Oceanlife Water Salinity Check









PSU (g/kg - ppt - ‰)
g/l (value read on the densimeter) (?)
(value read on the refractometer) (?)
mS/cm (value read on the conductivity meter)


Instructions for the correct measurement of salinity by using the refractometer (with and without automatic temperature compensation - ATC) or the densimeter or conductivity meter.

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 the temperature. It is the most important parameter.
Sea water has a salinity of 35 PSU.
The biggest mistake we can make is to confuse the salinity measured in PSU, then in g/kg, with the density measured in g/liter.
The most common mistake that is made is to salt the water according to the liters of water and not based on the Kg of water. This raises doubts about the quantity of salts needed to properly salt the water.

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.

Interpretation errors
  1. Having a water at 35 PSU means that the water has a salt content of 35g per kg of water, not every liter.
  2. One liter of sea water at 25°C weighs 1.02334 kg, thus containing 35.82 g of salts. As the density depends on the temperature, to have a salinity of 35 PSU, the quantity of salts per liter of water must also be slightly different depending on the water temperature.
  3. The only unit invariant with the temperature is the salinity expressed in PSU (g/kg), since the weight does not change with the temperature.


Measurement tools

Densimeter

Until some time ago, the most used method was that of the densimeter. Through a floating pole, thanks to the principle of Archimedes that regulates the float of the bodies, the higher the density, the more the shaft of the densimeter is pushed out of the water. A graduated scale indicates the density of the water.
This method has the defect that the measurement carried out depends on the temperature, so it takes some methods to convert the result obtained from the calibration temperature of the densimeter, to the water temperature that we have measured.

Refractometer

It is the currently most used method. Thanks to the principle of refraction, a ray of light is refracted through salt water with an angle that is dependent on salinity and temperature. The temperature of the instrument and that of water are the parameters that most influence the measurement.
Most of the refractometers on the market should have automatic temperature compensation, which means that, once calibrated to the temperature indicated in the instructions, the salinity measurements are brought back to that temperature, generally between 10 and 30°C.
For example, if the instrument is at 20°C and the calibration solution at 20°C, the reading will be reported as if the measured water were at 20°C for each sample temperature between 10 and 30°C.

Conductivity meter

It is the most widely used method in industry or for process automation or in aquarium computers. Conductive meters usually have a temperature compensation but it is not optimized for sea water measurement unless explicitly specified. In this regard it is necessary to correct the reading to obtain the correct salinity value.

Measurement errors

Behind the apparent simplicity of use of the refractometer, there are insidious errors that lead to completely incorrect readings of salinity, resulting in a more than significant change in the parameters of the water.

  1. Many refractometers on the market do not have temperature compensation or compensation does not work. To find out if your refractometer is (yet) compensated, take some water from the aquarium and measure it. Then place both the water and the refractometer in the fridge for a few minutes (do not run the refractometer and the water below 10°C) and repeat the operation. If the two measures are different, then the refractometer is not compensated in temperature, so you will have to correct the measurement according to the temperature at which you calibrated the refractometer.
  2. Refractometer calibration. Although in the instructions it is reported to calibrate the instrument with pure water bringing the reading to zero, it is better to calibrate the instrument with a solution of 35 PSU. This will reduce the measurement error, since the instrument is calibrated around the measurement we will carry out.
  3. Calibration temperature. An instrument with ATC (automatic temperature compensation) shows the salinity at the calibration temperature. A tool without ATC needs manual compensation.
  4. Sample temperature. Take water from the aquarium at 24 ° C and place it on an instrument at 20 ° C will bring the water quickly to a temperature between 24 and 20 ° C, indefinite. It is better to take some water and wait for it to reach the instrument temperature (environment), in this way we will have almost the certainty of the measured water temperature, being able to make the necessary corrections in a sensible way.
  5. The measurement scale and conversion from PSU to specific gravity that appears within the refractometer only applies to the reference temperature of the refractometer, in almost all cases at 20°C. The wording "d 20/20" means that the measurement and conversion scale is valid for samples measured at 20°C and for a calibration of the instrument performed at 20°C, which leads to the equivalence of 35 PSU to an SG of 1.0266.
  6. The index of refraction also depends on the composition of the water. Unfortunately this is a parameter of not easy predictability, for this reason, the reading made by refractometer will always have a certain degree of uncertainty.
To understand the extent of the most common errors, let's take a few examples.

Example 1 - Calibration of the refractometer made in winter at 20°C and measurement done in summer with an instrument at 27°C

Salinity sample
Instrument temperature
Calibration temperature
Salinity measured
35 PSU
27°C
20°C
32.4 PSU

We take an instrument without ATC, a refractometer at 27°C and a calibration sample at 35 PSU. If the refractometer has been calibrated at 20°C and has no ATC, instead of scoring 35 PSU will score 32.4 PSU. The result is logical as the water expands as the temperature increases, so the "apparent" salinity will be lower.
In the first measure, with a measure of 32.4 PSU, we would be led to put more salt, with the consequence of salting the water too much.

