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Refractometer vs digital salinity meter

Updated: 22 October 2026

You are choosing between a refractometer and a digital salinity meter for a marine tank, and the digital number feels as if it ought to be the accurate one. In this amazon.co.uk set that feeling is the trap. A refractometer reads light through a few drops. A pen infers salt from conductivity, and the pens sold here are mostly 5-in-1 pH meters with a salinity channel UK reviewers do not trust for a reef. Neither is a Hanna seawater tester unless the label says so. The twelve-row table is the [salinity meter comparison](/hub/measuring-equipment/salinity-meter). Listings were read on 21 October 2026. Products Compared did not run a lab. A qualifying purchase can earn a commission at no extra cost to you.

You are choosing between a refractometer and a digital salinity meter for a marine tank, and the digital number feels as if it ought to be the accurate one. In this amazon.co.uk set that feeling is the trap.

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.What each instrument measures
  2. 2.What you maintain
  3. 3.What the reviews in this set show
  4. 4.When to leave both for a named seawater meter

What each instrument measures

An optical refractometer measures refractive index from two or three drops. A conductivity meter is the laboratory habit for true salt, which Hanna's retailers say about Hanna's own method, not about the £14 prisms or £30 pens here. They can disagree, because the methods differ. D-D says a brine-scaled hobby refractometer can sit about 1.5 ppt off natural seawater. The optical titles here say 0–100 ppt. They do not say brine, natural seawater, or PSU, so you do not inherit that correction unless a known sample shows you need it. The steps are on the how to use page. A pen here converts conductivity to salinity beside a separate pH electrode. That is not Hanna's HI-98319, calibrated at 5.00 and 35.00 ppt, at £108.36 including VAT. Do not copy Hanna's IP65 rating onto UIUZMAR or YINMIK.

What you maintain

The refractometer has no battery on the HXTDBY listing, and the other optical rows never mention one. You maintain a clean, dry prism, daylight or a lamp, and a screwdriver. XRCLIF and the Agriculture Solutions WL0020-ATC kit list a case, a pipette or dropper, a cloth and that screwdriver. The ongoing cost is RO or deionised water. You do not immerse the body. The pen wants a wet probe, pH buffers at 4.00, 7.00 and 10.01, and power. UIUZMAR UI-09A puts powder in the box. YINMIK YK-5SEU claims six packs. Waterproofing is not stated. A dried pH electrode fails as a pH meter first. TEKCOPLUS at £115.99 is a Wi-Fi monitor, not a pocket pen, and eight reviews do not make it a Hanna.
Agriculture Solutions Agriculture Solutions WL0020-ATC

Testsieger

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

What the reviews in this set show

Optical complaints are about the view. QWORK buyers report a tilted line. One HXTDBY buyer could not zero below specific gravity 1.005, and one XRCLIF buyer saw deionised water wander from 1.000 to 1.007. If you cannot see the line, stop. Pen complaints hit the salt channel after pH has been praised. On UIUZMAR UI-09A a UK reef reviewer said pH was fine and salinity was not close. On YINMIK YK-5SEU a UK reviewer found salinity off against a Hanna instrument. Use a pen that holds pH for pH. For a monthly marine water change, take the optical meter. For pool ppm, take the pen. Pool targets stay on the aquarium and pool page. Units come first in the buying guide.

When to leave both for a named seawater meter

Leave both when you want conductivity calibrated at 35 ppt. Hanna's HI-98319 is £108.36 including VAT. A seawater refractometer from D-D, TMC or Red Sea is the optical version, at about £35 to £53 in the shop notes. Do not step from a £14 refractometer to a £30 5-in-1 and call it a Hanna. Digits are not the criterion. The criterion is whether you can calibrate at the point you care about. If two careful readings disagree and animals are already failing, ask a fish-health professional rather than buying a third screen.

Common questions

Does a digital number mean it is more accurate?
No. A digital salinity meter in this set converts conductivity with whatever formula the pen uses, and UK reviews show that number missing reef salinity while pH looks fine. A refractometer can be wrong too, especially on a brine scale or with a blurred line. Accuracy here means agreement with a known sample, not the presence of digits.
Why would I still want a refractometer if Hanna exists?
Because a Hanna HI-98319 is £108.36 including VAT and a usable optical kit in this set is £13.99, with nothing to charge. The refractometer is the right tool for a desk check you will zero. Hanna is the right tool when you want a conductivity seawater meter and you will buy the 35 ppt calibration. They are not the same purchase.
Will the pens in this comparison match a Hanna?
The file does not show that they do. One UK YINMIK review said salinity and temperature were off against a Hanna instrument. One UIUZMAR reef review said salinity was not close. Some pond owners are happy with pH and a salt dose they already understand. That is a different standard from matching Hanna at 35 ppt.
Do refractometers need batteries?
HXTDBY states that no battery is needed. The other optical listings do not describe batteries. You need light and a calibration screw. The pens and the TEKCOPLUS monitor are the powered instruments, and the monitor's listing says batteries are not required without explaining the other supply.
What extra consumables does a pen need?
pH buffer, often at 4.00, 7.00 and 10.01, plus whatever storage the probe manual asks for. Powder is in the box on UIUZMAR UI-09A and claimed as six packs on YINMIK YK-5SEU. A refractometer needs RO or deionised water and, for a reef you care about, a check near 35 ppt. Neither consumable fixes a unit that will not calibrate.
Can I immerse either tool?
Put drops on the refractometer. Do not dip the body. Dip the pen's probe, and do not assume the whole pen is waterproof, because these listings do not state a rating. Hanna's IP65 line, including the warning against full submersion, applies to the HI-98319, not to these ASINs.

Related pages

Best salinity meter

Salinity meter

Aquarium salinity meter vs pool salt tester targets

Salinity meter buying guide: choose ppt, ppm or a pen

How to use a salinity refractometer: zero, then read

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)