measuring salinity

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Reef Keeper
Jul 5, 2003
When culturing saltwater organisms, less than a 10 percent variation in salinity (± 5% or ± 2 ppt) is often required. The measuring methods cover a wide range of cost, convenience and accuracy. Basically, salinity is the sum of all dissolved ions in the water (not just sodium and chloride), expressed as parts per thousand (grams per kilogram).

One measurement method is to titrate chlorinity (chloride) and calculate the salinity based on the assumption that the composition is the same as seawater. Usually titrations are based on volumes which can introduce a 2.5 percent error if not done correctly. It is much more time- consuming than other methods and used as a means of checking other techniques.

Density is the basis of the hydrometer measuring method. Salinity can then be determined from tables based on density (specific gravity) and temperature. There are two types of floating glass hydrometers, 15°C/4°C and 60°F/60°F. Temperature correction from another table must be applied before reporting or converting to salinity. (Note that the salinity conversion tables are different for the two types, so be sure to use the table designed for the style being used.) If in doubt, take measurements in a hydrometer tube after adjusting the water and hydrometer to the calibrated temperature.

The plastic Sea Test hydrometer (HYD10) is temperature compensated for the 60-90°F range. These low cost instruments are calibrated to ± 0.001 SG in the mid range.

Refractometers are also density-based. They provide fast, accurate (± 1 ppt) salinity measurements. Generally, temperature-compensated models are suggested. Cost and care are the biggest drawbacks. Calibration using certified seawater is best; however, fresh water is a reasonable alternative for general aquaculture uses. Electrical conductivity is another method of determining salinity. Conductivity measurements are best at 25°C and any difference from this must be compensated for. If there is a question about temperature compensation, simply take a reading, change the temperature of the water by 5 or 10 degrees, and measure again. It should give the same salinity. Some units give readings in milliSiemens (MS) and require a salinity conversion table. Others read directly in salinity. Regular calibration with a standard solution is recommended.