The Puffer Forum

Still using a creaky old swing-arm box-type hydrometer? Or a delicate glass hydrometer that shatters at a moment's notice?

You know the score: Constant washes in vinegar to keep them working properly: fill to the top / line, poke with stick, water goes everywhere... hope for the best.

Here's me measuring the salinity of my GSP tank... 1.021. Or thereabouts, thanks to the helpfully blunt pointer (NOT). A bit on the low side for my liking, as my GSPs are in full saltwater.



Look at the mess!



There is a better way: Refractometers. These things are just as they sound -- they use a glass prism to refract light. The angle of refraction depends on the salinity of the sample.




Look through the eyepiece to see scales of salinity and specific gravity.



(it is much clearer than this in real life - very hard to get a shot of the scale!)

Let's test the GSP tank again. Very simple to use: Put a few drops on the glass, and close the cover:


Look through the eyepiece:



1.026 - 1.027! Perfect!

Compare the reading: 0.006 difference! that's a huge difference for any marine or brackish tank. Do you have a brackish tank where you are varying the salinity for F-8s or GSPs? Would you be shocked to know that your SG of 1.018 could really be 1.024? Or 1.012?

Now, refractometers, if not calibrated properly, can be as unreliable as hydrometers... but in general, if looked after properly, they are more accurate.... and, as you can see, they are much, much easier to use. IN addition, on auto-temperature compensation refractometers (which is what you're likely to find), temperature calibration is not an issue.

Looking for one more thing to buy for your tank? Look no longer! In my opinion, a refractometer is about the best investment you can make. Since I purchased this about a year ago, I have been much more confident about the salinity of my tanks, and I will never use a swing-arm hydrometer again. And it's a cheap investment, at well under $50.

$42.99 plus shipping here.

Nice article! I think I paid around $35 on Ebay.

Are the electric ones accurate?

Electronic conductivity meters can be accurate, as long as you calibrate them regularly as they tend to drift. I believe they are also affected by alkalinity (?).

I swear by my refractometer. Best investment I've made by far. Swing arms can vary minute by minute, depending on temperature...no thanks!!

Boooooo to hydrometers!

Swing-arm boxes are quick and dirty, good for ballpark estimates. Upkeep and cleaning and wear are all issues there.

Refractometers are not carefree either, nor are conductivity meters. Both devices do usually specity care requirements (glossed over with swing-arms) and calibration requirements (completely ignored by swing-arms), so that is big plus for both, if they are followed.

Any measurement device is only as good as the technique used to operate it. Sloppy technique with any device gives sloppy (read as: inaccurate to worthless) results. GIGO.

I bought my refractometer (new) over ebay for $28. Calibration is totally easy and I'm impressed by the precision of these things. It would be nice to have a refractometer with a smaller range and larger gradations, but this will do!

Finally, I bought one online and recieved it today!

BTW, do you have to use distilled water to calibrate it or can I use deionized?

Someone just replied to my question about distilled water or deionized and deionized is fine apparently.

This is an old post, I have sorted it out a while ago but thanks anyway.

I've been using a swing-arm (instant ocean brand) for a while and recently bought a refractometer from DrsFosterandSmith. Anyway, after calibration, the two are giving me the exact same reading 1.016. I do always rinse my swing-arm hydrometer with tap before and after readings, /shrug.

I'll still be using my refractometer of course, but I just wanted to post my experience so people could make educated consumer decisions. Reading this and other threads made me go "OMG must buy refractometer NOW..."

-Pyratt Pete

I think they are important. Too many cases of false readings, and temp adjustments are a PITA.