Plumbing question and salinity question

[gradoj]

I have a couple of questions:

1. Lots of pumps have 1/2" connectors. What size pipe/tubing do most people use for returns from their sumps?

2. What do people use to measure salinity - refractometer, specific gravity measuring device or a conductivity meter?

3. I found a refractometer on ebay but I'm not sure if it is suitable. It boasts a range of 0-10% and an accuracy of 0.10%. I assume this is measured and specified as a refractive index. Can anyone tell me what these values would correlate to in specific gravity?

TIA