DIY Salinity probe calibration solutions

[Randy Holmes-Farley Verified Community Expert]

Was able to do the calibration. It did measure the 40ppt and 8ppt stock solutions after calibration correctly so seemed that calibration was ok, but when I measured a reference fluid of 33ppt, it still showed 21.2 ppt, 2.12% and 1.014 for that 33ppt saltwater solution. So I assume it does not measure seawater ppt but some other salinity?

Found a "data sheet" of the product: http://www.tools.in.th/manual/Catalog-AR8012.pdf

What standard do you have that is 33 ppt? Maybe it was not made correctly.

You probably also calibrated the refractometer with it, so matching the refractometer may be circular logic if that standard is off.

 

[Bramzor]

What standard do you have that is 33 ppt? Maybe it was not made correctly.

You probably also calibrated the refractometer with it, so matching the refractometer may be circular logic if that standard is off.

Have you seen my last update? https://www.reef2reef.com/threads/diy-salinity-probe-calibration-solutions.729812/#post-7599395
The standard that I'm using is the DSR Reference. So I'm certain that one is correct. Also checked with my tank. My refractometer has never been calibrated yet but still measuring correctly after 2 years. Have 6 different methods to measure salinity and they all measure the same value. The problem was using the 40ppt calibration solution as it seems that it measures it in a different way and need a saltwater solution to calibrate it for saltwater.

 

[Randy Holmes-Farley Verified Community Expert]

. The problem was using the 40ppt calibration solution as it seems that it measures it in a different way and need a saltwater solution to calibrate it for saltwater.

I don't think that can be true.

I'm concerned the standard you have is off. And a refractometer that has never been calibrated sounds dicey at best. Commercial standards can be off for many reasons, including someone buying it, using some, adding DI water to refill, and return for a refund.

Anyway, to rule this out, I'd suggest making my 35 ppt standards that are specific to each device (refractometer and conductivity) and verify if each is actually reading correctly.

Easy insurance to be sure there are no systemic issues:


Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com

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

 

[Bramzor]

You mentioned before that a meter can measure different things right? You mentioned seawater ppt and sodium chloride ppt. I think you are on the right path.

Pretty sure the standard is not off. Also because it now matches my tank and I assume I wouldn't have a lot of SPS left if I would be running my tank on 21ppt right? Also I have a calibrated salinity probe on my Apex reporting stable 34ppt on my tank. That is why I do not calibrate my refractometer because it's not my main method for testing, only when it would show to be off which it never did.

So based on all that evidence, it is safe to say that calibration using the standards mentioned did not work (I have no clue why as the method is for calibrating EC). It seems the calibration is for a different salinity while calibration using the reference solution of DSR did provide a valid calibration. Maybe the probe has a different math to calculate salinity and therefore, needs to be calibrated in a different way? So with real seawater salt instead of sodium chloride? Makes any sense?

 

[Randy Holmes-Farley Verified Community Expert]

I don't know why you are resistant to doing the calibration check I suggested. It would eliminate any concerns.

Probes like this and the APEX only measure conductivity. What units they report and how they are calibrated impact the values, but it is not measuring something different.

The same is true of refractometers. Most that hobbyists use (probably you too) are not true seawater refractometers, so when calibrated as directed by the manufacturer with RO/DI water will NECESSARILY be inaccurate.

That is why I developed the idea of a 35 ppt standard, to make these refractometers function to measure 35 ppt seawater salinity just as well as a true seawater refractometer. Now many companies sell standards for it.

 

[Bramzor]

I don't know why you are resistant to doing the calibration check I suggested. It would eliminate any concerns.

To make a 3.29 weight percent sodium chloride solution, dissolve 6.20 grams (1 teaspoon) of Morton's Iodized Salt in 182 mL (182 g) of fresh water (making a total volume of about 184 mL after dissolution of the salt). This solution can be scaled up as desired.

So I did make the solution of 6.20 grams of NaCL and 182mL of RODI water.
When measuring this using the salinity (conductivity) meter, it gives me 39ppt. (When measuring this using refractometer it shows 29.)

So my concerns remain.

When I measure my tank water using the salinity meter and using refractometer, or when I measure a reference salinity fluid that I have, they both give about the same result 34ppt. So my guess is that what I measure is correct, but the solution described above is flawed in some way. I have a Flow Hydrometer and a Swing arm Hydrometer too and other methods to confirm salinity levels if needed.

 

[Randy Holmes-Farley Verified Community Expert]

So I did make the solution of 6.20 grams of NaCL and 182mL of RODI water.
When measuring this using the salinity (conductivity) meter, it gives me 39ppt. (When measuring this using refractometer it shows 29.)

So my concerns remain.

When I measure my tank water using the salinity meter and using refractometer, or when I measure a reference salinity fluid that I have, they both give about the same result 34ppt. So my guess is that what I measure is correct, but the solution described above is flawed in some way. I have a Flow Hydrometer and a Swing arm Hydrometer too and other methods to confirm salinity levels if needed.

If you made the solution as I described, it is suitable to determine that both of your devices are out of whack, and in different directions.

That 35 ppt DIY solution is not made for a refractometer. As I show in the article, it will read much lower in a refractometer than a conductivity meter. It will correctly read as ~ 31.5 ppt on a refractometer. So the refractometer reading (29 ppt) is too low by about 2.5 ppt (assuming you made it correctly). That refractometer should be recalibrated, IMO.

The conductivity meter is reading high by 4 ppt. Did it read the 40 ppt solution correctly after calibration?

 

[Bramzor]

The conductivity meter is reading high by 4 ppt. Did it read the 40 ppt solution correctly after calibration?

After calibrating it using the 40ppt solution? Yes, it did read the 40ppt calibration solution as 40ppt and even the 8ppt solution correctly. Problem was that everything else was wrong.

 

[Bramzor]

Calibrated the probe using the 35ppt DIY solution and measured my tank:

Salinity probe: 31.5ppt
Refractometer: 33ppt
Calibrated Apex salinity probe: 33.9ppt

33ppt DSR Reference using Salinity probe: 29.7ppt
So unless salinity can drop over time in a closed bottle... Something is most definitely out of whack.

Will compare it tomorrow with a Hanna Instruments 35ppt salinity calibration fluid to figure out what the value should be.

 

[Randy Holmes-Farley Verified Community Expert]

Calibrated the probe using the 35ppt DIY solution and measured my tank:

Salinity probe: 31.5ppt
Refractometer: 33ppt
Calibrated Apex salinity probe: 33.9ppt

33ppt DSR Reference using Salinity probe: 29.7ppt
So unless salinity can drop over time in a closed bottle... Something is most definitely out of whack.

Will compare it tomorrow with a Hanna Instruments 35ppt salinity calibration fluid to figure out what the value should be.

Sorry, I’m confused. How did you calibrate the salinity probe with the 35 ppt, 53 mas/cm standard? Doesn’t it think that is 40 ppt?

 

[Bramzor]

Sorry, I’m confused. How did you calibrate the salinity probe with the 35 ppt, 53 mas/cm standard? Doesn’t it think that is 40 ppt?

The salinity meter allows a calibration towards a range of values. Low range, this is 5-8ppt and for the high range, it's 32-40ppt. So I made your solution and calibrated it towards 35ppt to get the above measurements.

 

[Randy Holmes-Farley Verified Community Expert]

The salinity meter allows a calibration towards a range of values. Low range, this is 5-8ppt and for the high range, it's 32-40ppt. So I made your solution and calibrated it towards 35ppt to get the above measurements.

After you did whatever you did, did it read that 35 ppt diy as 35 ppt?