water flow

Maybe this is silly question, but that's consistent with my knowledge of plumbing. I'm planning to replace the thermostatic cartridge on my newport brass shower. (“Cartidge ASM-Thermo In-Wall” Part No 1-102). A job I've had plumbers do pretty regularly, as there's lots of sediment in my water...

I just replaced an old toilet with a American Standard Champion toilet, nothing wrong with old one besides discoloration and never had this problem. After the toilet is flushed it seems to go to proper level when fill valve shuts off as shown in picture1, then over time the level goes to...

So I got a fancy new water flow machine to check out some problems we've been having with our waste water system and it came with a larger set of plastic transducers an slightly smaller set thats made of stainless. I'm just wondering if anyone could tell me the difference?