I'm copying a question and the answer that might be useful regarding the Watson Smith type 140:
"We have number of the above I/P's used in our gas network. It has come to my attention that instead of using compressed air as a supply we are using methane gas we transport through the network. My question is would this invalidate the certification,
"Yes, the type 140 is suitable for methane as process gas and it includes a feature called "captured bleed" which allows you to pipe the vent of the converter away to a safe place.
Please see the instruction manual (page 7) for further details."