Refractometer Calibration Liquid Possibly Faulty

[bif24701]

The thing I still don't understand is the natural seawater is always 1.025, and that's all I've been using so my tank should be the same. But when calibrated with the fluid, the results come in at 1.023. How can this be unless the solution is faulty?

The NSW tests at 1.023 AFTER calibration? Or the calibration fluid is 1.023?

If the NSW is testing 1.023 after calibration that means that your calibration fluid is high exactly the same amount mine is.

That means there is a problem with the solution like mine and we should bring this to the manufacture, like ASAP!

 

[ksc]

The thing I still don't understand is the natural seawater is always 1.025, and that's all I've been using so my tank should be the same. But when calibrated with the fluid, the results come in at 1.023. How can this be unless the solution is faulty?

Seawater can vary quite a bit....

"Seawater has an average density of 1.027 gm/cm3, but this varies with temperature and salinity over a range of about 1.020 to 1.029. The density is typically indicated by a density index s which is the departure of the density from 1.000 multiplied by a thousand.
http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/seawater.html

 

[bif24701]

I think it's the solution. I have a older bottle of the same stuff that tests perfect. The new bottle is high, exactly equivalent to OPs

 

[Piranhapat]

Try Accurasea by twolittlefishes...

 

[Randy Holmes-Farley]

Hey all,
I've been using a ATC refractometer for quite some time now. I've always calibrated it using R/O D/I water, but just yesterday I went out and bought myself a Refractometer Calibration Solution by AquaCraft, and from there I've spun myself a massive web of confusion...

Apparently, according to the solution, it has been calibrated completely wrong. I adjusted it to 1.026/ 35 ppt as instructed, afterwards I tested the R/O D/I as sort fail proof paranoia practice, and the blue line is below the 1.000 mark... why? Is the solution faulty? I'm so confused. If the solution is correct, then my salinity has been wrong this whole entire time.

FWIW, that is what is expected for a brine (sodium chloride) refractometer. That is why you use a solution and should not use RO/DI water. :)

 

[Randy Holmes-Farley]

The thing I still don't understand is the natural seawater is always 1.025, and that's all I've been using so my tank should be the same. But when calibrated with the fluid, the results come in at 1.023. How can this be unless the solution is faulty?

Natural seawater can have a range of salinity, but 35 ppt seawater has a specific gravity of 1.0264. :)

 

[Randy Holmes-Farley]

But the RO/DI should always be marked at 1.000, right?

No, it does not and should not (even if perfectly made and perfectly calibrated to read seawater) unless it is a true seawater refractometer, which most are not. That's the problem.

I explain the exact reason it should have a fixed offset at zero in this article:

Refractometers And Salinity Measurement
http://www.reefedition.com/refractometers-salinity-measurement/

from it:

Imperfect Refractometer Use: Scale Misunderstanding and Salt Refractometers

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. If a manufacturer does not claim it is a “True Seawater” refractometer, it very likely is a brine refractometer.

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. This figure shows the measured salinity reading for seawater being about 1.7 ppt higher than it really is.

 

[Randy Holmes-Farley]

Why do u say never use RO water to calibrate?

It is more than just being far from the range needed. It is a systemic error in brine refractometers. See above discussion.

 

[ksc]

Interesting. Even reading the manuf.'s description it's not really clear if this is a "true seawater" refractometer. @Randy, what do you think?

The VEE GEE STX-3 is a handheld, portable, water-resistant salinity refractometer that measures salt concentration in parts per thousand (PPT) in the 0 to 100% range and specific gravity (SG) of seawater in the 1.000 to 1.070 range, has automatic temperature compensation (ATC), and a durable metal body with rubber grip. A salinity refractometer measures the concentration of salt, such as sodium chloride, magnesium and calcium sulfates, in liquids and displays the results. Handheld refractometers, also known as sugar meters, concentration meters, or Brix meters, are commonly used in the culinary, food and beverage, pharmaceutical, educational, geological, chemical, and ecological industries, and for water-testing applications.

 

[Brew12]

Interesting. Even reading the manuf.'s description it's not really clear if this is a "true seawater" refractometer. @Randy, what do you think?

