The main problem with most of these wall thickness gauges is they are supplied with a very cheap dial caliper. Once I got that figured out I purchased a high quality Starrett dial caliper from eBay. Since then my readings have been much more consistent and I have a lot more faith in the results. The Manson design has nothing I could fault, except at that price it would have to have the very cheapest gauge available.

When an old man dies a library burns to the ground. (Old African proverb)