DLG Clamp On Ground Earth Resistance Tester With USB Connection DI-120

How can it test a standalone ground rod? The description states "...measurements on multiple point ground systems". I thought there had to be a loop.

what is the manufacture phone number or address to contact if need unit calibrated?

does it come with a calibration ring? This is a separate piece that has a known resistance and you clamp around it to verify the meter is working.

will this test stand alone ground rods, also grounding wire from ground rod to fuse box and is there technical support?