The VEE GEE STX-3 is a handheld, portable, water-resistant salinity refractometer that measures salt concentration in parts per thousand (PPT) in the 0 to 100% range and specific gravity (SG) of seawater in the 1.000 to 1.070 range, has automatic temperature compensation (ATC), and a durable metal body with rubber grip. A salinity refractometer measures the concentration of salt, such as sodium chloride, magnesium and calcium sulfates, in liquids and displays the results. Handheld refractometers, also known as sugar meters, concentration meters, or Brix meters, are commonly used in the culinary, food and beverage, pharmaceutical, educational, geological, chemical, and ecological industries, and for water-testing applications.

They sure could be more obvious about it, but I believe it is. This is from the manufacturers website.
"Scale: Salinity Specific Gravity of Seawater"

I should also add that for other refractometers they make it only says "Salinity" and refers to them as NaCl refractometers, not seawater.

 

[Randy Holmes-Farley]

Interesting. Even reading the manuf.'s description it's not really clear if this is a "true seawater" refractometer. @Randy, what do you think?

The VEE GEE STX-3 is a handheld, portable, water-resistant salinity refractometer that measures salt concentration in parts per thousand (PPT) in the 0 to 100% range and specific gravity (SG) of seawater in the 1.000 to 1.070 range, has automatic temperature compensation (ATC), and a durable metal body with rubber grip. A salinity refractometer measures the concentration of salt, such as sodium chloride, magnesium and calcium sulfates, in liquids and displays the results. Handheld refractometers, also known as sugar meters, concentration meters, or Brix meters, are commonly used in the culinary, food and beverage, pharmaceutical, educational, geological, chemical, and ecological industries, and for water-testing applications.

If it doesn't clearly say true seawater refractometer, it most often is not. In this case, it is hard to know for sure.

 

[aaron23]

Hmm had no idea this was the case about brine refractometers. Thanks randy I will look into this further!

 

[stevo01]

I have an ATC refractometer. Origionaly I made the mistake of recalibration to RODI water. I recently bought the solution posted in the thread. And found my refractometer was calibrated .002 under. In the solutions directions it states that if you own an ATC refractometer to allow an extra 45 seconds for the reading.

Also wondering something. I usually add 4-6 drops on the prism, flip the cover down, and gently push the cover down. I notice a difference in refractometer reading if I do this, or don't do this. Don't know which is the right way tho. I think the cover should be flat as possible allowing only a certain amount of liquid on the plate. Please let me know. Thanks!

 

[bif24701]

Randy,
How do digital refractometers compare to analog in accuracy? Like the MA877?

 

[ksc]

I was just thinking there would be no need to have a 0-100 scale if it was a "true seawater" refrac.. But once a "salt" refrac. is calibrated using a seawater calibration solution, it should be reading correctly. I hope this is right or it will cost me another $65...

This one looks good:https://reefbuilders.com/2010/07/12/dd-seawater-refractometer-scaled-calibrated-seawater-saltwater/

 

[bif24701]

I was just thinking there would be no need to have a 0-100 scale if it was a "true seawater" refrac.. But once a "salt" refrac. is calibrated using a seawater calibration solution, it should be reading correctly. I hope this is right or it will cost me another $65...

This one looks good:https://reefbuilders.com/2010/07/12/dd-seawater-refractometer-scaled-calibrated-seawater-saltwater/

Nice, but it won't work for hypo. Really considering the Milwaukee

 

[stevo01]

Also wondering something. I usually add 4-6 drops on the prism, flip the cover down, and gently push the cover down. I notice a difference in refractometer reading if I do this, or don't do this. Don't know which is the right way tho. I think the cover should be flat as possible allowing only a certain amount of liquid on the plate. Please let me know. Thanks!

Can someone please tell me the answer thanks.

 

[Randy Holmes-Farley]

Can someone please tell me the answer thanks.

How big of a difference?

 

[bif24701]

Can someone please tell me the answer thanks.

Yea sure




Yes.

 

[stevo01]

How big of a difference?

I want to say it's a couple thousandths. I'll post the exact ammount when I get home.