Refractometer
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saltyphish
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I paid $30 for mine at dr fosters and smith. Works quite well.
Randy Holmes-Farley
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Be sure to get an appropriate calibration fluid, such as the in point 53 mS/cm solution (or make a good DIYsolution). Calibration with RO/DI may not be accurate for all types, despite what the manufacturer may say. 
Dixie_reefer
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I ordered mine from bulk reef supply and it comes with calibration fluid. Paid $48 shipped.
kschweer
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Be sure to get an appropriate calibration fluid, such as the in point 53 mS/cm solution (or make a good DIYsolution). Calibration with RO/DI may not be accurate for all types, despite what the manufacturer may say.
Do you have an easy "recipe" for calibration fluid?
Randy Holmes-Farley
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Yes. Note this recipe will only read exactly 35 ppt with a refractometer, not a hydrometer or conductivity probe. I have different recipes for those.
Here's my refractometer recipe:
Make a 3.65 weight percent sodium chloride solution by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified fresh water. That amount roughly corresponds to ¼ cup (73.1 g) of Morton's Iodized Salt dissolved into 2 liters (2000 g) of water (giving very slightly more than 2 L of total volume).
For a rougher measurement in the absence of an accurate water volume or weight measurement:
1. Measure ¼ cup of Morton's Iodized Salt (about 73.1 g)
2. Add 1 teaspoon of salt (making about 79.3 g total salt)
3. Measure the full volume of a plastic 2-L Coke or Diet Coke bottle filled with purified fresh water (about 2104.4 g)
4. Dissolve the total salt (79.3 g) in the total water volume (2104 g) to make an approximately 3.65 weight percent solution of NaCl. The volume of this solution will be slightly larger than the Coke bottle, so dissolve it in another container.
How to Use a Refractive Index Standard
One simple way to use this refractive index standard is to measure it with a refractometer, and just remember what setting the standard came to. That setting represents S=35 seawater. Hopefully, the reading of the refractometer at that point will be similar to the properties of 35 ppt seawater (specific gravity = 1.026 - 1.027, or S=35, depending on the units). Simply using it as the target salinity for the aquarium is a fine way to go.
Alternatively, one can actually calibrate the refractometer using the standard by adjusting it until it reads the appropriate setting. Exactly how to adjust it depends on the refractometer, but often it is as simple as turning a screw.
Generic Refractometers are around the $40 range, Name Brand $60-$70, Digital $110 - $130. - Refractometers - Water Testing - Controllers & Testing
Randy Holmes-Farley
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kschweer
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You're welcome.
Happy Reefing.
One more quick question. The scale that I use only displays tenths not hundredths. If I were to make the solution with 3.6g of salt to 96.4g of water would it be off by much? Sorry if this is a stupid question.
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Randy Holmes-Farley
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That's a good question!
Assuming it is accurate, 3.6 g would give ~34.5 ppt (sg = 1.0260) and 3.7 g would give ~35.5 ppt (sg = 1.0268), so that does not seem like a big deal to me, given the range of salinity that is OK for a reef tank.
Here's what I'd do: add salt to 3.7 g. Then take a tiny bit off until it just clicks back to 3.6 g. That is likely in between 3.6 and 3.7 g.
Assuming it is accurate, 3.6 g would give ~34.5 ppt (sg = 1.0260) and 3.7 g would give ~35.5 ppt (sg = 1.0268), so that does not seem like a big deal to me, given the range of salinity that is OK for a reef tank.
Here's what I'd do: add salt to 3.7 g. Then take a tiny bit off until it just clicks back to 3.6 g. That is likely in between 3.6 and 3.7 g.
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redfishbluefish
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You ideally want to use 53.0 mS/cm calibration solution. HERE is but one example from BRS....all the big suppliers carry these, as well as your LFS.
