Why is the red sea refractometer scale incorrect??? Anybody?!?!

[Lionfish hunter]

The red sea refractometer scale has 1.000 sg equal to 2 ppt. How is this possible? It is not off on my specific instrument, the actual scale on the box is the same. What is more confusing is they have you calibrate it at 0 ppt, which is waaay below 1.000! 1.000 is pure fresh water, you are already started off with a sg error if calibrated this way. How can this possibly work for hypo and be accurate? Red sea has not responded to my email in the time they stated they would, and I am at a loss for words. How can an expensive refractometer from an established company have such a massive simple mathematical error? I have attached a picture of my actual refractometer at 1.000 sg. I will stay out of arguments on how good or bad the red sea refractometer operates as I have a strong opinion on that, just want to know why this is.

 

[piranhaman00]

Get a veegee

 

[Randy Holmes-Farley Verified Community Expert]

Sadly, its just not made properly if it looks like the one in the Red Sea web website picture. They show 35 ppt = 1.025 sg, which isn't true. The whole left side needs to move down aby about 1.5 ppt.

Seawater Refractometer - Red Sea Salinity test for marine aquariums

Red Sea’s Seawater Refractometer offers exceptional accuracy, for measuring the absolute salinity of seawater Specifically designed for saltwater aquariums

www.redseafish.com

 

[Lionfish hunter]

Sadly, its just not made properly if it looks like the one in the Red Sea web website picture. They show 35 ppt = 1.025 sg, which isn't true. The whole left side needs to move down aby about 1.5 ppt.

Seawater Refractometer - Red Sea Salinity test for marine aquariums

Red Sea’s Seawater Refractometer offers exceptional accuracy, for measuring the absolute salinity of seawater Specifically designed for saltwater aquariums

www.redseafish.com

This is red seas response. There was more about temperature and cleaning the refractometer that were irrelevant, but this was the only part of the response related to their issue.

"The specific gravity of 0 ppt water at 25 degrees Celsius will have a specific gravity of 0.99768 therefore the lines of S.G and PPT are not aligned."

I thought pure fresh was 1.000 or at least very close to it. Does pure fresh water have a specific gravity this low? Either they are wrong, or all refractometer scales are way off besides theirs.

 

[Lionfish hunter]

Their response accounts for the 2 ppt error perfectly. But are they wrong is the question. And if they are as I believe they are, how is this possible with a company like red sea!!

 

[KrisReef]

What is the error reported for this instrument? That defraction scale span is huge with very low accuracy. Not very impressive. Thanks for posting

 

[Lionfish hunter]

What is the error reported for this instrument? That defraction scale span is huge with very low accuracy. Not very impressive. Thanks for posting

with the naked eye, 1.000 sg equals 2 ppt on their scale. They came up with the explanation above to account for this. Interested in what Randy has to say about it.

 

[Randy Holmes-Farley Verified Community Expert]

The world is a complicated place when you look too deeply at anything.

I think Red Sea may be confusing density (pure fresh water at 25 deg C has a density of 0.9970470; very close to what they told you) and specific gravity, and if true, is sad that they make such an obvious mistake. Not impossible, though, as I have received clearly incorrect info from them in the past when asking tech support to explain their assertions.

The alternative is that they are making some nonstandard assumptions.

Let's assume they are perfectly correct, and let's see where it leads us.

What is the specific gravity of totally pure fresh water?

What is specific gravity, EXACTLY?

"B. SPECIFIC GRAVITY TERMS 1. In simplest terms, specific gravity is the ratio of the mass of a given volume of material to that of the equal volume of water at the same temperature"

from:

https://dot.ca.gov/-/media/dot-media/programs/engineering/documents/californiatestmethods-ctm/ctm-106-jun2008-a11y.pdf

It is thus the density of the fluid being measured divided by the density of pure fresh water.

Sounds very much like the specific gravity of pure fresh water should be exactly 1.0000000000000000000000000000000000000000000000000000 by definition.



But looking in much greater detail, there is more to it. I discuss these complications in great detail here:

Chemistry and the Aquarium: Specific Gravity: Oh How Complicated!

This month, Randy explains and reviews specific gravity.

reefs.com

The primary complication is temperature.

The specific gravity of pure fresh water at any temperature, when comparing to pure fresh water at the same temperature will, by definition, be exactly 1.00000000000.

BUT, if somehow the two solutions are at different temperatures, then the sg can be quite different from 1.00000000. For example, pure fresh water at 25 deg C has a density of 0.9970470 g/cm3. Pure fresh water at 20 deg C has a density of 0.9982067 g/cm3.

Thus, if you asked about the specific gravity of pure fresh water at 25 deg C, using 20 deg C as the standard for measurement, then it would be 0.9970470/0.9982067 = =0.9988382166.

What could Red Sea be thinking that the sg of pure fresh water at 25 deg C could be identified as 0.99768?

In other words, what temperature of standard are they comparing to?

IF the sg at 25 deg C is truly 0.99768 as they claim, then what is the density of the water standard it is being compared to?

It would have to be X, with
0.99768 = 0.9970470 g/cm3 divided by X g/cm3
solving for X we get the density of the "standard" to be 1.000634 g/cm3

At what temperature does pure fresh water have a such a density? NEVER.

The density of pure fresh water at normal atmospheric pressure is NEVER above 1.000000 g/cm3, and it is only at 1.0000 g/cm3 at ~4 deg C.

So Red Sea could claim correctly, within some rounding errors, that the specific gravity of 25 deg C fresh water is 0.99768 IF they are comparing to the standard of pure fresh water at 4 deg C.

That is not a totally absurd thing, since that is done in some scenarios. Sometiems that can be called "relative density". But it is also certainly not the norm for the aquarium hobby or most other industries.