Example 2 - Calibration of the refractometer made in summer at 27°C and measurement done in winter with an instrument at 20°C

Salinity sample Instrument temperature Calibration temperature Salinity measured
35 PSU
20°C
27°C
37.2 PSU

We take an instrument without ATC, refractometer at 20°C and calibration sample at 35 PSU. If the refractometer has been calibrated at 27 ° C and has no ATC, instead of scoring 35 PSU will score 37.2 PSU. The result is logical because the water contracts with decreasing temperature (up to about 4°C), so the "apparent" salinity will be higher.
In the first instance, with a measure of 37.2 PSU, we would be led to dilute the water, with the consequence of having values of calcium, magnesium and KH well below the norm.

Effects of incorrect salinity on the parameters

An incorrect measurement of salinity leads to errors even macroscopic in the preparation of water and therefore in its composition. Taking again the two previous examples, trying to correct the measured salinity, we would bring the water to a real salinity of 37.6 PSU in the example 1 and of 32.8 PSU in the example 2.

Salinity
37.6 PSU
32.8 PSU
Element
ppm
ppm
Calcium
451
394
Magnesium
1482
1293
KH
8.6
7.5
Potassium
430
375
Strontium
8.6
7.5


Techniques for a correct measurement without ATC

Calibration refractometer at 20°C - classic method

Calibration
  1. Bring the refractometer and calibration solution to 35 PSU at 20 ° C.
  2. Place a few drops of calibration solution on the lens of the refractometer and gently crush with the appropriate cover panel taking care that the solution does not present bubbles and is uniformly distributed over the entire surface.
  3. Calibrate the instrument with the appropriate screw in order to match the reading at 35 PSU (SG 1.0266 at 20°C).
Measurement
  1. Leave the refractometer for at least 10 minutes in the environment in which the measurement is carried out and determine the ambient temperature.
  2. Take a little bit of water to be measured and allow it to bring to room temperature.
  3. Place a few drops of water to be measured on the lens of the refractometer and gently crush with the appropriate cover panel taking care that there are no bubbles and is uniformly distributed over the entire surface.
  4. Read the specific gravity value and enter its value in the program above, inserting the ambient temperature as the instrument temperature and the calibration temperature 20°C.
  5. After clicking on "Calculate", the correct salinity value will appear.

Refractometer calibration at room temperature

This system provides for the calibration of the instrument every time the measurement of salinity is carried out but does not require corrections with the software.

Calibration
  1. Bring the refractometer and the calibration solution to 35 PSU at room temperature
  2. Place a few drops of calibration solution on the lens of the refractometer and gently crush with the appropriate cover panel taking care that the solution does not present bubbles and is uniformly distributed over the entire surface.
  3. Calibrate the instrument with the appropriate screw in order to match the reading at 35 PSU.
Measurement
  1. Take a little bit of water to be measured and allow it to bring to room temperature.
  2. Place a few drops of water to be measured on the lens of the refractometer and gently crush with the appropriate cover panel taking care that there are no bubbles and is uniformly distributed over the entire surface.
  3. Read the salinity value.

Measurement by densimeter

Unfortunately, on the market it is increasingly difficult to find precision densimeters but we believe that it is still the simplest method and that it is less suitable for measurement or interpretation errors..

Measurement
  1. Take some water to be measured and place it inside the densimeter measuring cup.
  2. Place the density meter in the water to be measured and read the density value.
  3. Measure the temperature of the water in which the densimeter is immersed.
  4. Enter the density value read in the program above, entering the measured water temperature and the reference temperature of the densimeter, which is usually 20°C.
  5. After clicking on "Calculate", read the salinity value.

Measurement using conductivity meter

Also this method lends itself to fewer errors compared to the use of the refractometer but the direct reading of the conductivity must be corrected according to the temperature.

Calibration
  1. Calibrate the instrument with a solution close to 53 mS/cm according to the instrument manufacturer's instructions.
Measurement
  1. Read the conductivity value given by the instrument and insert it in the program above, entering the measured water temperature. The calibration temperature is to be considered equal to that of the measured water.
  2. After clicking on "Calculate", read the salinity value.



References:
  1. Unesco – IES-80 - Algorithms for computation of fundamental properties of seawater – 1983
  2. Equation of state of Seawater
  3. Density and the Equation of State
  4. Seawater Density (salinity and temperature effects)
  5. Density of Seawater
  6. A global algorithm for estimating Absolute Salinity
  7. Practical Salinity Scale – 1978


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