Geoprobe® 3.75 in. Rods: A Real Game Changer! Use the 3.75 in. rod system to properly install 2.0 in. prepacked monitoring wells with your 7822DT in conjunction with collecting continuous soil cores. The 3.0 in. ID of the rods leaves enough annular space to properly grout back the well. Geoprobe Systems® offers a number of probe rod sizes from 1.25 in. to 6.0 in. Below you will find the specific inner and outer diameter as well as weight for the 3.75 in. probe rods.