Measurement

Audit an aquarium conductivity-to-salinity conversion

Reproduce a declared PSS-78 calculation with the correct temperature and pressure basis, then keep numerical reproducibility separate from sample-matrix applicability.

By FishyMag · Published November 12, 2026

Worked arithmetic example · Not a clinical review or treatment recommendation.

Quick answer

A salinity algorithm can reproduce a number without establishing that the number answers the question for your aquarium’s ionic mixture. In the declared reference example, 42.914 mS/cm at 14.99640086 °C ITS-90 and zero sea pressure gives Practical Salinity approximately 35. The calculation does not independently validate a non-seawater mixture.

AI-generated illustration of two different probes mounted in one aquarium water compartment; no conversion inputs or result can be read from the image. — AI-generated editorial illustration. Also used in related FishyMag guides. Not a photograph of FishyMag testing or a treatment demonstration.

Reproducing the calculation does not validate every water mixture. The figure’s data and explanation are available below. — Original FishyMag explanatory diagram. Worked cases are identified in the article. Manufacturer statements and published study values retain their cited scope. The illustration is not a product test or treatment instruction. View full-size image · Download infographic (SVG)

Identify the documented algorithm rather than guessing from a label

YSI lists PSS-78 as the salinity method for CastAway-CTD. That gives the reader a named scale to investigate. This example uses the independently documented GSW implementation; it does not claim bit-for-bit reproduction of YSI firmware. The GSW function takes conductivity in mS/cm, temperature on ITS-90 and sea pressure in dbar. Feeding it a conductivity already normalized to another temperature while also supplying the original in-situ temperature would describe a different input packet.

Reproduce one fully declared conversion

The reference temperature is 15 °C on IPTS-68, expressed as 15/1.00024 = 14.99640086 °C on ITS-90. Local GSW 3.6.23 returns 34.9999999241, rounded here to 35.0000. Six official GSW examples were independently reproduced to numerical precision before constructing this case. Those software checks are not physical meter tests.

Reproduce one fully declared conversion: worked table
Input or output	Declared value	Audit result
Conductivity	42.914 mS/cm	Unit matches function input
Temperature	14.99640086 °C ITS-90	15 °C IPTS-68 converted explicitly
Sea pressure	0 dbar	Not entered as one atmosphere in dbar
Implementation	GSW 3.6.23 SP_from_C	Original local calculation retained
Output	SP approximately 35.0000	Numerically reproduced, not an aquarium suitability claim

Make the second packet an applicability decision

A fictional second sample has the same measured conductivity but a documented non-seawater ionic composition. The function can still return a number. That does not establish its equivalence to the sample’s salt mass fraction or validate the intended aquarium interpretation. Keep algorithm output, reported quantity and matrix coverage in separate fields. The published GSW PSS-78 description also states a 2 < SP < 42 range and a modified low-salinity formulation below 2; a software extension is not blanket validation of every freshwater or brackish mixture.

Do not replace one out-of-domain calculation with another

The same audit applies when someone proposes transferring a freshwater conductivity-to-solids regression into brackish water. Recover its calibration population, conductivity and composition range, and actual quantity predicted. A computable extrapolation is not validation. This guide supplies the completed algorithm-path example, not an invented freshwater regression or a new true salinity for the second sample. Request the missing domain evidence instead of borrowing a familiar conversion factor.

Use your own details

Audit an aquarium conductivity-to-salinity conversion worksheet

Download the editable text file, fill it in on your device and keep it with the relevant record. This blank worksheet records your information; it does not verify entries or approve treatment.

Instrument model and firmware or reporting documentation
Measured conductivity with mS/cm versus S/m basis
In-situ versus reference-corrected conductivity
Temperature value and ITS-90 or IPTS-68 convention
Sea pressure versus absolute pressure
Algorithm, version and numerical range
Matrix or composition applicability evidence
Computed output and separately bounded validation claim

Download worksheet (.txt)

Worked example files

These files contain the article’s worked example. Read its source and scope notes, and keep your own current records before opening an example in a tool.

Download the input and implementation checks

Put it into practice

Your checklist

Match conductivity, temperature and pressure conventions before calculating.
Record numerical reproduction separately from matrix applicability.
Withhold an interpreted sample quantity when its conversion domain is unsupported.

Keep the original report basis →

Sources and scope

This article distinguishes manufacturer statements from FishyMag’s own workflows and arithmetic examples. Linked formulation records retain their own evidence dates; publishing this article does not refresh those records.

AI-assisted writing and development. Not hands-on product testing or veterinary review. How we publish →

FishyMag reports what manufacturers say, quoted and linked. It does not diagnose illness or recommend a treatment, and it never infers that a product is safe from the absence of a warning — where no statement exists, it says so.

Formulations change. Always read the label on the bottle in your hand, and contact the manufacturer if your tank contains anything valuable or unusual.