Canreef Aquatics Bulletin Board - refractometer calibration

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- - refractometer calibration (http://www.canreef.com/vbulletin/showthread.php?t=86257)

Quote:

Originally Posted by rayjay (Post 716020)

Actually, I haven't tried it myself because I have no use for a refractometer.
Without checking on the math of it, I believe the error is .0017 or thereabouts but I don't remember low or high just off hand. The answer is in that article.
There is NO WAY that an accurate refractometer that measures NaCl in water like the standard refractometer does, can read accurately throughout the scale measuring sea water with the many different salts involved as the refractive index is affected. If your NaCl refractometer calibrated with water is reading correctly at normal salt aquaria ranges, then it is a poor quality that has errors that just happen to read correctly at that range for some reason but it is scientifically not possible for a quality refractometer to do so.

From your article:

Quote:

First calibrate the refractometer in pure freshwater. This can be distilled water, RO (reverse osmosis) water, RO/DI water, bottled water and even tap water with reasonably low TDS (total dissolved solids). Calibrating with tap water that has a TDS value of 350 ppm introduces only about a 1% error in salinity, causing readings in seawater to read a bit low. So 35 ppt seawater (specific gravity = 1.0264) will read to be about 34.7 ppt, and will show a specific gravity of about 1.0261

So even with tap water the error is very small, if you can read the difference between 1.0264 and 1.0261 on a refractometer you've got skills beyond the average person.

And yeah it's the crappy refractometers work better...

Quote:

Very inexpensive refractometers can be prone to errors and may need to be checked in a solution matching seawater, not just pure freshwater.

The article actually states calibrating with water is perfectly fine. It only suggest using a natural seawater calibration fluid once to simply verify the quality of unit, cause yeah obviously there is a lot of cheap crap out there and once can't be too careful.

Quote:

Calibration is usually performed by putting the freshwater on the refractometer, letting it sit for at least 30 seconds so it comes to the same temperature as the refractometer, and adjusting the calibration screw until it reads a value appropriate for freshwater (e.g., refractive index = 1.3330, salinity = 0 ppt, specific gravity = 1.0000). Normally, this step is a quick and easy procedure, and may often be all that is required IF the refractometer has been verified

Another fine example of how people use long boring articles to exaggerate facts into myths, and more annoyingly they never even actually read the article or this case even use the equipment in discussion.

A refractometer calibrated with 0 TDS water @ room temp. is more than accurate for mixing saltwater, I've seen people with systems anywhere from 1.022 to 1.027 and everything is living.
Lets face it, we're mixing seawater not rocket fuel.
Swing arm testers should be banned, they are a waste of recycled plastic.

You need to read the article fully so you understand each segment and don't let your opinions reflect in what you are seeing.
First of all, the article isn't mine, it is by a respected chemist in the medical field who just happens to spend a lot of time researching the hobby and helping hobbyists on RC.
Quote:
First calibrate the refractometer in pure freshwater. This can be distilled water, RO (reverse osmosis) water, RO/DI water, bottled water and even tap water with reasonably low TDS (total dissolved solids). Calibrating with tap water that has a TDS value of 350 ppm introduces only about a 1% error in salinity, causing readings in seawater to read a bit low. So 35 ppt seawater (specific gravity = 1.0264) will read to be about 34.7 ppt, and will show a specific gravity of about 1.0261
You are taking this out of context. This error is specifically the error between calibration with freshwater and calibration with TDS of 350 ppm.
The section you didn't see states:

Quote:

Refractometers can lead to incorrect readings in additional ways and, again, these issues abound for reef aquarists. One is that many refractometers are intended to measure sodium chloride solutions, not seawater. These are often called salt or brine refractometers. Despite the scale reading in ppt (‰) or specific gravity, they are not intended to be used for seawater. Unfortunately, many refractometers used by aquarists fall into this category. In fact, very few refractometers used by hobbyists are true seawater refractometers.

