Hi. I have a combination csst and black pipe gas line system and it has a leak. At 10psi it will drop 4 psi in an 18hr period but at 5psi it will only drop .5psi in the same amount of time. Either way, there doesn't seem to be enough air leaking to make bubbles using a detergent/water mix. Is there a better way to detect such a slow leak?

I've taken apart the black pipe, put the appropriate pipe putty on it and retightened it very hard. Every time there is still a leak this is driving me nuts.

Thanks for your help!! Keith