Don't Trust Cheap Refractometers!

[Randy Holmes-Farley Verified Community Expert]

It pretty easy. It's by definition, grams / volume (mL) edit: only works for aqueous solutions.

That oversimplifiction is why I asked. I know how to do it, and I was concerned you were doing it incorrectly based on your comments, and that appears to be the case.

One CANNOT assume freshwater is 1.000, because it is not except at 4 deg C.

To get from density to specific gravity, you have to look up the density of freshwater at the same temperature that you measured the density of your sample.

Then divide the density you determined by the density of fresh water at that temperature.

 

[Randy Holmes-Farley Verified Community Expert]

Well I have definitely observed a behavior with my Milwaukee meter that shows the temperature correction might need some work. My meter will show a 0.001-0.002 difference if the meter itself isn't reading close to the same temperature as my tank.

I run at 25°C. Ambient is about 21-22°C. If I don't warm up the meter a bit so that the temperature reads close to 25, I get higher than usual readings. Which makes sense. Colder sea water is denser than warmer sea water. The meter doesn't seem to do a good job correcting for it.

Edit: spelling

It is certainly possible that some or many devices get the correction wrong, especially if they are generic devices not specifically designed for seawater.

 

[Atrumblood]

That oversimplifiction is why I asked. I know how to do it, and I was concerned you were doing it incorrectly based on your comments, and that appears to be the case.

One CANNOT assume freshwater is 1.000, because it is not except at 4 deg C.

To get from density to specific gravity, you have to look up the density of freshwater at the same temperature that you measured the density of your sample.

Then divide the density you determined by the density of fresh water at that temperature.

I do know that fresh water isn't exactly 1.000sg, however it is common practice to assume 1 mL of water is equal to 1 gram in laboratory based industry. It's how we calibrate our micropipettes and other liquid handling equipment.

 

[Randy Holmes-Farley Verified Community Expert]

I discuss the density/specific gravity complicatiosn here;

and note this sentence:

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

This month, Randy explains and reviews specific gravity.

reefs.com

The density of pure water at 20 °C is 0.998206 g/cm3, and at 60 °F it is 0.9990247 g/cm3. While these seem close to 1, and are often simply claimed to be 1.00 in many contexts, the difference can be substantial.

If you measure density of to 1.0220 g/cm3 at 20 deg C, and assume the density of fresh water is 1, you get a sg of 1.0220, but when correctly dividing by the density of fresh water at that temp, the sg becomes 1.0220/0.998206 = 1.0238.

 

[Randy Holmes-Farley Verified Community Expert]

I do know that fresh water isn't exactly 1.000sg, however it is common practice to assume 1 mL of water is equal to 1 gram in laboratory based industry. It's how we calibrate our micropipettes and other liquid handling equipment.

It is incorrect and not common in seawater determinations. I show the error above.

 

[Atrumblood]

It is incorrect and not common in seawater determinations. I show the error above.

Ok. That was a big oversight on my part. My bad. However Im fairly certain that my method still provides an accurate measurement since I am filling the cylinder with 25°C water from the tank. Even if the water cools the weight isn't going to change, just the observed volume shrinks. Is that correct?

 

[doubleshot00]

Frustrating!! Purchased a cheap Amazon refractometer and Fritzs calibration solution, calibrated the refractometer and have been going off of that. I happen to pull out the old hydrometer and check using it and it was saying the SG was 1.023 much lower than the refractometer.

Now that I didn't trust what I was seeing I decided to purchase a Hanna salinity tester and the ole hydrometer was right. My "calibrated" refractometer was saying the SG was 1.027 and the Hanna salinity checker confirms the hydrometer is correct and is reading SG of 1.023.

Moral of the story, with one of the most important parameters in the tank, don't trust a cheap refractometer, go with old proven equipment or new technology with proven accuracy! Won't make that mistake again! Now to slowly raise the salinity!

i had the same refractometer and calibration fluid. Mine worked for about 2 weeks then had huge swings.
I bought a Milwaukee. Lessons learned here.

 

[Randy Holmes-Farley Verified Community Expert]

Ok. That was a big oversight on my part. My bad. However Im fairly certain that my method still provides an accurate measurement since I am filling the cylinder with 25°C water from the tank. Even if the water cools the weight isn't going to change, just the observed volume shrinks. Is that correct?

In general, the method will work with a high quality volumetric flask, but probably not very well with a graduated cylinder.

Jim Welsh has a nice discussion of the requirements here (starting at post 11)

Salinity determination by density

The subject of various alternative means of determining salinity previously came up in this thread where I dabbled (in what I now see was an amusingly over-simplified way) with the idea of using a couple of simple tools to determine salinity by calculating the density of your tank water. I have...

www.reef2reef.com

 

[Atrumblood]

In general, the method will work with a high quality volumetric flask, but probably not very well with a graduated cylinder.

