Aquarium salinity converter

Specific gravity compares the density of the sample with a reference-water density. Practical Salinity Scale values are derived from electrical conductivity and are dimensionless, although aquarium displays often label them “ppt” because the numbers are close to historical parts-per-thousand salinity.

A refractometer does not directly count grams of salt, and a hydrometer does not directly measure conductivity. Each instrument converts an optical, buoyancy, or electrical response using its calibration model.

The calculator uses the UNESCO EOS-80 seawater density polynomial at atmospheric pressure. It computes seawater and pure-water density at the entered temperature, then forms their same-temperature ratio for SG. The reverse conversion is solved numerically.

At 25°C, practical salinity 35 corresponds to about 1.0264 under this definition. SG 1.025 at 25°C corresponds to about 33.19, close to the simple hobby approximation but based on the full density relationship.

Hydrometers are manufactured for a stated reference temperature or density ratio convention. A sample read at another temperature may require the correction table supplied with that hydrometer.

Remove bubbles from the float, keep it away from the container wall, read the correct side of the meniscus, and rinse it after use. Swing-arm pivots and trapped bubbles can introduce additional error.

Automatic temperature compensation adjusts the instrument response, but it does not eliminate the need for calibration or guarantee that a sodium-chloride scale matches seawater. Follow the specific device manual.

Use the calibration standard specified by the manufacturer and allow the sample and instrument to equilibrate as directed. Record the device and calibration date when a small shift could change husbandry decisions.

Evaporation removes water but leaves salts behind, so salinity rises unless evaporation is replaced with fresh water. Salt creep and wet skimmate can remove salts, while water changes can move the system toward the new batch salinity.

Investigate surprising readings by repeating the sample and checking calibration before changing the aquarium. A fast correction based on a faulty measurement can be more harmful than the original drift.