The salinity of freshwater is less than 0. 5 parts per thousand (ppt), while the salinity of saltwater is about 35 ppt. [1] X Research source For a cheaper, but somewhat less accurate, method, try a hydrometer. If you are measuring the salinity of soil, use a conductivity meter.
Note: Salt refractometers are used to measure sodium chloride dissolved in water. Seawater refractometers are used to measure the mixture of salts typically found in seawater or saltwater aquariums. Using the wrong one may result in a reading approximately 5% off, which may be acceptable for non-laboratory purposes. [2] X Research source Refractometers are also designed to compensate for the expansion of a specific material due to temperature changes. [3] X Research source
Note: If you have not yet used your refractometer, you may wish to calibrate it first for a more accurate reading. This process is explained at the end of this section, but you may wish to read the following steps first so you are more familiar with how to work the device.
A damp tissue may work if you don’t have a cloth flexible enough to reach the whole surface of the tiny prism.
A new, high-quality refractometer may only need calibration once every few weeks or months. A cheaper, older refractometer may need to be calibrated before each use. Your refractometer may come with calibration instructions that specify a certain water temperature. If none are included, use room temperature distilled water.
This method cannot be used for solid material. If you are measuring the salinity of soil, see the conductivity method instead. For a more accurate measurement, use the inexpensive evaporation method, or the faster refractometer method.
For a slightly more accurate reading, you may heat or cool the sample to the temperature the hydrometer was built for. Be careful not to overheat the water, as steaming or boiling can significantly alter the specific gravity.
Do not submerge glass hydrometers entirely, as this can mess up the reading.
The water curve is called a meniscus, and is a phenomenon caused by surface tension, not salinity. [8] X Research source
This table is for hydrometers calibrated at 77 °F (25 °C) The water sample temperature is given in ºC. These charts and calculators will also vary based on the liquid, but the vast majority are used for salt water.
Some aquarium hobbyists like to use both a conductivity meter and one of the other tools described on this page, in order to confirm their salinity reading.
S=Siemens, an electrical conductivity unit. mS=milliSiemens
In laboratory conditions, you may be required to let the mixture settle for thirty minutes, or use a more accurate “saturated soil paste” method which can take over two hours. These are rarely done outside of a laboratory environment, however, and the method above is still reasonably accurate.
If your EC meter has neither of these features, it may come with a chart which you can use to manually correct the reading based on the water’s temperature. [10] X Research source
Respectively, these units stand for milliSiemens per centimeter, deciSiemens per meter, or millimho per centimeter. The mho (inverse of an ohm) is an old-fashioned name for the Siemens, but is still used in some industries. [11] X Research source
Note: Whenever looking up EC ranges for a particular plant, find out which method that source used to test the EC. If the soil is diluted with 2 parts water, or with just enough water to make a paste, instead of the 1:5 ratio used here, the numbers may be significantly different.
Some calibration solutions may come with a “check solution” to test after calibration. If the check solution’s conductivity is inaccurate, your EC meter may be broken.
