refractometer calibration

[sphelps]

Originally Posted by Zoaelite

I'm not near my tank, anyway you can do the same with salt water and see if that changes the reading a measurable amount?

I'll put the calibration fluid in the fridge and post later

[rayjay]

If you have a seawater refractometer then calibration with water should provide a good reading at normal salt water ranges. However, it is still recommended that one use calibration fluid.
If you have a salt water refractometer (NaCl) which is most common in the hobby, there will be an error at normal salt water ranges if calibrated with water.
Randy Holmes-Farley on Refractometers and Salinity

[sphelps]

Originally Posted by rayjay

If you have a seawater refractometer then calibration with water should provide a good reading at normal salt water ranges. However, it is still recommended that one use calibration fluid.
If you have a salt water refractometer (NaCl) which is most common in the hobby, there will be an error at normal salt water ranges if calibrated with water.
Randy Holmes-Farley on Refractometers and Salinity

I was waiting for someone to state they'll be error and post that exact article. This is exactly why I started this thread.

May I ask how much error you would expect and if you've actually tried it yourself?

[rayjay]

Originally Posted by sphelps

May I ask how much error you would expect and if you've actually tried it yourself?

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.
I don't use a refractometer because I started over 18 years ago when everyone around here used either floating hydrometers or SeaTest swing arms.
As I already had a complete set of hydrometers from when I worked in a lab, that was my first choice, but I switched to using swing arms when I broke the floater that checked the range of the aquaria. I purchased a new Fisher Scientific certified calibrated floating hydrometer and a new columnar cylinder for measuring the s.g. in (I broke both hydrometer and cylinder at the same time). I also bought two SeaTest swing arms to use for everyday use so I wouldn't break another glass floater.
I used the certified hydrometer to check the swing arms, and one read slightly high and the other was slightly low.
With this older style you could remove the swing arm and so I shaved a touch off the weight in the arm of the high reader to make it read accurate at 1.026, and shaved a touch off the float material of the arm to allow the reading to rise to 1.026.
Now, I'm still using the swing arms, rinsing well after each use, soaking overnight in white vinegar once a week, and check once or twice a year against the certified, and it still reads accurately.

[mike31154]

I've always used RO water to calibrate or check my refractometer. Could never bring myself to bother ordering & trying calibration fluid despite reading all the dire warnings & horror stories on various forums. I've also used a floating hydrometer as well as the swing arms. Tank has been running for 4-5 years with no apparent issues related to out of whack salinity.

The two swing arms I have both yield different readings with the same test water. I don't like them, they're crap, even though I put correction marks on them using the refractometer reading as a calibration standard. In a pinch I'll use one & go by the mark I put on them, although I'm not sure why I haven't thrown them in the trash.

I broke the floating hydrometer a while back, but actually really liked it. Old tech but very accurate. The only thing you need to watch for is solution temperature since the floaters have no temperature compensation like most refractometers do. The floaters are designed, calibrated & tested at a specific temperature & the instructions usually include a temperature compensation chart. Provided the solution you're testing is around 20C, the floating hydrometer will be bang on. I use one regularly for my wine & beer making efforts, I think I recall using it on the tank water & it was accurate.

Ah yes, the refractometer. I have the instructions for mine & the calibration procedure calls for distilled water, no mention of any other claibration fluid. RO or RODI water should be a fair substitute I would think. My refractometer is temperature compensated, so provided I leave the solution on the prism glass for at least 30 seconds to adjust to the ambient temperature of the refractometer, the reading will be accurate. I've read the arguments about refractometer scales not being linear, but from my own experience, if I have a reading one notch above zero with RO water, then test tank water at 1.026, if I tweak the refracto to read zero with RO water, a subsequent test of tank water gives me 1.025. Quite linear I would say. No need to waste time & $$$s on calibration fluid that checks your refractometer at 1.026 or whatever.

Here are the calibration instructions for my refractometer verbatim:

1) Open daylight plate, and place 2-3 drops of distilled water on the main prism. Close the daylight plate so the water spreads across the entire surface of the prism without air bubbles or dry spots. Allow the sample to rest on the the prism for 30 seconds before going to step #2. (This allows the sample to adjust to the ambient temperature of the refractometer).
2) Hold daylight plate in the direction of a light source and look into the eyepiece. You will see a circular field with graduations down the center (you may have to focus the eyepiece to clearly see the graduations). The upper portion of the field should be blue, while the lower portion should be white.
3) Because this instrument is equipped with Automatic Temperature Compensation, the ambient working temperature of the room must be 20C (68F) whenever the instrument is recalibrated. Once calibrated, shifts in ambient temperature within the acceptable range, should not affect accuracy (10-30C). Using distilled water as a sample, look into the eyepiece and turn the Calibration Screw until the boundary between the upper blue field and the lower white field meet exactly on the 0.0 ppt (or 1.000 spec. grav.).

So it appears quite important that the calibration be carried out at the proper ambient temperature 20C. No mention of calibration fluid whatsoever.

Photos of readings years ago with the two swing arms & the floating hydrometer using the same tank water. Don't recall what the refractometer read exactly, but do remember that the floating hydrometer read virtually the same.

Coralife swingarm


IO Deep Six swing arm


Made in Taiwan floating hydrometer


IIRC, the refractometer read right around 1.025 with the same water. Result... Coralife low by 2-3 points, IO a tad more than 1 point low & floater just a fraction under refracto reading. Unlikely that I'll be ordering any calibration fluid.

[sphelps]

Originally Posted by rayjay

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:

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...

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.

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.

[Cal_stir]

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.

[rayjay]

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:

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.

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.

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.