The LP-2750 is very sensitive to changes and can identify which part of your pool system is leaking by doing two tests. One with the pumps on, and the other with the pumps and equipment off.
Once you complete and log the results of both, you need to compare the results. If the test with the pumps on has a higher leak rate, then your leak will be in the return side of the system. If the results are nearly identical, this means it is in the structure of your pool, like a crack or light fixture leak. If you have a high leak rate with the equipment off, it is signaling that your leak is in the suction side of the pool system.