A pit depth gage (PD-3004) is used to measure the depth of pitting in a thread’s roots.
Inspecting and quantifying a root’s pit depth is a recommend practice in API 7G-2 “Recommended Practice for Inspection and Classification of Used Drill Stem Elements”.
Simply zero the PD-3004 in a root (or a non-pitted surface), and then position the gage into the pit. The dial on the gage will indicate the depth.