Salinity vs Specific Gravity: PPT, PSU, SG & Conductivity for Reef Tanks

Reef keepers often describe the same water as:

• 35 ppt;
• about 35 practical salinity / PSU on many hobby displays;
• around 1.026 specific gravity on a reef instrument;
• roughly a seawater-range conductivity value;
• or a value derived from refractive index.

Those numbers are related, but they are not interchangeable labels for one direct measurement.

The instrument may be sensing conductivity, refractive index or density and then converting that result to a display unit.

For hardware selection, use the aquarium salinity tester guide. For calibration details, use the reef refractometer calibration guide. If the tank itself needs adjustment, use reef salinity too high or low.

The terms at a glance

Term	What it represents	Typical instrument path	Main interpretation issue
salinity / ppt	salt-content style concentration convention	direct label or converted value	hobby “ppt” and practical salinity are often mixed conversationally
practical salinity	conductivity-ratio based PSS-78 quantity	conductivity meter / algorithm	formally unitless even though “PSU” is widely written
specific gravity (SG)	density ratio relative to a reference	hydrometer or calculated display	depends on reference temperature / scale convention
conductivity	ability of water to conduct electricity	EC/conductivity probe	strongly temperature dependent, usually compensated
refractive index	optical bending property	analog/digital refractometer	converted to salinity or SG by an instrument scale/algorithm

1. Salinity in ppt: useful shorthand, but define what the instrument means

Historically, ocean salinity was described as grams of dissolved salts per kilogram of seawater, which maps naturally to parts per thousand (ppt).

In aquarium use, “ppt” is also commonly printed on refractometers and electronic salinity testers.

The complication is that a modern instrument may not literally weigh dissolved salts. It may measure:

• conductivity;
• refractive index;
• density/buoyancy;

and then convert that measured property into a salinity-style number.

So a display that says 35 ppt is the output of a measurement model, not proof that the instrument directly measured 35 grams of dry salt.

2. Practical Salinity and PSS-78

Modern oceanographic Practical Salinity is defined through the Practical Salinity Scale 1978 (PSS-78).

NOAA reference material explains that practical salinity is calculated from a conductivity ratio relative to a defined reference solution.

That means Practical Salinity is formally unitless.

“PSU” — practical salinity units — is still widely used on instruments, data systems and aquarium documentation because it is convenient, but the PSS-78 quantity itself is not a mass unit such as grams per liter.

This is why ppt and PSU are not formally the same definition.

Around normal seawater, however, their displayed values are often numerically similar, and aquarium manufacturers may let one device show either value.

Treat that numerical similarity as a practical convenience, not as proof that the definitions are identical.

3. Conductivity: the electrical route to salinity

Seawater contains dissolved ions that conduct electricity.

A conductivity meter measures electrical conductivity and uses temperature information plus a conversion model to report salinity.

The Hanna HI98319 is a clear aquarium example: it measures conductivity and can display the result as:

• ppt;
• PSU;
• S.G.;
• plus temperature.

Hanna also specifies automatic temperature compensation and a 35.00 ppt calibration standard for this model.

That does not mean ppt, PSU and SG are three separate sensor measurements. They are alternate outputs derived from the meter’s conductivity-based measurement system.

4. Refractive index: the optical route

A refractometer uses the way light bends through the sample.

The Milwaukee MA887 measures refractive index and temperature, then converts the result to:

• PSU;
• ppt;
• S.G. (20/20).

That last label matters.

“S.G. (20/20)” carries a reference convention in the name. The display is not simply a temperature-free universal number.

Analog seawater refractometers do the same general job with an optical scale rather than a digital algorithm.

5. Specific gravity is a ratio with reference assumptions

Specific gravity compares the density of the sample with a reference density.

That sounds simple, but aquarium confusion starts when a bare number such as “1.026” is quoted without stating:

• the reference temperature;
• the hydrometer/refractometer calibration convention;
• whether the scale is for real seawater or sodium-chloride brine;
• whether the instrument applies automatic temperature compensation;
• what the instrument manufacturer means by its SG scale.

Because of those assumptions, there is no universal SG-to-ppt conversion that should be applied blindly to every instrument.

6. The D-D 35 ppt example shows the problem

D-D The Aquarium Solution documents the issue particularly clearly.

Its seawater refractometer is calibrated at 20°C. D-D states that 35 ppt seawater corresponds to about 1.0266 on that instrument’s SG scale.

D-D then compares the same salinity with approximately 1.0264 for a standard hydrometer referenced at 25°C.

The salt content did not change.

The displayed SG relationship changed because the reference convention changed.

That is why “35 ppt = 1.0264” needs context rather than being treated as a universal identity.

7. Temperature can change the proxy without changing salt mass

If a sealed water sample warms up, no salt has magically entered or left it.

But temperature changes physical properties used by instruments:

• density;
• refractive index;
• conductivity.

So temperature can change a raw measurement or the relationship between that measurement and a displayed SG/salinity value.

This is why modern meters and refractometers publish:

• calibration temperature;
• automatic temperature compensation range;
• sample-equilibration instructions;
• reference-temperature notation.

ATC is useful, but it does not erase the need to understand the instrument.

8. PPT versus PSU in aquarium practice

A practical way to read aquarium documentation is:

• ppt: common salinity-style display and historical mass-based language;
• PSU / Practical Salinity: PSS-78 conductivity-based practical salinity convention;
• near seawater salinity, the numbers are often close enough that hobby discussion may use them loosely.

But when accuracy matters, follow the instrument manual.

NOAA oceanographic references distinguish the historical ppt concept from PSS-78 practical salinity. Aquarium manufacturers likewise document their own conversion algorithms and display labels.

9. SG versus density

Specific gravity and density are related, but they are not synonyms.

