Can't find leak in Gas Pipes
We are renovating a home, our plumber took our existing gas lines, we moved where the stove was and removed an old bathroom heater. The rest of the lines stayed intact.
From the meter comes a long run to the furnace in the attic, then splits to the stove and hot water tanks.
All pipe is existing except the new run to move the stove from one side of the kitchen to the other.
Our gas is off.
Our plumber started with pressure at 50lbs - and then used the soapy water to test all connections. There were two old shut off connections, one at the hot water tank and one at the furnace that were bad, we changed those. We repressurized the system and in 2 days it was down to 0
Next weekend - re-charged and resoaped everything, couldn't find anything (so we removed the old bathroom heater) - it was previously left connected but thought this was probably the problem so we disconnected it.
Re-charged the system to 90#'s - resoaped everything - no bubbles. Held 90#'s for 2 hours or so, then dropped to 60 in 6 hrs, it was below 30 by morning and 0 by the end of the 2nd day.
We had a friend who used a stethescope kind of device to see if he could hear the leak. No luck.
Gauge is fine, we switched that out all three times.
How else can we find the leak? Someone mentioned a dye you can charge in the pipes and use a black light to see where the particles are escaping the pipe from. Said its used in automotive AC, I can't find anywhere that it is used for house gas and don't want to do something stupid.
Any ideas would be greatly appreciated. We have friends staying there and our plumber can only work on it on the weekends. They have yet to have a warm shower.
Please help