Refractometer

RhodeIslandReef

RhodeIslandReef

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Just got a new refractometer and its reading 1.031. I've always used a swing arm type and it reads 1.025.

Anyone have a formula for a certain amount of IO salt and water that will equal 1.025 so I can see if refractometer or swing arm is incorrect.

I think I remember seeing a formula on here to make salt water with regular ionized salt to calibrate that might be useful if someone has a link to that thread.
 
You refractometer should come with calibration instructions. Mine has been off, but never by much. The swing arm testers are notorius for being wrong.
 
I calibrated it prior to use. I found the info on using iodized salt and distilled water and tested it out. Found that the refractometer gave me a reading of 1.030 on the mix which was supposed to be around 1.026. Swing arm read 1.024. I calibrated the refractomer using the solution to 1.026 which the solution is supposed to be and then tested my tank water. It was 1.027.

Looks like i'll be changing out some water to lower the salt a bit. Not sure... but i think i'll be sending this refractometer back if it doesn't work correctly based on their calibration instructions.
 
I just got a refractometer and had the same result. The swing arm was saying 1.024 and the calibrated refractometer says 1.031. What a difference. What damage does that high salinity cause in the tank?
 
Mine was a marine depot type. I followed the instructions and calibrated it with distilled water and set it to zero. It was pretty close out of the box. I have slowly brought the salinity down to about 1.025. The lack of a LFS to verify my results since the hurricane is frustrating.
 
I just purchased a refractometer from Drs F&S. The instructions for calibration were to use distilled water and set the scale at 0.

RhodeIslandReefer - From my own experimentation with IO salt I got these readings with my refractometer after calibration

In 5 gallons of water
2.5 cups = 1.023 sg
add .5 cups = 1.028 sg

New 5 gallons
2.75 cups = 1.025 sg

Hope this helps.
 
That sounds about right, i'm going to make up 5 gallons and see if the refractometer reads anything close to that. I appreciate the info.
 
FWIW, it is also possible that there was a problem with the salt mix that you made to test it, as opposed to the refractometer. When I made it carefully, every refractometer that I tested at a recent Boston Reefers meeting (all the same brand, no-name blue one) were perfect once calibrated against DI water.
 
I just received my refractometer today. It was the $39.00 special from Marine Depot. I set it according to directions, using distilled water, to zero. I then mixed up a calibration solution according to Randy's instruction. 3.65 grams of table salt, and 96.35 grams of distilled water. I used an electronic scale to measure the salt and water. When I put this calibration solution on the refractometer it read the following...

Specific Gravity 1.031 Salinty 40

So, another valuable lesson learned. From reading this thread, it appears that there is a low cost refractometer being sold that if calibrated with distilled water, will not calibrate correctly. The only way to calibrate it correctly is to use a calibration solution of KNOWN salinity and specific gravity.

After calibrating it to a Specific Gravity of 1.026 and Salinty of 35 using the calibration solution formula Randy has provided us to use. I tested my tank. My tank water read...

Specific Gravity 1.024 Salinity 32

Earlier, when I had calibrated with Distilled water to zero, my tank read...

Specific Gravity 1.030 Salinity 39

So, if I had not made the Calibration Solution according to Randy's formula, I would have started lowering my Specific Gravity based on the refractometer reading, after calibration with distilled water to Zero. When in fact, I need to raise my Specific Gravity and Salinity after Calibration with the SOLUTION of KNOWN Salinity and Specific Gravity.

So when someone says, use a Refractometer, they are much more accurate than a hydrometer, what they are saying is meaningless unless the refractometer has been calibrated properly.

Dave
 
Now that I think about it. I think I will write to Marine Depot, and explain the them the situation and give them the link to this thread. Apparently, they have a batch of refractometers that will not actually calibrate to ZERO using distilled water. But, after calibration to a Specific Gravity of 1.026 and Salinity of 35 using Randy's formula for the Calbration Solution, the refractometer still reads ZERO when I put distilled water on the glass..

SO, I am confident that the refractometer is correctly calibrated now. It reads ZERO with distilled water, and it reads 1.026 with Salinity of 35 with the Calibration Solution made according to Randy's instruction......

