refractometer calibration

[sphelps]

Originally Posted by Nano

really?
but water quality varies where ever you go, so personally wouldn't use tap water for my refractometer
especially considering the poor water quality we have in Wetaskiwin

Yes really, my tap water isn't great here either, sitting at around 300ppm. However the point wasn't to tell you to use tap water but rather your RO water is perfectly fine for calibrating. At the end of the day do you feel best with but you are wasting your time bringing your refractometer to a LFS for calibrating. They will either use RO water or a calibration fluid, however unless the calibration fluid is new (more likely been sitting around for long time) you probably run more risk with that being contaminated.

As for temperature ideally what you use to calibrate should be close to your tank temp as far I as know, however IME this makes little difference as well as long as it's pretty close, ie room temp is fine.

I encourage anyone to try calibrating their refractometer with proper solution and comparing the results for themselves. I only only posted this to prevent the need for others to try such an experiment and put to rest the myth of inaccurate refractometers. Now I'm not saying there ins't error involved however the errors we are talking about are in the ppm range while our equipment measures in the ppt range with is 1000 times greater. You simply can't measure that kind of error with our refractometers nor can you even calibrate the device that accurately anyway.

[Zoaelite]

Originally Posted by sphelps

Yes really, my tap water isn't great here either, sitting at around 300ppm. However the point wasn't to tell you to use tap water but rather your RO water is perfectly fine for calibrating. At the end of the day do you feel best with but you are wasting your time bringing your refractometer to a LFS for calibrating. They will either use RO water or a calibration fluid, however unless the calibration fluid is new (more likely been sitting around for long time) you probably run more risk with that being contaminated.

As for temperature ideally what you use to calibrate should be close to your tank temp as far I as know, however IME this makes little difference as well as long as it's pretty close, ie room temp is fine.

I encourage anyone to try calibrating their refractometer with proper solution and comparing the results for themselves. I only only posted this to prevent the need for others to try such an experiment and put to rest the myth of inaccurate refractometers. Now I'm not saying there ins't error involved however the errors we are talking about are in the ppm range while our equipment measures in the ppt range with is 1000 times greater. You simply can't measure that kind of error with our refractometers nor can you even calibrate the device that accurately anyway.

I'm on board with calibrating with with RO water but in my person experience a cold refractometer can give an incorrect reading.
Colder instrument---> Salt water cooled---> Density drops---> Incorrect reading. That being said everything I have read says the amount is almost negligible, where's Kien when you need a science experiment done!!

[sphelps]

Yeah density obviously decreases with temperature but at a rate of less than one tenth of a percent per 10 degrees C. So if you're close it really won't make a difference. Not to say you should use boiling or close to freezing temp water to calibrate but using say 21C water to calibrate for a tank running at 24C really won't make a detectable difference.

To give an idea, 4C RO water from my fridge reads about 0.5ppt on my refractometer, just barely over the zero line. And that's a pretty significant temperature difference.

[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?

[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?

[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?

1.026 Calibration fluid reads 1.027 at 4 degrees C

[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.