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How to use a salinity refractometer

Updated: 22 October 2026

You have an optical salinity refractometer on the bench, or one arriving from amazon.co.uk, and the first number you need is a zero you can repeat. A fresh-water zero does not prove the 35 ppt end of the scale, and a fuzzy or tilted line is a fault rather than a knack you have not learned. This is how to use a salinity refractometer with the wide 0–100 ppt meters in the [salinity meter comparison](/hub/measuring-equipment/salinity-meter), using a Red Sea step order and D-D's brine warning. Products Compared did not run a lab. Listings were read on 21 October 2026. A qualifying purchase can earn a commission at no extra cost to you.

You have an optical salinity refractometer on the bench, or one arriving from amazon.co.uk, and the first number you need is a zero you can repeat.

Our top picks

Agriculture Solutions Agriculture Solutions WL0020-ATC

Best overall

Agriculture Solutions Agriculture Solutions WL0020-ATC
9.0/10about £13.99
View offer
UIUZMAR UIUZMAR UI-09A pen

Alternative

UIUZMAR UIUZMAR UI-09A pen
7.6/10about £29.99
View offer
XRCLIF XRCLIF refractometer

Best value

XRCLIF XRCLIF refractometer
8.1/10about £13.99
View offer
On this page
  1. 1.Before the first drop
  2. 2.Zero on fresh water
  3. 3.Check near seawater if the tank is a reef
  4. 4.When the reading is not usable
  5. 5.A short branch for pens

Before the first drop

Let the meter and the water reach the same temperature. A UK review of the Agriculture Solutions WL0020-ATC says the fresh-water zero has to match the sample, even when temperature compensation is claimed. XRCLIF prints ATC as 10–30 °C. Do not paste a Red Sea or D-D window onto every ASIN here. Rinse the prism with RO or deionised water and dry it. Tap water can shift the zero. Do not dunk the body. These listings are drop meters.

Zero on fresh water

Put two or three drops of RO or deionised water on the prism, close the cover without bubbles, and wait about 15 seconds, which is the Red Sea manual's pause. Focus until the boundary between the light field and the dark field is sharp. On a zeroed meter it sits on specific gravity 1.000, or 0 ppt. Turn the calibration screw until the line rests there, wipe the prism, and read fresh drops again. Bring the tank sample to the same temperature before you compare. A cold zero against a warm sample invents a swing. One XRCLIF review saw the fresh reading wander from 1.000 to 1.007. Zero that meter immediately before the sample.
Agriculture Solutions Agriculture Solutions WL0020-ATC

Testsieger

Agriculture Solutions Agriculture Solutions WL0020-ATC
9.0/10about £13.99
View offer

Check near seawater if the tank is a reef

Fresh water only sets the offset. It does not prove a natural-seawater scale at 35 ppt. D-D says 35 ppt seawater can match about 36.5 ppt on a brine scale. That is not an instruction to aim every 0–100 ppt meter here at 36.5. These listings do not say which scale they use. If animals depend on the number, check near the salinity you keep. Marine Aquatics sells Fritz salinity calibration solution at £7.99 for 60 ml, printed at 35 ppt and specific gravity 1.0264 at 25 °C. Pick that reference or another named one, and keep using it. Rinse and dry the prism. The unit split is on the aquarium and pool page.

When the reading is not usable

A tilted line, a scale you cannot focus, or a zero the screw will not reach is a fault. HXTDBY has a UK review that could not get below specific gravity 1.005 on RO water. You cannot subtract your way to a true zero from a line that will not move. Go back to the seller under the Consumer Rights Act 2015 if the meter cannot show its scale. If the line is sharp, the zero repeats, and a check fluid lands where that fluid says it should, you can mix with it. If livestock are already failing and a second careful reading disagrees, stop changing salt and speak to a fish-health professional.

A short branch for pens

A conductivity pen is not zeroed with that screwdriver. UIUZMAR and YINMIK listings talk about pH buffers at 4.00, 7.00 and 10.01, and salinity is a separate conductivity reading. Dip only the probe, then rinse it. These pages do not state a waterproof rating. A pen that holds pH and still misses reef salinity, which one UIUZMAR review reported, has told you the limit of that channel. The method comparison is on the refractometer versus digital page.

Common questions

Do I calibrate with tap water or RO water?
Use RO or deionised water for the zero. The Red Sea manual specifies deionised water, and tap water can sit off the mark because of dissolved minerals. Rinse the prism with that same fresh water before the tank sample, then dry it, so the sample is not diluted by the rinse.
How long do I wait before reading?
The Red Sea manual says about 15 seconds after the drops are on the prism. Give the boundary time to settle, then focus and read. If you are matching temperatures, the wait that matters more is the time the water and the meter spend reaching the same room, which is the habit D-D describes for their own instrument near 20 °C.
Should I dip the refractometer in the tank?
No. Put two or three drops on the prism. The optical meters in this set are not described as dip probes, and the Red Sea manual tells you not to immerse the body. A pen is the tool you lower into the water, and only the probe end.
What if it will not reach zero?
If the screw cannot put the boundary on 1.000 or 0 ppt with RO water, the meter is not usable. HXTDBY has a UK example stuck at specific gravity 1.005. Return it rather than applying a homemade correction. A zero that moves between 1.000 and 1.007, as one XRCLIF review found, needs a fresh calibration immediately before every reading you keep.
Do I set it to 35 ppt or to 36.5 ppt?
Zero it to fresh water. Do not set the screw to 35 or to 36.5. D-D says 35 ppt seawater can read about 36.5 ppt on a brine-scaled hobby meter. A seawater-scaled D-D or Red Sea is read as 35 after a normal zero. These 0–100 ppt listings do not say which scale they are, so do not add 1.5 ppt unless you have checked that meter against a known seawater reference.

Related pages

Salinity meter buying guide

Salinity meter

Aquarium salinity meter vs pool salt tester targets

Best salinity meter for a home reef or a salt pool

Refractometer vs digital salinity meter on your reef

Sources

Public UK sources from the research digest, last checked on 21 October 2026. Shelf prices and star lines are the amazon.co.uk figures captured with these twelve listings. The price bands are this set, not a market average.

  1. Hanna Instruments UK: HI-98319 salinity tester (retrieved 21 October 2026)
  2. Practical Fishkeeping: D-D seawater refractometer (retrieved 21 October 2026)
  3. D-D The Aquarium Solution: salinity and specific gravity (retrieved 21 October 2026)
  4. Hayward UK: salt concentration for salt chlorination (retrieved 21 October 2026)
  5. Camlab: measuring the salinity of seawater (retrieved 21 October 2026)
  6. HORIBA UK: aquarium water testing (retrieved 21 October 2026)
  7. Finest Aquatics: Tropic Marin high-precision hydrometer (retrieved 21 October 2026)
  8. GOV.UK: Measuring Instruments Regulations 2016 (retrieved 21 October 2026)