Better devices may tell you exactly what they mean when they quote specific gravity. The Tropic Marin floating glass hydrometer, for example, tells you the sample and the comparator are at 77 deg F (it is printed right on it). It will read 1.00000000 in 77 deg F fresh water.

So Red Sea may just have a poor understanding of what specific gravity of water is, or they may be comparing to the density fresh water at 4 deg C (which is quite nonstandard).

.

 

[Tobin VP]

I just spent an hour pulling my hair out on this topic. I usually only read ppt on my refractometer but today I took a sg reading. I too can confirm that my Red Sea Refractometer is printed on the glass with a 2 ppt (or .0015 sg) offset between the two scales.

I had just sent a note to them asking for clarity on this before finding this thread ... highly anticipating their reply.

Assuming that there ATC mechanics are working correctly (which it seems to) then for now I'm going to be sure to stick with the scale that I use for calibration only and convert post-facto if needed. It appears to be a nearly constant offset throughout the measurement range.

 

[Randy Holmes-Farley Verified Community Expert]

I just spent an hour pulling my hair out on this topic. I usually only read ppt on my refractometer but today I took a sg reading. I too can confirm that my Red Sea Refractometer is printed on the glass with a 2 ppt (or .0015) offset between the two scales.

It is shocking to me that Red Sea could get it this wrong. @Randy Holmes-Farley spends some time above showing what they "might" be thinking here ... but in my opinion there doesn't seem to be any reasonable explanation other than "oops - sorry we made a mistake - we'll fix that in future devices". And if they are trying to come up with their "own" standard it seems that they would spend a little time explaining that well so that there are no mistakes or confusion.

I had just sent a note to them asking for clarity on this before finding this thread ... highly anticipating their reply.

Assuming that there ATC mechanics are working correctly (which it seems to) then for now I'm going to be sure to stick with the scale that I use for calibration only and convert post-facto if needed. It appears to be a nearly constant offset throughout the measurement range.

If you calibrate it with a 35 ppt (sg = 1.0264) standard, then you can calibrate using either scale and just stick to that scale.

 

[Tobin VP]

@Randy Holmes-Farley : Yes ... agreed ... that's why I've been fine using ppt for calibration and then using ppt readings. It seems they are using the " at 4 deg C" sg ref standard.

But now I'm realizing that there is even more going on than I realized. I'm trying to work all of this out ... maybe you could help clarify for me.

I have always had a hard time getting my refractometer readings (in ppt) to match my apex with any degree of satisfaction. This causes me stability problems with water changes. Regardless of my care in calibrating both devices I seem to commonly have a 1-1.5 ppt difference. My refractometer always reading higher than apex.

I just found this in Red Sea's instruction manual as step #8 under use:

"8. Read the seawater salinity in ppt and/or the equivalent specific gravity (S.G.) at 25°C."

Does that mean the salinity reading is as if the water is at 25 deg C? Assuming that most reef tanks are near this temp are they trying to account for the fact that you are measuring at a typically lower ambient temperature?

Then there is this in the manual:

"Calibration: Alignment should always be carried out with the instrument at 22-25°C. Allow the refractometer to stand at this temperature for 30 minutes before alignment."

This is news to me that I'm suppose to regulate my refractometer to these temps before calibrating ... I'm often readjusting and have done so at ambient temps under 22 deg C. This could seemingly be the source of my errors. But also oddly they don't even suggest calibrating with something near 35 ppt. They suggest using RODI water at 0 ppt which is not near typical measurements. Maybe just for practical ease of use so users don't feel compelled to buy calibration solutions?

Furthermore, on their product page they say:

"Therefore, Red Sea’s Seawater Refractometer calibrated at 25°C/77 0F directly reads the absolute salinity of seawater with no need for temperature compensation factor)"

so I'm thinking that because they want me to calibrate at around 25 deg C that I never have to worry about making an adjustment to the temp in my actual tank. But then I read on and I find this:

"ATC – Automatic temperature compensation

Temperature is probably the biggest cause of wrong readings with a refractometer. Calibration and testing of water samples should be done after the refractometer and water sample have equalized at the correct temperature.

Automatic temperature compensation (ATC) compensates for a few degrees of difference between the ambient temperature and the calibrated temperature of the refractometer. Larger temperature variations will give an error of approximately 1.0ppt in the salinity reading.

ATC does not adjust the PPT reading of a refractometer to the temperature of the aquarium!"

So I do need to adjust? Lots of things going on here. We have ATC which I always assumed only accounts for refractive index variation due to temperature differences. Then it seems that they also want you to calibrate with your device near the temperature of your tank at 22-25 deg C, yet you may not in general take device (i.e. ambient) measurements near your tank temp - in my case they vary by around ~5 deg C (~10 deg F). I take readings in my room typically around 68 deg F. So then I was thinking that they are accounting for the fact that ambient device temp is going to be lower than tank temp? But then they warn that ambient temp should be within a few degrees of calibrated temp or you get a 1 ppt error - and furthermore they conclude by warning that ATC does not adjust to the tank temp.

My best guess at this point is:

I should calibrate with device at 22-25 deg C. If I do this AND my ambient temp is within a few degrees of this calibration temp then I should be reading salinity at an assumed 25 deg C tank temp. If not (which is typically the case for me) .. I likely need to adjust further ... up to 1 ppt further? But based on what correction equation?

@Randy Holmes-Farley:
I'm in the process of slowly working through all of this. If you have quick explanations to help I surely appreciate your input. I think I recall in the past reading some of your writing somewhere that you commented that you could never figure out why Red Sea required calibration near 25 deg C. So maybe its all just a mystery and I really need to attempt to take this all up with Red Sea.