Never tried an ultrasonic, but the next best thing is to shut down the compressor room and listen. Simple but effective, should be done on any maintenance after I "heard" how effective it was at locating leaks in comp. rooms.

Using the Ultra sonic would be logical.
1. Toggle off systems and let rack pump down to 1 comp.Shut down all compressors.
2. Wait a few minutes for pressures to balance and quiet down.
3. Listen for leaks with your own ears then use the ultra sonic to be sure and you,'l likely find any leak there is, just to be sure fire up the H-10 for backup.

Lots of people are scared to shut down a rack but as long as its done right and you do a proper pumpdown and gradual start up!!!!!!!!!!! you should be ok.
If you just come up and shut off some racks instantly you run the risk of damaging a compressor on startup.