Why can’t you run water through any meter (with the exception of the Model 234)?

Almost all of the Max meters have metal-on-metal contact in the flow stream. By running a non-lubricating fluid or compressed gas you run the risk of galling the mated surfaces. There is also high-carbon, stainless-steel ball bearings submerged in the flow and they are susceptible to rust if exposed to water and air. The Model 234 meter is the exception to this restriction as it has graphite-like pistons and no ball bearings in the fluid stream, so neither galling nor rust are an issue.