How to Convert a Refrigeration System from Mineral Oil to POE Oil Using a Refractometer

A practical guide to measuring residual refrigeration oil during a lubricant conversion

Converting an existing refrigeration system from mineral oil or another legacy lubricant to polyolester (POE) oil requires more than simply draining the compressor and adding new oil.

The objective is to remove as much of the original lubricant as practical and verify that the amount remaining is within the requirements established for the particular compressor, lubricant, refrigerant and application.

A refractometer provides a practical way to monitor that process.

This in-depth guide compares 1.800 nD and 1.81 nD gem refractive index fluids, explaining how each is used with gem refractometers like the GIA Duplex II and NI Supply RGM-700 for accurate gemstone identification. It highlights that while the 1.81 fluid offers slightly higher measurement capability, the more affordable 1.800 fluid effectively covers nearly all gemstones, making it the best value for most users. The article also explores the extremely narrow 1.800–1.810 RI range, listing the few gemstones affected and showing why the difference is minimal in real-world testing. Ultimately, it recommends a dual-fluid strategy for professionals while emphasizing the cost-efficiency and practicality of the 1.800 RI fluid for everyday gemology work.