Specific Gravity

[matt]

Refractometers sold for the aquarium hobby are meant to measure the refraction of NaCl, not seawater. Apparently there is a difference. Plus, they're pretty inconsistent in terms of quality control. There are all sorts of stories about varying results with them. Randy Holmes-Farley wrote a coupe of good articles about this. You can use a specific pinpoint conductivity calibration fluid to calibrate hobby level refractometers at 35PPT, which is the range they need to be accurate in. Calibrating with R.O. water only insures that they'll read zero PPT correctly.

Maybe the easiest cheapest answer is to have a big glass floating hydrometer and an accurate thermometer, or use the pinpoint solution to calibrate the refractometer. as several people have pointed out, there's an acceptable salinity range, so being exactly at 35PPT is not so important.

[cbianco]

Well, I am back down to 35ppm. I did a bunch of small replacements (salt water for fresh water). My corals are starting to open back up. My candy canes, that turned whitish, are gaining their red color back.

I would like to thank Tony again for sending the refractometer. Without him, I would have just bought another swing arm.

BTW... If you are looking for a used (slightly defective) Deep Six, I have one for sale, $20. :P

Christopher

[tony]

glad to hear you put it to use christopher, i checked mine again yesterday with an lfs and they were both the same so if nothing else the refractometers are much more consistent

you should add more to the price of that hydrometer, yours seems to be lighter than the other ones. that should demand a premium ;)

haha

[cbianco]

glad to hear you put it to use christopher, i checked mine again yesterday with an lfs and they were both the same so if nothing else the refractometers are much more consistent...

The refractometer has gone to very good use. I already have other people who want to "borrow" it. You know who you are MattK ;) .

...you should add more to the price of that hydrometer, yours seems to be lighter than the other ones. that should demand a premium ;)

haha

It's the "special edition" made out of light weight, non-accurate plastic.

Christopher

[matt]

An update; I finally got around to getting some of the pinpoint conductivity calibration fluid. Premium aquatics sells this as "salinity probe calibration fluid" and it's a known standard for correct refraction at 35PPT. So, if you buy this stuff ($3.45) you can calibrate your refractometer so that it is very accurate at typical reef aquarium salinity. Calibrating a refractometer to read zero with R.O. water doesn't ensure that it will be accurate at 35PPT, especially measuring ASW as opposed to NaCl dissolved in water.

Anyhow, after all the reading I did about this, I finally tested my refractometer, which is one of the sybon one. It was right on the money, which really surprised me.

[cbianco]

Good to know. Thanks for the info matt!

Christopher