Inline Refractometer Selections

I am in the process of designing a simple and cost effective system for mixing a solution of sodium hydroxide on the fly. One thing I want to do is monitor the concentration of the sodium hydroxide with reasonable accuracy and to have the ability to detect failures in the system before product is lost.

The dilution system will be diluting 50% sodium hydroxide down to 4% +/-.5% at a flow rate that will range between 10-70gpm depending on plant needs.

I have looked at mass flow meters for monitoring the SG but they run around $6,000 so I also looked at inline refractometers. While I have used handheld and benchtop refractometers I have no experience with inline models.

I have used multiple search engine's and have found several models so I do not need people to blindly posting links to refractometer manufactures. What I am looking for is input from people that have ACTUALLY USED inline refractometers and that can attest to ease of use, reliability and any short comings or strengths of a particular model.

For on the fly chemical dilution I am looking at using Dosatron proportional dosing pumps which operate off of feed water pressure. The goal is to be cost effective in the design....say less than $5,000 USD and to have it take up the smallest footprint possible. I literately have a 12inch X 24 inch area to work with but I do have a 23 foot ceiling so building tall is an option. The dilution system will be diluting 50% sodium hydroxide down to 4% +/-.5% at a flow rate that will range between 10-70gpm depending on plant needs. Any thoughts?