About this article
This article will explain why there can be differences between the ProfiLux calculated values of density and salinity from your conductivity probe and other conductivity – salinity/density calculators.
In order for the ProfiLux to provide the most accurate conductivity values, the respective sensor must be calibrated regularly. More info on Conductivity probe calibration and care can be found here.
Understanding how salinity and density is measured
In order for an electronic instrument to display the salinity and density of water, it must first measure the conductivity of water and its temperature for compensation. These two components of measurement are necessary because salinity and density of water are usually not measured directly with an electronic instrument.
Once these two components are considered, we can assume a salinity and density value.
Why is this assumed?
These values are “assumed” because there is no precise mathematical formula to convert conductivity to salinity and density. The conversion respectively a formula or table will only be 100% correct for a specific “water mix”, or to be more precise for only one specific ion combination.
What do these ion concentrations have to do with measuring salinity and density?
Every every aquarium contains different unknown combinations of ions, such as Cl, Na, Mg, etc. Each ion will also have its own specific weight. These two points are important to consider because different ion combinations might produce the same conductivity while having different weights, therefore resulting in different salinity and density.
For example, if you take a sample of water from two different aquariums with the same conductivity, both samples may display different salinity and density values because each water sample will contain a different combination of ions.
Calculators vs. ProfiLux measured values
Now that we’ve explained how varying ionic combinations in water play a role in the calculated value, we can explain why there are differences between the ProfiLux value and salinity/density calculators.
When you use a calculator to convert conductivity into salinity and/or density, it is important to know that these tools do the conversion using one specific sea water composition. In other words, these calculators do the conversion assuming one specific ion mix. Different calculators may also produce different results.
The calculator provides you with a value that DOES NOT consider the specific ion mix of your own water.
As soon as the actual measured water differs from the mix, which has been used as a basis for the conversion, you will see different results.
How does GHL take these varying differences into account?
GHL analyzed several calculators and tables which convert conductivity into salinity and density and created a formula which lies somewhere in the average of all them and is very close to the conversion table of “Copenhagen Standard Sea Water”.
That way, the conversion works best for the average sample and the deviation to other waters is as minimal as possible.
If a user is sure that the actual density reading of the ProfiLux is a bit different to reality, he can fine-tune the reading by entering an offset in the conductivity settings page.
To learn more about this standard, click here.