Jim Welsh has a nice discussion of the requirements here (starting at post 11)

Salinity determination by density

The subject of various alternative means of determining salinity previously came up in this thread where I dabbled (in what I now see was an amusingly over-simplified way) with the idea of using a couple of simple tools to determine salinity by calculating the density of your tank water. I have...

www.reef2reef.com

Well I do have high quality flasks in my home lab. I can try a comparison. Though I have to wonder if a kitchen scale with an accuracy of 1 gram can even resolve the difference between a flask and a cylinder.

Either way I am able to get three different salinity measurement tools to agree with each other. This is likely due to all of these methods being "hobby grade".

Perhaps I'll save a few months of tank budget and pick up a quality salinity meter to really see if my method is adequate.

Sorry if I frustrated you Randy. As always, I appreciate the wealth of information you bring to these discussions.

 

[FishTruck]

Well I do have high quality flasks in my home lab. I can try a comparison. Though I have to wonder if a kitchen scale with an accuracy of 1 gram can even resolve the difference between a flask and a cylinder.

Either way I am able to get three different salinity measurement tools to agree with each other. This is likely due to all of these methods being "hobby grade".

Perhaps I'll save a few months of tank budget and pick up a quality salinity meter to really see if my method is adequate.

Sorry if I frustrated you Randy. As always, I appreciate the wealth of information you bring to these discussions.

Could you not compare your product to a test solution of 1.025 water and thin spit ball the conversion? Assuming, you can, and want to, consistently re-create your own test solution. I understand it still wont be perfect, but, it would be fun to see what you get.

 

[Randy Holmes-Farley Verified Community Expert]

Could you not compare your product to a test solution of 1.025 water and thin spit ball the conversion? Assuming, you can, and want to, consistently re-create your own test solution. I understand it still wont be perfect, but, it would be fun to see what you get.

what solution with a specific gravity of 1.025 are you referring to?

 

[Atrumblood]

what solution with a specific gravity of 1.025 are you referring to?

Yea I never said anything about a calibration solution. My method relied on gravity and temperature only. I know that you have a post on making calibration solutions.

I have some ACS grade sodium chloride at home that I could make one with however.

Since I am not the expert here, can you tell me why only sodium chloride is used for a calibration solution instead of a salt mix for sea water determination?

 

[FishTruck]

what solution with a specific gravity of 1.025 are you referring to?

I was referring to a commercially available calibration solution. Atrumblood could use a refractometer on both his own test solution and a purchased calibration solution to see how much he is off.

BUT, I just realized Atrumblood was not trying to make calibration fluid, he was just using water. Also, I think it is better to test your equipment in the range you are trying to measure (not 1.000 but 1.025 or so).

NEVERMIND - LOL!!

 

[Randy Holmes-Farley Verified Community Expert]

Yea I never said anything about a calibration solution. My method relied on gravity and temperature only. I know that you have a post on making calibration solutions.

I have some ACS grade sodium chloride at home that I could make one with however.

Since I am not the expert here, can you tell me why only sodium chloride is used for a calibration solution instead of a salt mix for sea water determination?

Salt mix is fine, but they always carry substantial moisture, so you have to make it to about where you want, then independently determine it’s salinity.

For potassium chloride (the material that the PSU salinity scale is actually based on) or sodium chloride, they naturally have little moisture and can be dried if you want (not true of salt mix) so you can know the salinity just by the mass of solids and liquid.

 

[Randy Holmes-Farley Verified Community Expert]

I was referring to a commercially available calibration solution. Atrumblood could use a refractometer on both his own test solution and a purchased calibration solution to see how much he is off.

BUT, I just realized Atrumblood was not trying to make calibration fluid, he was just using water. Also, I think it is better to test your equipment in the range you are trying to measure (not 1.000 but 1.025 or so).

NEVERMIND - LOL!!

The actual reason I asked is you said sg of 1.025 but most standards are 35 ppt, like that one, with sg = 1.0264.

Note there is also a typo on it. It is 53 mS/cm, not the 1000x lower 53 uS/cm.

 

[Derrick0580]

I bought my refractometer at petco. Calibrated it using distilled water and checked it against my hygrometer and it was .01 off. It read 1.026 where as the hygrometer read 1.025. Possibly with actual calibration fluid it may be spot on but for now I just mix to 1.027 on the refractometer to make up the discrepancy!

 

[Ef4life]

I was once frustrated with refractometers and calibration solutions but never again. I bought the tropic Marin hydrometer. It’s great but very fragile being thin glass.

 

[Tuan’s Reef]

I was once frustrated with refractometers and calibration solutions but never again. I bought the tropic Marin hydrometer. It’s great but very fragile being thin glass.

thats exactly what i did. no more solutions to worry about.