The rate at which CO2 dissolves into water depends on the area of the CO2/water contact. This is a very small area. Consider that CO2 from the atmosphere has the total footprint of the tank as the contact area. This is more like drop checker! It would be interesting to find out how much CO2 gets into the water.
Yeah, about the surface area. Maybe an oversized Tee, like 2inches diameter then reduce down to 1/2 inch for the hoses. I'll have to find some transparent PVC for the vertical to see where the CO2 and water meet.
A larger Tee fitting would seem to work the best. I wonder about the turbulence of the water flowing through, caused by the change in diameter, and the lack of flow through the side of the tee with CO2. I can imagine that the water in that leg would be whirling around, up and down, and let big CO2 blobs into the water flow. But, a lot of things I can imagine are nothing more than my imagination, too.
I've used these check valves from clippard, http://www.clippard.com/part/MCV-1BB
previously in my CO2 setup but they both failed. I guess when in contact with water (dirty aquarium water), it fails. I have a spare one but would a cheap check valve from the fish store be better?
I have been using these check valves, https://www.usplastic.com/catalog/item.aspx?itemid=23369&catid=489 in my CO2 system for about 3 years, with no failures yet. I have had, one in my tank, in the water, with CO2 going through it for all of that time, and it still works fine. They are made of plastics that are highly chemical resistant. Shipping costs for buying them drive up the cost substantially, but I still like them. I mistakenly bought the 3/16 inch size the last time, so it can be difficult to slip tubing over them, but they still work. The 1/8 inch size works much better.