Reef Aquarium Salinity: DIY Calibration Standards

[Randy Holmes-Farley Verified Community Expert]

I have a digital kitchen scale that measures in grams. If I take your 3.65g salt to 96.35g of purified water and multiply by 3 it gives me 10.95 (round to 11g) salt to 289.05 (round to 289g). This small rounding shouldn't hurt the equation too much as even decent digital scales have some +/-. The resulting fluid should give me a calibrating fluid extremely close to 1.0264 on my refractometer..........correct?

Yes and no. The errors you post are no issue, but 11 g could be 10.50 or 11.50, and that’s a big difference. I’d get a better scale, or use much larger amounts.

 

[Mac8128]

Yea, that’s what I was afraid you would say. Thanks, will do

 

[RobertK]

Randy-

Thank you for this informative article. It has different instructions for making a refractometer standard and a specific gravity standard, which confuses me.

I have a TM hydrometer and a VeeGee refractometer. I would like to use them both in different situations and be able to rely that I will get the same (or similar enough) results with either device.

Should I:
a) make two different calibration standards for the two different devices, or
b) make a SG standard to confirm that the hydrometer is accurate, then calibrate the refractometer to the hydrometer using the same solution, or
c) do something else?

Thanks as always for your help!

 

[BeanAnimal]

I would first check the hydrometer. Other than temperature, it does not deviate.

If it is accurate I would use the same solution to calibrate the refractometer.

I would then sanity check. Hydrometer reading in tank water, and see of the refractometer matches.
From that point on you can test tank water with hydrometer from time to time and then set refractometer to match.

This does not check the trueness of the hydrometer, as you only used one standard against it. But it does give you a single point of calibration against one standard and a refractometer that can be calibrated to match.

 

[Randy Holmes-Farley Verified Community Expert]

Randy-

Thank you for this informative article. It has different instructions for making a refractometer standard and a specific gravity standard, which confuses me.

I have a TM hydrometer and a VeeGee refractometer. I would like to use them both in different situations and be able to rely that I will get the same (or similar enough) results with either device.

Should I:
a) make two different calibration standards for the two different devices, or
b) make a SG standard to confirm that the hydrometer is accurate, then calibrate the refractometer to the hydrometer using the same solution, or
c) do something else?

Thanks as always for your help!

A. Making two different solutions allows you to test two different types of devices directly, and is preferable.

B. This is not appropriate. The sg standard is not appropriate for a refractometer.

C. You could calibrate one device with its appropriate standard, measure some tank water, and use that as a standard for all devices. This is less desirable since each such transfer to a new fluid has its own potential errors. But it is accurate enough for reefing.

 

[RobertK]

Thanks Bean and Randy!

I made both solutions as Randy advised.

Using the SG standard my TM hydrometer read 1.0264 at 77° F. My understanding is that this means the hydrometer is accurate and does not need to be tested against a standard any further.

Using the refractometer standard my VeeGee read 1.026/35 ppt. My understanding is that this means the refractometer is properly calibrated at present and should be recalibrated from time to time.

Checking the tank water with both devices I then got 1.0261@77° with the hydrometer, and 1.025/34 ppt with the refractometer. The latter measurement appears to be within the margin of error of the refractometer, which VeeGee lists as +/-0.001 SG or +/-1%. But it also says to calibrate it using distilled water at 20°C/68°F - should I do that?

 

[Randy Holmes-Farley Verified Community Expert]

Both devices are good to go.

How had you calibrated the veegee before measuring the standard?

 

[RobertK]

How had you calibrated the veegee before measuring the standard?

I assumed that the hydrometer was correct (without checking it against a standard). I measured SG of tank water with the hydrometer, then set the refractometer to the same reading.

 

[Randy Holmes-Farley Verified Community Expert]

I assumed that the hydrometer was correct (without checking it against a standard). I measured SG of tank water with the hydrometer, then set the refractometer to the same reading.

Ok, then since we now know the hydrometer was good, you used the method C I said above and that’s fine.

