Help pls - Refractometers conflicting - Red Sea vs Other

[GimpyFin]

I was actually looking for first-hand testimonies of this unit. Seems like you have been impressed with it. Could you clarify whether you have ever checked it against a reference solution or actual sea water?

Or could it be just giving the "impression" that it's accurate because it's "consistent", yet maybe consistently wrong by a couple of ppt?

The data sheet for the product indicates that the "resolution" is 0.001, and the "accuracy" is 0.002. It unfortunately does not say if that is SG, PSU, or ppt. If it was ppt, that would be unbelievably good, likely impossible. If it is SG, it's nowhere near acceptable...

I need to be able to distinguish between 1.024 and 1.026 with accuracy, that tolerance would simply be too wide.



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I have not checked it against actual ocean water or a reference solution like you get with an analog refractometer. It measures all 3 (SG, PPT, or PSU, whatever you prefer) and it also has an auto temp compensator. My best reference for it's accuracy would be Diver's Den since I order many things from there. The Wisconsin facility is generally kept at 1.025 for both corals and fish. Every time I get something from there and test the salinity in the bag, it reads 1.025 on the Milwaukee. Not exactly a reference solution, but I'd deem that as being pretty accurate (Unless Diver's Den is also way off.) Every six months or so I will just re-calibrate it to see if it changes and it does not. (Test my tank water, re-calibrate w/ ro/di, test tank water after to see if it reads the same.)

 

[Starvin Marvin]

Every six months or so I will just re-calibrate it to see if it changes and it does not.

I like the sounds of that, thank you for sharing.



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[running scott]

I too use the Milwaukee digital refractormeter. I really like its ease of use. I wouldn't use water directly from the ocean to decide wether it's accurate or consistent, since I use NSW for my water changes and its consistently inconsistent! It rAnges between 1.024 to 1.035.

 

[Sounds Fishy]

My Solution to a NEWLY Made problem..

5
They just work.. Easy to read and 110% accurate in any Room temp. No Calibration Required....

I wasted money buying refract-o-wrong-meter..

My two cents . Sometimes the simplest tools are the best...

When I started with the marine hobby in the 1970's,the only tool available was a hydrometer ,for reading salinity.They work great.
Now I don't know 100% if my refractometer is set right?

 

[Optionman]

i also use the Milwaukee digital. its great, easy to use, and comes with 1.000 and 1.025 test solutions to calibrate. returned my RS.

 

[PilotMaximus]

I use the brs no name refractometer. The cheap one. I check for calibration every few months and in over 2 years it has never needed a twist. I check against the calibration solution and a lab grade hydrometer I too have had for over 20 years.

 

[Starvin Marvin]

I'll be interested in seeing which model they might have.

For those interested, I did receive a technical recommendation from a gentleman at Cole-Parmer.

He recommended that I use an instrument which is calibrated at 60'F and then apply a correction factor using the following table:

From this chart, the relevant portion for my Reef Tank is this:

It states that interpolation is required for temperatures and SG readings not specifically listed in the chart.

I did some calculations and found that it's not a linear formula and there is a sharper curve the higher the temperature becomes.

However, when I plugged in the SG correction factors for the temperatures of 20'C and 30'C (which skips 1 charted interval), and then solved for the mid-point of 25'C, the results were very accurate so within this Reef Tank range I am happy to interpolate as if it was linear.

This iPhone app is called Interpolator and was easy to use.

The correction factors are within a narrow range and don't change much at all between an SG reading of 1.000 (RO/DI water) and 1.100 (Salty soup with corals not very happy).

At 27'C and 1.000 SG, the factor is 0.0022

At 27'C and 1.100 SG, the factor is 0.0023

So the SG isn't really a factor, you don't even have to worry about moving up or down on the chart depending on what you read since it's only a 0.0001 difference.

The temperature is what makes the difference and needs to be carefully checked prior to testing with a NIST calibrated thermometer I would think. Or at least one you have checked in boiling water and ice water to see how far off it is.

You can see below that if I use 1.024 SG as the reading I'm getting on the Hydrometer (quite likely to be the case), the correction factors change by 0.0005 depending upon a temperature swing of 2'C.

At 27'C and 1.024 SG, the correction factor is the familiar 0.0023

However at 25'C and the same 1.024 SG, the factor drops down to 0.0018.


I will simply use 0.002 as my factor when the temperature is between 26'C and 27'C.

Since my tank ranges between 26.5 to 27.5, this seems like a reasonable temperature that the water will be at the time of testing in the glass tube.

So I will be looking for a reading of 1.024 SG on the Hydrometer, to tell me I'm at 1.026 in my tank.

Cole-Parmer recommended the following Instruments for my application:

I would agree with this since our target measurement range is in the middle of the 1.000-1.050 scale, and it appears to have 0.0005 graduation lines which is great.

I also asked them for a suitable test tube and was recommended this product:

I think I will place the order and see how it differs from the cheaper Amazon.ca Hydrometer and Test Tube that I already ordered while awaiting this reply. The Amazon Hydrometer was $2.50 and the test tube was $30.

But I would like to have one in my water mixing room as well so hopefully it will be suitable.

I'm assuming the lab-grade stuff shown above will be top notch quality, for those prices.

Now for the big question... do I need a secondary correction factor to account for all the "stuff" in Seawater which causes refractometers to read Seawater differently than Brine? Or is that only an issue with refractive index and not a problem when reading SG with a floating Hydrometer?




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[bertoni]

That's interesting. I'm looking forward to hearing how you like it, and how it compares to the other instruments.

 

[Starvin Marvin]

Well - I got the Hydrometer from Amazon for $2.50.

It's Horrible (yes, capital H)

You can hardly see the graduations, it floats lopsided so the meniscus of the water squishes up against the side, the thermometer inside is a glued on piece of paper (not that I would use it anyways) and it's light as a feather.

At best - it's useful to see if you are in the ballpark +/- 0.004 SG.

So if you don't like the taste of salt, you could maybe use it to remind yourself whether you already added salt to a bucket of RO water.

Also there is zero information included or posted about it, so there is no way to know which temperature it was calibrated at, meaning you can't apply the proper correction factor.

Can't wait for the good ones to arrive.

I did however get some Pinpoint 35 ppt solution to calibrate my Red Sea and No Name Refractometers.

After doing so - I tested my tank which was around 36 ppt of salt (based on a freshly calibrated GHL probe) at the time due to an auto top off failure that I discovered, so I wanted to cross check with the freshly calibrated Refractometers.

Red Sea showed 35 ppt with the Pinpoint solution, and 31 ppt on tank water.

No Name Refractometer showed 35 ppt.

I think - it's time to strip these down and sell the component parts on eBay. They are utterly useless to me, there seems to be no rhyme or reason to the discrepancies between them and each other or any other measurement of my water.

Hydrometer is my last hope! And until then the GHL probe is what I'm putting my faith in, I'll just have to do a weekly calibration to be on the safe side.


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