i am desperate about the gas and the water
I tried on both meters the get a signal with a sensitive hall sensor, but no succes. Also tried to read a number-wheel with a IR emittor-receiver but I cannot get a clear pattern from it.
Here I ask for help : are there suggestions to be able to read those meters? Is there a possibility to read the strips at a number-wheel with a fine beam emittor en a receiver?
I submit that you might as well put your own sensor in the pipe, rather than mess around with somebody else's.
I am using the same water flow turbines as Vittorio. They are working fine. You test their operation by gently blowing through them. Therefore they can work as gas meters as well.
While this proves the point, I don't know how suitable they are in terms of operating range. You might find that a turbine designed for a low flow rate like thishttp://www.omega.com/Manuals/manualpdf/M5171.pdf
is more suitable. Another is the Swissflow. Indeed there is a swag of small hall effect turbines around that are no good for mains water pressure but may actually be better suited for gas. The type Vittorio mentioned is only about $15 and thus may be worth getting just to try it out.