I've been working on a design for a counter pressure auto bottler (ie counter pressure bottle filler that starts filling a bottle once the bottle is on, but stops when the bottle is removed.)
It should allow for 1 handed, easy operation. If needed you can add CO2 purging too, but obviously you'll need valves for that.
Being that I live in the UK, all measurements on this plan are in metric, although I'm certain you lovely peeps who haven't switched will be able to find suitable parts. Also because I live in the UK, most of the parts are copper because stainless steel is cray expensive (did I use cray right? I hear young people say it all the time, but I don't know what it means).
Anyway here is the plans (not to a great deal of scale, but it gives you an idea)
Right, the basic idea is the same as any other Counter pressure system, except at the top you have a single check valve which is on upsides down ie the flow of beer is going against the check valve, so that if you activate the flow to the bottler, nothing will happen because the check valve is closed, it will only open if a force is applied from the other side.
You can use a double check valve, but you'll need to remove one of the check valves, so it's pointless really.
From the check valve, beer will run down the 6mm stainless pipe (used stainless here because I have some) into the bottle. The contents are isolate via various reducers. Any gas in the bottle is expelled via the 10mm pipe. Not shown on here is the #2 bung which will sit at the top of the bottle, but bsically the 10mm copper pipe will run into the bung, and allow any gas to escape. This is similar to all Counter Pressure Systems.
The pressure release system would be out the right hand side of the equal Tee, but is not shown because there are a wide variety of options from PRV's to needle valves.
Here's where the magic happens (not tricks, tricks are what whores do)
There is a length of stainless rod which runs from the check valve to about 4mm outside of the 6mm stainless pipe. This can be attached to the check valve by heating the rod and melting it in if the check valve is plastic and for extra security use some glue which won't poison you.
So, when the bottler is inserted into the bottle for the first time, you adjust the bung position so that it seals the bottle and the bit of rod is pushed up. Then, you can activate the flow of beer. This will fill the bottle. Once done, remove the bottle and the check valve will seal again and the flow stops.
Hey presto, 1 handed, cheap and easy to use counter pressure auto bottler.
I've got to go offshore tomorrow so I haven't had time to build it, but I'll let you all know how it goes when I get back next week.