Refractometer calibration @ 18C

[Dburr1014]

Which is the correct way to do this?

My basement is at 18 Celsius. This is where I keep my refractometer, I don't believe it is an ATC. I checked my tank, and it's been at 1.030 probably because of my error. I've been slowly trying to drop it to 1.026 but because of my error my auto water change is also at 1.030.

So I pulled out the directions for my refractometer to calibrate it. It says to calibrate it at 20 degrees Celsius if you have an ATC. I don't believe mine is. So it also says in the directions to calibrate it with RO water and I did. I also have some accurasea from 2 little fishes. It's a couple years old but it's been stored in my basement with the cap on. Checked my refractometer and it's about 1.029. How do I know which is correct? I'm assuming that this is because of the two degrees Celsius temperature difference. Is there some sort of compensation I should be doing here?
The reason why I noticed my sg is off is cuz my Joker acan has been closed up for two weeks and I couldn't understand why. So I checked my sg. But now that the sg is lowering its starting to open up again.

 

[Randy Holmes-Farley Verified Community Expert]

Does the refractometer have a name or anything on it?

If it's a brine refractometer, as many are, it will be inherently inaccurate if calibrated with RO/DI, despite being recommended by the manufacturer.

Most refractometers sold to hobbyists are ATC, even if they are brine refractometers. The requirement to calibrate an ATC refractometer at a specific temp doesn't make sense to me, but I know they sometimes state it. Any temp in the ATC range will work fine (how could it not?).

My recommendation is to buy or make a quality 35 ppt standard, then you can use your refractometer at that calibration temp (even if 18 deg C) to match 35 ppt seawater and take all your measurements under the same conditions.

 

[Dburr1014]

Does the refractometer have a name or anything on it?

If it's a brine refractometer, as many are, it will be inherently inaccurate if calibrated with RO/DI, despite being recommended by the manufacturer.

Most refractometers sold to hobbyists are ATC, even if they are brine refractometers. The requirement to calibrate an ATC refractometer at a specific temp doesn't make sense to me, but I know they sometimes state it. Any temp in the ATC range will work fine (how could it not?).

My recommendation is to buy or make a quality 35 ppt standard, then you can use your refractometer at that calibration temp (even if 18 deg C) to match 35 ppt seawater and take all your measurements under the same conditions.

No name on the refractometer.
It does say for salt water and brine.
Funny that the directions have atc and non-atc standards but there is nothing noted on the refractometer itself on what it is. I did get it from another hobbiest, perhaps it once did have a name on it??
I will look up how to make my own. I have seen your work on how to do it.

 

[Dburr1014]

@Randy Holmes-Farley
I need some clarification.
I found this thread on reef2reef; that states, 1tsp per 163.66 mls water. I made it but doesn't seem correct. After calibrating my tank is 42~43 s.g.



Then I found this article that says 161 mls of water to 1tsp.
Which is correct?

I have not made the second recipe yet but it will give me slightly more saline solution to calibrate. Which should give me a lower s.g. Of the tank water.
Please advise.

Thank you in advance

 

[KStatefan]

What are you using to measure salinity?

Randy's article has how to make standards for refractometers, hydrometers and conductivitity meters. Make sure you are looking at the correct section.
The first one looks like the ratio for a conductive standard and the second looks like a specific gravity standard.

 

[Eric R.]

Out of curiosity, are you weighing the salt to 6.20g on a scale or measuring a tsp?

 

[Duncan62]

@Randy Holmes-Farley
I need some clarification.
I found this thread on reef2reef; that states, 1tsp per 163.66 mls water. I made it but doesn't seem correct. After calibrating my tank is 42~43 s.g.



Then I found this article that says 161 mls of water to 1tsp.
Which is correct?

I have not made the second recipe yet but it will give me slightly more saline solution to calibrate. Which should give me a lower s.g. Of the tank water.
Please advise.

Thank you in advance

The 35 solution is 8 bucks on Amazon.

 

[Randy Holmes-Farley Verified Community Expert]

Since I wrote both of those, they are both correct, but for different types of devices.

Be certain to use the recipe appropriate for the type of device being used.

 

[Dburr1014]

Yes, I'm using a refractomer.

The 35 solution is 8 bucks on Amazon.

I have two little fishies cal solution. I think it's not right. Just trying to verify.

Since I wrote both of those, they are both correct, but for different types of devices.

Be certain to use the recipe appropriate for the type of device being used.

Thanks randy, need to go back and make sure again.

 

[Dburr1014]

After doing a whole bunch of reading, testing, calibrating and recalibration... I forgot I have an aquaforest magnesium test kit that I recently purchased. That has a solution standard in the Box unopened. The bottle claims that the salinity is 33% +/- 1.
I'm going to say that's right. So I calibrated my refractometer to that. I checked the two little fishy solution and that was high. Probably due to evaporation. Then I check the solution I made per the recipe in my 4th post. That came out to 1.020 or 32 ppt. I don't know why I followed the recipe exactly.
Then I checked my tank and it was 1.026 or 35 PPT.

 

[Randy Holmes-Farley Verified Community Expert]

I do not think I ever recommended a standard recipe using 1 teaspoon. Too much potential error. my recipes use larger amounts. You or someone else scaled it down to that.

 

[Randy Holmes-Farley Verified Community Expert]

http://reefkeeping.com/issues/2004-06/rhf/index.htm

This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified fresh water. That amount roughly corresponds to ¼ cup (73.1 g) of Morton's Iodized Salt dissolved into 2 liters (2000 g) of water (giving very slightly more than 2 L of total volume).

 

[Dburr1014]

I do not think I ever recommended a standard recipe using 1 teaspoon. Too much potential error. my recipes use larger amounts. You or someone else scaled it down to that.

Well that would explain why it's off. That was just a thread I found on Reef2Reef asking for a scaled-down version of your recipe that you answered.
No worries.