water ?