Fortunately for aquarists, the differences between a salt refractometer and a seawater refractometer are not too large. A 35 ppt sodium chloride solution (3.5 weight percent sodium chloride in water) has the same refractive index as a 33.3 ppt seawater solution, so the error in using a perfectly calibrated salt refractometer is about 1.7 ppt, or 5% of the total salinity. This error is significant, in my opinion, but not usually enough to cause a reef aquarium to fail, assuming the aquarist has targeted an appropriate salinity in the first place. Figure 23 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in salinity.

You appear to be picking sentences out of the article and not using them in the context of the part of the article they are found in.

Quote:

IMO, it's only boring to those that aren't really interested in the whole facts and while I agree that in the great scheme of things, the .0017 error isn't going to cause your tank to fail, it is there, and it's hypocritical IMO to crucify use of swing arms by someone who ignores the limitations of many refractometers.
From the quote I pasted from Randy's article, it appears you haven't adhered to your own policy of "they never even actually read the article", (at least in it's entirety).
As for me not using a refractometer, it is because I know their limitations and I know that a certified calibrated hydrometer is more dependably accurate than a refractometer, and that I have no need to use a refractometer because it isn't going to make my tanks run any better.

Each method of measurement in the hobby has its limitations. From my personal experience, the swing arms appear to be the least reliable & messy to use. The floating hydrometer & refractometer are easier & less of a mess. Only thing for the floater is that it's useful to draw a water sample & place it into a tall cylinder to get the best results. Trying to read it while bobbing in the tank is not a good idea. The refractometer is dead simple, a couple of drops of water, easy to read scale thru the eyepiece. My floater is broken, my swing arms gathering dust in the basement. I'm stuck with the refractometer which I calibrate periodically with pure water. So much simpler to use when I'm trying to match my freshly mixed water to tank water before water changes.

[quote=rayjay;716120]You need to read the article fully so you understand each segment and don't let your opinions reflect in what you are seeing.
First of all, the article isn't mine, it is by a respected chemist in the medical field who just happens to spend a lot of time researching the hobby and helping hobbyists on RC.

I am a huge fan of Randy, but sometimes it's just too much science.

Quote@IMO, it's only boring to those that aren't really interested in the whole facts and while I agree that in the great scheme of things, the .0017 error isn't going to cause your tank to fail, it is there, and it's hypocritical IMO to crucify use of swing arms by someone who ignores the limitations of many refractometers

I check my refractometer regularly, I keep it clean, I always test @ tank temp. and I allow 30 seconds for temp. compensation, purchased calibration fluid only to find out I didn't need to, that's not ignoring its limitations.
I've had 4 swing arms, all off by at least 3 points, 2 were 7 ponts apart, they are all in the garbage because the are not worth their weight in recycled plastic and yes they should be crucified.

Each segment only outlines possible sources of error in a worst case scenario, not confirmed or "for sure" types. These types of articles are thorough and based on theoretical events and not actual. That's why at the end of the day you'll see conclusions are vague yet still suggest calibration with freshwater is perfectly acceptable despite all the errors listed.

In addition if you look into the matter further you will find numerous cases of hobbyists experimenting finding the same results as I have. Some have even purchased rather expensive so called seawater refractometers and compared them to other types and getting identical results.

At the end of the day articles like this should be taken with a grain of salt, not to be used in a black and white scenario saying things like there's no way it'll be accurate cause this article lists sources of error. The parts I quoted previously where to reflect and taken from a what I believe was more concluding section of the report, it was certainly not nitpicking certain sections and stating them as fact. I've read the article numerous times, however I followed what I mentioned above, using it as good source of information on the subject but not the only one.

The simple matter is stating refrectometers need a special fluid for calibration is a myth, while based on some fact it's an exaggeration on those with unverified assumptions.

Quote:

Originally Posted by sphelps (Post 716137)

I agree 100%