Tom is working on getting someone to make just that type of pH probe, converting it to a CO2 probe. This only works if the response time of the device is a lot faster than our drop checkers. So, it requires that the pH probe be in a very tiny amount of reference KH water, and have a very large ratio of membrane area to water volume. (Obviously, it has to be done with a gas permeable membrane to have any hope of achieving the rapid response time.) If the response time is too long the pH controller would be constantly overshooting and undershooting the desired pH, so as to be totally useless.

Also, any membrane device that is left in the water for very long will see the membrane pores becoming increasingly plugged, due to biofilm. So, this would not be a "stick it in and forget it" device.