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        Review: D-D Natural Seawater Refractometer



While browsing the D-D/Deltec booth, last weekend at MACNA, I noticed a refractometer that looked to be a little out of the ordinary. Most of us are aware that practically all refractometers used in the hobby are designed and calibrated for use with a simple salt (sodium chloride) or brine solution. They are not made with the refractive index of natural seawater in mind.

The problem stems from the fact that, although NSW's largest component is sodium chloride, other elements in seawater cause a refractive index that deviates from that of a simple salt, or brine solution. To add to the confusion, refractometers are designed to be calibrated and report readings according to refractive index, but for the sake of the aquarist are marked in specific gravity and and parts per thousand (ppt). According to Holmes-Farley, a 35ppt solution of NSW will have the same refractive index as a 36.5ppt solution of sodium chloride. So, if we want an accurate reading with "normal" brine solution based refractometers, we must calibrate them with a 36.5ppt sodium chloride solution for a reading of 35ppt. As far as I know, such as solution doesn't exist commercially, but no fear, Holmes-Farley gives a formula for exactly this solution (see references, below).

A natural seawater refractometer

Enter the new D-D refractometer, designed according to D-D,  "Specifically for aquarium use for measuring the salinity of natural sea water".  D-D call the model H2Ocean. It comes in a handy plastic case...

... which includes a nicely designed foam insert that provides protection for the instrument and accessories. Speaking of accessories, they include a plastic pipette, for sample collection, and a miniature, flat head, screw driver to adjust the scale when calibrating the instrument.

The build quality of the refractometer is equal to most of the different brands available to reef hobbyists. The knurled handle fits well in the hand and the instrument, as a whole, has a solid feel.  You don't notice anything special about the refractometer until you bring it up to your eye. After you adjust the focus to compensate for your eyesight characteristics by screwing the eyepiece in or out, what you immediately notice is a very clear scale. And the scale, itself, looks different than what we're used to in the hobby. After a moment, you realize that the scale caters only for the range we need as reefers - from 20 ppt to 40ppt (1.015 to 1.030 SG). This makes the scale relatively large and very easy to read when compared to other models.

The clarity of the scale is a large plus point, in my opinion, when deciding which make or model refractometer to purchase.

(ATC) Automatic Temperature Compensation

The refractive index of a solution varies as the temperature of the solution changes. As with most refractomers that produce their readings based on the refractive index of the solution being tested, the H20cean has ATC. There is a bi-metal strip inside the instrument that moves the scale to compensate for the change in refractive index as the ambient temperature changes. This means that once correctly calibrated, the H2Ocean can be used in environments where the temperature is within the range of the ATC limit, which (according to D-D) is 50 to 86 Fahrenheit.


As with most measurement based instruments, the H2Ocean must be calibrated before it can be used. It seems that 20deg centigrade (68deg Fahrenheit) is the standard at which almost all refractometers are designed to be calibrated, and the H2Ocean is no exception. In Florida, where I live, finding a room temperature of 68deg is not easy. Especially in summer - somewhere around 74deg is probably average.

D-D advises the the refractometer should be allowed to equilibrate to the room temperature for about 30 minutes before calibration should be attempted. They do stress the point that the H2Ocean is made using a copper body which tends to acclimate itself quicker than most other metals.

I won't go into a lot of detail as far as the steps involved in the actual calibration is concerned. D-D describes the process very well and thoroughly. I will note that they say that calibration should be done with distilled or RO water. This, of course, means that calibration must be done using the zero point of the scale.

Now this instrument is not an expensive piece of equipment designed and manufactured for exacting laboratory use. It's intended for hobbyists like you and I. As such, it most likely won't be entirely accurate if calibrated for zero ppt and the readings, in practice, tend to fall at the other end of the scale (35.0 ppt).  Keeping this in mind, I would strongly suggest using a solution that allows calibration to 35.0 ppt, instead of distilled or RO water were we have to calibrate for zero ppt. Although such calibration solutions are available commercially (PinPoint), I personally have always used a DIY 36.5 weight percent sodium chloride solution (36.5 grams of table salt and 963.5ml of distilled or RO/DI water) andthen  calibrated for 35.0 ppt at the prevailing ambient temperature where I normally do my testing, and I would recommend that anyone having access to such a calibration solution do the same. Your reward will be increased accuracy.


After calibrating the H2Ocean (which was straight forward and easily achieved) and a week of using it on a daily basis, I feel it's possible to state that:

  1. build quality is satisfactory to good - as it is with the majority of hobbyist refractometers.
  2. it is easy to calibrate and the instructions on how to achieve calibration are excellent.
  3. the instrument comes with all necessary accessories, including an above average foam lined case.
  4. the clarity of the scale is very good. The H2Ocean is much easier to read than most hobby refractometers.
  5. the price is acceptable and in line with what is being acquired.

In conclusion, here are a couple additional pics:



Sam says...
I love this item. Unfortunately I recently dropped mine and the acrylic/plastic flap to cover the water broke off. Any idea if I can get that part replaced? Thanks
GlassReef: Actually I broke mine, also. I contacted Deltec and was told replacements were not available. I used super glue to fix it - still works fine.
1st July 2014 1:18pm
Arnold says...
hello nice
8th February 2011 1:06pm
Postal says...
Nice review. Your site has tons of potential. How long before RC buys you out? Grin
GlassReef: Well, it may be a while before that happens.Cool
7th October 2010 9:01pm
Reefrubble says...
Very good review . Awesome Pics as well .

Keep up the good work . Best site on reef tank info .

GlassReef: Hi Ted! Thanks for the kind words. This is not a bad little refractometer. As I mentioned in the review, I especially like the clearness/sharpness of the scale - makes it much easier to read than other models I've owned.
2nd October 2010 9:01am
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