Salinity control

The correct way to check salinity

35 PSU is a calibration liquid for refractometers consisting of sea water reproduced in the laboratory.

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Software

Here you can find the most complete salinity conversion software on the internet

salinity converter

methods of measuring salinity

The correct measurement of salinity is of fundamental importance for having stable values of calcium, magnesium, carbonates and in general of all the elelations present in sea water.
The study of the evaluation of the exact density of sea water has been going on for several decades and is a complex polynomial function of salinity, temparature and pressure.
We understand what the main water parameters are and how they interact with one another.

Salinity
To simplify, salinity (S) is the concentration of salts measured in PSU (Practical Salinity Units). It is expressed in g/kg and is invariant with temperature. It is the most important parameter.
Tropical seawater has a salinity of 35 PSU.
One of the most common mistakes is to confuse the salinity which is measured in PSU (therefore in g/kg) with the density which is measured in g/liter.
Although the mistake is minimal, salting water based on liters of water and not based on kg of water is not a scientifically correct practice. This is due to the fact that, like all other materials, water changes volume based on temperature and 1 liter of water weighs differently depending on temperature.
Other units of measurement of salinity are ppt (parts per thousand) or ‰ (per thousand).

Density
Density (D) is a function of salinity (in PSU), temperature and pressure. It is measured in Kg/m3 or g/liter.
With a salinity of 35 PSU and a temperature of 25°C, sea water has a surface density of 1023.34 g/liter.
With a salinity of 35 PSU and a temperature of 20°C, sea water has a surface density of 1024.76 g/liter.
Pure water, at 25°C and atmospheric pressure, has a density of 997.048 g/liter.

Specific Gravity
Specific gravity (SG) is defined as the ratio between sea water density and pure water density. It is a dimensionless measure, so it is not followed by any unit of measurement. It varies with temperature changes.
With a salinity of 35 PSU and a temperature of 25°C, sea water has a specific gravity at the surface of 1.0264.
With a salinity of 35 PSU and a temperature of 20°C, sea water has a specific gravity at the surface of 1.0266.

Conductivity
By simplifying, the conductivity of a fluid is its ability to carry electric charges. The salinity can be calculated starting from the conductivity through a complex polynomial equation that has like other variables the temperature and the pressure.
With a salinity of 35 PSU and a temperature of 25°C, sea water has a conductivity of 53.07 mS/cm.
With a salinity of 35 PSU and a temperature of 20°C, sea water has a conductivity of 47.91 mS/cm.

Examples of measurement errors

Example 1 - Refractometer calibration made in winter at 20 °C and measurement made in summer with instrument at 27 °C

Real sample salinity Instrument temperature Calibration temperature Measured salinity
35 PSU 27 °C 20 °C 32.4 PSU

Let's take an instrument without ATC, refractometer at 27 °C and calibration sample at 35 PSU. If the refractometer has been calibrated at 20 °C and has no ATC, instead of marking 35 PSU it will score 32.4 PSU. The result is logical as the water expands with increasing temperature, so the "apparent" salinity will be lower.
With a measurement of 32.4 PSU, we would be inclined to put more salt, with the consequence of salting the water too much.

Use of the refractometer at room temperature without ATC

Unfortunately the use of the refractometer is the most used system as these instruments are low cost, but it is also the method that leads to more errors.
To avoid errors, our system requires the instrument to be calibrated every time the salinity is measured and does not require further corrections.
Although the refractometer calibration instructions provide for zero point calibration with osmotic water, calibration on 35 PSU leads to more accurate readings.

Calibration

  1. Bring the refractometer and calibration solution to 35 PSU at room temperature, wait until the temperatures have stabilized.
  2. Place a few drops of 35PSU calibration solution on the refractometer lens and gently squeeze it with the special cover panel, taking care that the solution does not show bubbles and is evenly distributed over the entire surface.
  3. Calibrate the instrument with the appropriate screw in order to make the reading coincide with 35 PSU.

Measurement

  1. Take a little water to be measured and allow it to warm to room temperature.
  2. Place a few drops of water to be measured on the refractometer lens and gently squeeze it with the special cover panel, taking care that there are no bubbles and that it is evenly distributed over the entire surface.
  3. Read the salinity value.

Available formats

35 PSU

30 ml
150 ml

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