Hey Everyone, just saw this thread and although its a year old now, I figured I'd chime in.
I use a bubbler liquid level sensing system in my apartment setup using an arduino to read the voltages from an MPX5010dp differential pressure sensor (0-3ft of height is 0-5V). This works well as you have one port reading the pressure of the height of liquid, the second is open to atmosphere for reference.
I'll be using this technique to do automated gravity readings with two bubbler tubes spaced a few inches vertically apart from eachother. I'll feed the system initially with an aquarium air pump however may eventually use C02 (from either a bottle or a C02 bath created by the fermentation of the beer itself
The bubbling of the system as previously mentioned is not really an issue as you can filter this out with software.
Reasons I will be using the bubbler are:
1. no concern over crud or krausen or any other debris accumulating on the tubes
2. Already use a single bubbler tube for liquid height measurement on my brewery so integrating this will be simple
3. Its very simple (not as simple as an optical sensor in the airlock) and its output directly measures gravity not back calculating it from bubbles or mass flow of C02 or weight etc.....
I'm sure I'll learn a thing or two while doing this project so I'll be sure to keep everyone up to date with how I am progressing on this.
As this is a learning forum, please challenge anything i've said above if it doesn't seem to add up