It is a fine experiment to see what happens if you calibrate with pure fresh water, but it’s not going to give a more accurate value than it gives now, and may be worse.

 

[RobertK]

Ok, then since we now know the hydrometer was good, you used the method C I said above and that’s fine.

It is a fine experiment to see what happens if you calibrate with pure fresh water, but it’s not going to give a more accurate value than it gives now, and may be worse.

I probably won't bother calibrating the refractometer with pure fresh water if it won't give a more accurate value. But I guess this demonstrated the margin of error of the refractometer, given that I just calibrated it against the hydrometer a few days ago and they now disagree by 0.001 SG.

Last question: Is it ok to save the refractometer standard in an airtight jar? How long does it stay good for?

 

[Randy Holmes-Farley Verified Community Expert]

I probably won't bother calibrating the refractometer with pure fresh water if it won't give a more accurate value. But I guess this demonstrated the margin of error of the refractometer, given that I just calibrated it against the hydrometer a few days ago and they now disagree by 0.001 SG.

Last question: Is it ok to save the refractometer standard in an airtight jar? How long does it stay good for?

As long as it does not evaporate, it is good forever. :)

 

[RobertK]

Great, thanks Randy!

 

[BeanAnimal]

B. This is not appropriate. The sg standard is not appropriate for a refractometer.

That means I misspoke above. I did not realize that there were two. So much for reading I guess.

 

[Randy Holmes-Farley Verified Community Expert]

That means I misspoke above. I did not realize that there were two. So much for reading I guess.

Since the diy standards are pure sodium chloride, they cannot be made to match all of refractive index, conductivity, or density of 35 ppt seawater at the same time. Only one at a time.

Of course, one standard can match 35 ppt on one device and some other value on a different type of device that is close to, but not exactly 35 ppt. I have not tried to determine what those different numbers are, but it’s a fine approach if someone did it.

 

[RobertK]

Thanks again Randy and Bean. I don't know why this was so confusing. Now that I know the hydrometer is accurate I don't think it needs to be checked against a standard again. In practical terms I think that the simplest thing is for me to adjust the refractometer from time to time so that it matches the hydrometer, as you both suggested.

 

[Randy Holmes-Farley Verified Community Expert]

Thanks again Randy and Bean. I don't know why this was so confusing. Now that I know the hydrometer is accurate I don't think it needs to be checked against a standard again. In practical terms I think that the simplest thing is for me to adjust the refractometer from time to time so that it matches the hydrometer, as you both suggested.

Sounds good.

Happy Reefing!

 

[kisember]

According to the tables in the opening post, the S=35 water has 1.0264 specific gravity at 25 Celsius and 1.0266 at 20 Celsius so measuring tank water at any room temperauture is just fine, far below the margin of error imho.
Using the calculator made by @BeanAnimal available at here however shows 1.0256 (S=33.7) for the 1.0266@20C reading, which I think is out of the acceptable range.
Is there an explanation I missed for these different numbers, or the data is wrong?
Thanks for both the detailed article and the calculator!

 

[Randy Holmes-Farley Verified Community Expert]

According to the tables in the opening post, the S=35 water has 1.0264 specific gravity at 25 Celsius and 1.0266 at 20 Celsius so measuring tank water at any room temperauture is just fine, far below the margin of error imho.
Using the calculator made by @BeanAnimal available at here however shows 1.0256 (S=33.7) for the 1.0266@20C reading, which I think is out of the acceptable range.
Is there an explanation I missed for these different numbers, or the data is wrong?
Thanks for both the detailed article and the calculator!

Specific gravity is not a hydrometer reading. The latter changes a lot with temperature while actual specific gravity does not.

 

[Ryan - Serious Reefs]

If anyone finds typos or mistakes or other issues in the article, please post them here so I can fix them. :)

I Just wanted to thank you for these. They are immensely helpfull :)

 

[Randy Holmes-Farley Verified Community Expert]

Thanks!