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Swing arm hydrometer vs. refractometer vs. floating hydrometer


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Thought this might be an interesting thread since it comes up pretty frequently. There are plenty of posts here on specific gravity and salinity, so, what about measuring it? We all know that there are many levels that are acceptable to keep one's marine tank specific gravity at. Whatever you choose, stability is probably much more important than the actual number, unless that actual number falls into a range that may stress or actually harm your livestock.


There are basically four ways that most of us measure our specific gravity:

1) Swing arm hydrometer

2) Floating hydrometer

3) Refractometer

4) Salinity monitor (since very expensive, will focus on the above)


How do we judge them? Well, there are basically three things to keep in mind:

1) Precision: what level can we measure the s.g, or to how many digits

2) Accuracy: how close are we to the true value of what we are measuring

3) Reliability: if we take the measurement multiple times, will we come up with the same or close result on repetition


Swing Arm Hydrometer


This is probably the most commonly available method for measuring a tank's s.g. and most likely the type to be purchased by a newcomer to marine aquariums. This hydrometer functions by a moving swing arm that will float to a given level at a given specific gravity. The hydrometer must be immersed in the tank to fill with your sample. The s.g. is read at the pointer of the swing arm.






1) Very inexpensive ($5-6)

2) Readily available

3) Easy to read




1) Have to immerse the entire hydrometer in the tank to float the arm; requres 50-100ccs of SW; ?messy?

2) Bubbles can get stuck to the float arm rendering it inaccurate

3) Needs to be calibrated to ensure accuracy (usually against a refractometer)

4) Although precise, not always accurate or reliable: often can be off by 0.003-0.004

5) Can be affected by changes in temperature


Manufacturers include Instant Ocean, Deep Six and SeaTest.


Floating Glass Hydrometer


Many companies manufacture these and they can run from inexpensive to very expensive (for professional models). These hydrometers are placed in a sample of your tank water and then the s.g. gravity measurement is read off the side of the hydrometer at the meniscus. Tropic Marin manufactures this model:








1) Simple to use

2) Not as expensive as a refractometer, typically run $20-30




1) Accurate, reliable, but not always precise

2) More expensive than a plastic hydrometer

3) The ideal way to measure is to put the hydrometer in a cylinder with your SW. Inconvenient. Otherwise, the bobbing float away method is used.

4) Can be affected by changes in temperature




A refractometer is usually a small hand held instrument that looks like a small telescope. It has a flattened,angled area on the end where you place 2-3 drops of your sample, drop a cover down and then hold the refractometer up to the light to get a reading.




The refractometer functions on the basis of refraction. Light striking a different density material will refract or bend. The light showing thru a refractometer will bend to differnt lines on the scale based on the density of the water (specific gravity is on the left, salinity on the right):






1) Fun to use

2) Precise, accurate, reliable

3) Easy to use

4) Small sample needed

5) Calibrated for differences in temperature

6) Easy to check calibration....use RO water to zero; adjusts with a small screw driver supplied




More expensive, usually in the $100-$200 range (however, Drs. Foster and Smith carry a good one at about $40)


OK, then, which one should I buy?


It's a personal choice based on your wallet and the emphasis you place on the importance of maintaining the safety and stability of your marine tank. A refractometer is the preferred method for measuring your tank's specific gravity among most serious reefers. It is easy to use, but, more importantly, it is precise, accurate and reliable. Considering the cost of any marine tank setup, a $40 investment is probably invaluable when it comes to the well-being of your corals and livestock.


Hope this helped. SH

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nice post. Fun to use though? lol anyways i got a refractometer from premiumaquatics and it was 39 bucks the quality is just as good as any other I've seen, it has auto temp adjustment and is dead accurate, I highly recommend it over a hydrometer , you will thank yourself for seonding the extra 20 bucks or so.

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My $40 refractometer works just as good as the $375 one I've used at the hospital...and THAT one just looks at urine!

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I would definately recommend just spending the extra $30 and getting a refractometer, you'll end up getting it in a month anyways, so why waste $10


On the bobber style, you're not supposed to drop them in the water, it throws off the measurement, and if you do it too hard, it will break, that's why I had to buy a refracto

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Refractometers are definitely worth the $40-$50 for their ease-of-use and quickness in measuring S.G./salinity. Hydrometers are usually off by .002 for every 10 degrees F above 60F that you keep your aquarium. So, if your aquarium is kept at 80F and you get a reading of 1.021 from your hydrometer, you're probably closer to 1.026. Refractometers don't have this problem -- once they're zero'ed out/calibrated, they'll give an accurate reading at any temperature.

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