Dave
 
Ok, I wrote to Marine Depot.. here is a copy of what I wrote. I hope it makes sense to them....

"Hi,

I received my Refractometer today. Thanks for the great service, I did not expect to get it until after the weekend.

However, there is a problem. According to the instructions with the refractometer, calibration should be done with distilled water to ZERO.

This does not work. If you calibrate the Refractometer to ZERO using distilled water, it will then incorrectly report a specific gravity of 1.026 as a specific gravity of 1.031 ...

This has been verified by several people now, and you can see this at this forum on Reef Central....

http://www.reefcentral.com/forums/showthread.php?s=&postid=6348509#post6348509

If the refractometer is calibrated to a solution of known Specific gravity of 1.026 it will then be calibrated correctly. It will read the specific gravity correctly of 1.026 and will also read ZERO for distilled water. But if you calibrate it to ZERO using distilled water, it will read that same Solution, KNOWN to be a Specific Gravity of 1.026 as a specific Gravity of 1.031 ..

I do not believe there is anything wrong with the Refractometer, I only believe that the calibration mechanism can not accurately be set to ZERO when using distilled water on the glass. Probably because there is a large ZERO point when a solution of a specific gravity of ZERO is used. SO probably you could tighten the set screw too much or too little and still be getting a reading of ZERO .... But if you are using a solution of KNOWN Specifci Gravity of 1.026 the refractometer can be set to that accurately, and still read ZERO when using distilled water on the glass.

I hope this makes sense...

I believe you should contact everyone who purchased this refractometer, possibly only from the last manufactured batch even, and explain this to them. Otherwise, if they calibrate with distilled water, they will be lowering their Specific Gravity and Salinity mistakenly.

Thank you,

Dave Brader"


I hope it makes sense to all of you too.......

How about it Randy, does this make sense to you????

Thanks, Dave
 
It is a very odd problem. Sort of like there is a lot of slop in the positioning of the zero point.

It makes sense to me in that you well described the problem. I don't know enough about how they manufacture refractometers to understand how it might come about.

In any case, I'm glad my solution was useful. :)

You used table salt, right, not salt mix salt?

Folks making it with coke bottles and such have more uncertainty in what they did, but it sounds like you did it very carefully. Assuming you did what you said accurately, there is no way that the solution can have a sg of 1.030. It can be low if the salt was wet, but it cannot be too high due to that or any other typical uncertainty.

Let us know what they say. I spend quite a bit of money there. :)
 
Yes, I used Table Salt, and Distilled water, measured on an electronic gram scale, that was also calibrated :)

And you are exactly right, there is just a lot of slop in the ZERO point on the set screw... I am sure it is a manufacturing defect, and I am also sure that as long as the refractometer is set to a Calibration Solution of KNOWN salinity, it will work just fine. I do not believe there is anything wrong with the optical part of the refractometer...

But it certainly can not be just coincidence that so many people set it to ZERO with distilled water and get a reading of 1.031 exactly... I sure am glad I happened to read that thread. Of course, I am a fanatic about my equipment and I am worried about the hundreds of people out there who may have one of these refractometers and have no idea they are not calibrated properly. I was actually starting to put together a plan to lower my salinity when I read this thread........ and went ahead and checked the salinity with your Calibration Solution.... and I know better.... I actually was a little bit wary of buying the cheap refractometer, but I know quite a few people who use them with no problem.

I guess the bottom line is, no matter what equipment we buy, we just can not assume it works properly without checking Calibration properly... that goes for PH meters, ORP meters, Hydrometers, Refracs..... all of them....

I can't imagine what people did before they had the resources we have today with the internet.....

Dave
 
I guess the bottom line is, no matter what equipment we buy, we just can not assume it works properly without checking Calibration properly... that goes for PH meters, ORP meters, Hydrometers, Refracs..... all of them....

:thumbsup:

I agree.
 
I recently had a similar problem. Both the glass and swing arm hydrometers were reading 1.025. Well... I bought a refractometer and it was reading 1.030. Over 3 weeks I brought my SG down to 1.025. All of my fish are regaining their color and the fins are healing. The corals are doing much better and the coraline algae is starting to grow again.

In my case, I had been using B-Ionic without doing regular water changes, which did not help the matter.

-Alfred
 
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