Water level readings are taken at the top of the casing with accuracy to within 1/100 of a foot (3mm). They utilize a durable polypropylene storage reel with a rugged solid aluminum frame. Their highly accurate Polyethylene-coated steel tape is available in either standard/English or metric unit increments. The 5/8" (1.59cm) probe is made with stainless steel conductors for durability, and it can be serviced in the field. The 3/8" (.95cm) is not replaceable.