Marine refrigerant charging basics (R134a, R410A)

Marine AC and refrigeration use R134a (older systems, being phased out globally), R410A (current mainstream), or increasingly R32 (new-build). Refrigerant charging is EPA-regulated: US law requires a Section 608 Type II or Universal card to buy, handle, or discharge these refrigerants. Charging a system without proper equipment ruins it in weeks, humidity contamination, oil loss, or wrong charge amount all cause premature failure. Proper charge procedure requires: (1) Recover any existing charge into a recovery cylinder (never vent). (2) Vacuum the system to 500 microns with a two-stage vacuum pump and micron gauge, the micron gauge is non-negotiable, standard gauges lie. (3) Hold the vacuum for 30 minutes to prove no leaks and moisture is boiled off. (4) Charge by weight per the nameplate (not by pressure, pressure only tells you charge state after temperature stabilizes). (5) Once charged, run and log superheat + subcooling values against manufacturer spec. R134a and R410A are NOT interchangeable, different oils, different operating pressures (R410A runs 50-70 percent higher). Never mix. Diagnostic rule: if a marine AC "needs charging" every year, it has a leak. Find it with UV dye or an electronic sniffer before charging again, repeat charging without leak repair is illegal, expensive, and doesn't work anyway.