Reef tank gadgets are always popular

The Amazingly Simple Aquarium Refractometer

One of the items on my wish list this past year was a refractometer.

Easy to use refractometer

For years and years, I have always used one of those swing-arm style hydrometers (specific gravity meters)…and I was convinced that refractometers weren’t worth all the hassle. Well, this year I put the old refractometer on the list and my wish came true. And with a bit of trepidation, I unboxed it.

refractometer unbox

The first thing I saw was a screwdriver…which made me grumble under my breath (assuming it required assembly).

refractometer open
the screwdriver isn’t that intimidating…but I didn’t want to have to assemble anything

The instructions were carefully stowed (hidden) under the foam insert attached to the lid.

But I was amazed at how easy it was to use.

how to use a refractometer
easy to use
  • Step 1 Use the eyedropper to get a small sample of water
  • Step 2 Lift up the plate at the top.
  • Step 3 Place 2 drops on the prism
  • Step 4 Put that plate thing down
  • Step 5 Look through the eyepiece
  • Step 6 Read the measurement (it is the line where the blue and white fields converge)

how to read a refractometer

Right out of the box, without calibration, the piece gave me the same reading as my good old fashioned swing arm specific gravity meter. I was blown away at how easy it really was. It took 2 drops of water and 30 seconds. I’m not sure if you’re like me and ever thought you didn’t want to be bothered…rest assured, refractometers are super-easy to use and I’m not a handy sort of guy.

Despite how easy they are to use, refractometers make use of some pretty cool science.

How a refractometer works

The refractometer bends light, man, it’s out of this world. But seriously, it does. The business-end of the instrument is a prism…and it refracts (bends) the light. Dissolved salt in the water has a direct impact on how much the light bends (refracts). Some smart person carefully measured and figured out the relative refractive indexes at the various salt concentrations we care about–and put a handy-dandy grid on the refractometer. So you can just put a couple of drops on the prism and take your reading.

To avoid getting that rainbow effect (like on the cover of a Pink Floyd album), the Prism actually restricts the light getting in (which is why it’s only blue and white). So based on the refractive index of the light traveling through your water sample (the meager two drops), you are able to determine, with a fair degree of precision, what the salinity and specific gravity of your aquarium water are. Pretty cool.

Why use a refractometer?

Having a clear understanding of the salinity of your aquarium water is extremely important. You generally want to keep your salinity (the measurement of salt content in your water) around 33 ppt. That corresponds (under normal conditions) to a Specific Gravity (sometimes abbreviated OG) of 1.025. The two most common methods of measuring this are with a specific gravity meter or a refractometer.

In some respects here, I’m just passing on some myths that I’ve read before, so take it with a grain of salt…but I’ve read before that the swing-arm style hydrometers are notoriously inaccurate (or at least run the risk of being inaccurate). I can speak from first hand experience that bubbles get under the arm (affecting the reading) and the arm gets stuck a lot, but all-in-all I was surprised to see they both provided the same reading in my own individual experiment (n=1).

So…the real reason to use it is clearly to be cool. Let’s face it, if you’re in this hobby, you’re probably a gadget junkie–so get your fix and get a refractometer. I was also surprised at how reasonably priced they are. The one I got was about $25 bucks. Can’t beat that with a stickfish…can’t beat that with an acropora frag…ah, never mind.

In case you want to know, here
is the one my wife got me (it’s an affiliate link to amazon):

And here is Dark Side of the Moon, a great album to listen to while you’re bending light.






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