Here's the basic layout of the shop. The circle with an "x" in it represents a ball valve.
Below is a pic of the chart in ThingSpeak. The slopes of the different curve segments are calculated below:
So it appears the best the system can do is around 2 psi/hr. That's with as little of the system as possible in the mix, i.e. best case. That said ,there's still the compressor check valve, a water separator/filter, air line quick disconnect fitting, and 3 ball valves in the mix. It's possible the air line quick disconnect fitting could be the bulk of it, however the filter also has an auto drain feature which could be leaking too.
UPDATE: I've been opening up each end point separately and it looks like the Bridgeport is kinda a problem. I'm pretty sure it's the Kurt Power Drawbar IN/OUT switch - I knew it was leaking, but it's leaking more than I wanted. I think the rest of leaks are probably a collection of minor things I'll have to find by soapy water, etc.
A couple of years of ago I finally added hard lines to my air compressor throughout the shop. What a nice upgrade! No more hoses all over the floor. However, it seems my copper sweating skills are maybe not what they should be as it seems to leak a fair amount of air. After I switch off the compressor it takes something like 4 hours for the system to depressurize.
I felt I could do better.
But how to isolate the problem? And how to make it a complex project?
Well, that's easy. Just build a wireless network connected microcontroller with a pressure sensor, an SD card for logging, and a graphical display. Have all your data get collected in two ways - 1) local to the SD Card and 2) upload to ThingSpeak.com.
My thinking is to attach the logger to different segments of the air line. As I move away from the compressor - therefore adding more pipe joints, valves, equipment - I should be able to deduce from the pressure decay rates what the relative leak rates are per section. That should give me an idea of what to work on. Additionally, I can verify my repairs in quantitative terms.
So after a few days of cobbling things together
Here you can see it's attached to my air lines - running about 115psi.
Close up of the mess. The screen is an older Arduino TFT (now discontinued) with an SD card socket. The smaller chip above that is an ESP8266 WiFi chip. For $5 you can't beat it!
The ESP8266 is a 3.3v part which needs some juice when transmitting. I happen to have had lying around an old XBee shield that I had previously added a 2 channel level shifter to. It took care of both issues as it also has a robust 3.3v power supply.
Here's a link to the ThingSpeak page where I'm uploading the data : https://thingspeak.com/channels/6008
I'll have to update this as start logging the system and try to figure it out.