Density has units such as kg/L or g/cm³.

Specific gravity is a ratio and therefore has no physical unit.

A hydrometer scale effectively turns buoyancy/density behavior into a specific-gravity reading under its design/reference assumptions.

This is another reason a density table, an SG scale and a salinity table cannot simply be mixed without checking the reference conditions.

10. Why two instruments can disagree even when both are working

Two salinity instruments can differ because they use different:

• physical measurement methods;
• calibration standards;
• temperature compensation;
• SG reference conventions;
• seawater versus NaCl algorithms;
• resolution and accuracy specifications.

Before deciding one is wrong:

1. calibrate each instrument according to its own manual;
2. clean the sensor/prism;
3. let temperature stabilize as instructed;
4. test the same representative sample;
5. compare the published accuracy limits.

Use the reef refractometer calibration guide for that workflow.

11. Do not convert blindly

Do not convert blindly from a random SG chart to ppt, or from ppt to SG, when you do not know the chart’s assumptions.

Before converting, identify:

• instrument type;
• manufacturer;
• SG reference/calibration temperature;
• seawater versus NaCl scale;
• whether the value is measured or already converted by firmware;
• the instrument’s calibration state.

A generic linear “SG = 1 + ppt × constant” shortcut may look convenient but hides the temperature/reference behavior that causes much of the aquarium confusion.

For that reason PetGearReport does not publish a context-free SG↔ppt formula here.

12. 35 ppt examples: what is safe to say

A useful reef statement is:

35 ppt seawater is commonly displayed around SG 1.026 on aquarium instruments, but the exact SG number depends on the instrument’s reference convention.

A more precise manufacturer example is D-D’s:

• 35 ppt → about 1.0266 on its 20°C refractometer scale;
• the same salinity → about 1.0264 on a 25°C-referenced hydrometer example.

That conveys the relationship without pretending one SG number is universal.

13. Which display should you use?

For routine reef keeping, consistency is usually more useful than switching units constantly.

Choose the unit that:

• your instrument documents clearly;
• your salt manufacturer or husbandry references use;
• lets you verify calibration confidently;
• you can record consistently over time.

PPT/practical salinity is often easier for cross-reference because it avoids some of the SG reference-temperature ambiguity.

SG remains completely usable when you understand the instrument scale.

14. How this connects to reef maintenance

Mixing new saltwater

Use the reef saltwater mixing guide. Measure the finished batch rather than relying only on dry-salt mass.

Correcting a salinity error

Use reef salinity too high or low. Verify the measurement before making a large correction. For transparent exchange-volume math after verification, use the reef salinity adjustment calculator.

Managing evaporation

Evaporation removes water while dissolved salts remain. A correctly configured aquarium auto top-off system replaces the missing freshwater and helps keep salinity stable.

Quick interpretation table

You see	Ask
35 ppt	Is this a manufacturer’s ppt conversion or another salinity definition?
35 PSU	Is the device reporting PSS-78 practical salinity?
SG 1.026	What is the reference temperature / instrument scale?
53-ish mS/cm	Is the conductivity raw or temperature-compensated, and what salinity algorithm is used?
two different readings	Are both instruments calibrated and using comparable reference assumptions?

Sources and methodology

PetGearReport has not physically compared hydrometers, refractometers and conductivity meters against a laboratory salinometer, measured temperature-dependent density, validated manufacturer conversion algorithms or created an original SG↔ppt conversion table.

This guide uses oceanographic definitions plus current manufacturer documentation to explain why aquarium instruments can express related seawater measurements in different ways.

• NOAA — Ocean Explorer salinity/conductivity educational material and NOAA practical-salinity datasets — historical ppt context, conductivity-based modern salinity measurement and practical salinity reporting.
• NOAA / PSS-78 reference material — Practical Salinity defined through conductivity ratio and formally unitless.
• D-D The Aquarium Solution — Refractometer instructions — 35 ppt SG examples at 20°C and 25°C reference conventions and explanation of calibration-temperature effects.
• Hanna Instruments — HI98319 Marine Salinity Tester and manual — conductivity measurement, ppt/PSU/S.G. display modes, automatic temperature compensation and 35.00 ppt calibration.
• Milwaukee Instruments — MA887 instruction manual — refractive-index measurement and conversion to PSU, ppt and S.G. (20/20).

Follow the exact instrument manual whenever its scale, temperature reference or conversion method differs from a generic aquarium chart.

Frequently asked questions

Are ppt and PSU the same thing?

They are often numerically similar around normal seawater and aquarium instruments may offer both displays, but they are not formally the same definition. Practical Salinity under PSS-78 is derived from a conductivity ratio and is unitless, while ppt historically describes parts per thousand by mass.

Does 35 ppt always equal 1.0264 specific gravity?

No universal statement is safe without the reference assumptions. D-D shows the same 35 ppt seawater as about 1.0266 on its 20°C-calibrated refractometer scale and about 1.0264 for a 25°C-referenced hydrometer. Check the instrument scale and reference temperature.

Why do salinity meters show ppt, PSU and specific gravity from one sensor?

A meter can measure one physical property and mathematically convert it to several display units. Hanna HI98319 measures conductivity and can display ppt, PSU or S.G.; Milwaukee MA887 measures refractive index and converts that optical measurement to PSU, ppt or S.G. (20/20).

Does temperature change the amount of salt in the water?

Temperature alone does not add or remove salt from a closed sample, but it changes density, refractive index and conductivity. That is why instrument temperature compensation, calibration conditions and SG reference temperature matter.

Which unit should I use for a reef tank?

Use the unit your correctly calibrated instrument documents clearly and stay consistent. PPT or practical salinity can be easier to compare across references, while SG remains useful when its instrument/reference assumptions are known.