Good quality floating glass Hygrometers work within +/- .001 error of measurement. I use two to compare readings as I use tall glass containers to put a sample of the water.
I've seen incredibly variability in readings from swing arms. It looks like either hot weather or high atmospheric humidity somehow tampers its accuracy (I live in the Caribbean Basin).
I also use calibrated PinPoint Salinity Monitor but they loose calibration easily (meaning time consuming task) but that is indeed a conductivity meter capable of working in saltwater (most conductivity meters are manufactured for Freshwater only).
Indirect ways of measuring salinity is by testing Ca or Mg levels of the actual mix and comparing the results with expected levels of known Salt mixes (e.g. Coralife at 1.026 needs 25-26 drops of Ca reagent to change color).
I only use RO processed drinking water, with <20 TDS. Too many undesirable elements are present in my tap water.
Pepetj
Santo Domingo
