Huge discrepancy between refractometer and digital salinity meter

[Ardeus]

I recently got a digital salinity meter (Scionix) to replace my old refractometer.

I callibrated it with the fluid included in the package and I noticed that my refractometer was way off by comparison, so I began to slowly adjust the salinity of my tank. I also callibrated the refractomter with the fluid.

In the last few days I began noticing that something was not right with the corals.

I suspected the salinity could be the problem, so I made my own callibration solution with freshwater and table salt (963 gr of water + 37 gr of table salt).

I tested it with my refractometer: 1.025
I tested it with the digital salinity meter: 1.031

I tested the tank again:

Refractometer: 1.024
Salinity meter: 1.025

When I test the refractometer with rodi water it shows a value probably around 0.995. When I test rodi water with the digital salinity meter it shows 1.000.

What the heck is going on???

The nearest LFS is over 200kms away.

 

[4n2xtacy]

That's crazy... tg hus is where the old school comes into play. I have a hydrometer and a refractometer. Every once in a while, once every 2 months, I calibrate my hydrometer and compare with my calibrated refractometer to make sure my refractometer is on point.

 

[Waters]

I own both a digital (Milwaukee) and a refractometer and I can can I always trust the refractometer with calibration solution over the digital. The digital always reads high and would rarely give me consistent readings, even back to back.

 

[Marksreef]

You need fresh calibration fluid of 35ppt.
I’m not too sure about this table salt thing but I wouldn’t trust it.
If your new digital meter came with soulution, it most likely is 35 ppt
I would trust that first BUT the solution could be bad too.
If you ordered the digital meter online, I would contact them for some more solution or buy a bottle of 35 ppt. solution n

 

[Retro Reefer]

I just went through this same issue when I received my new Hanna salinity tester, after careful calibration it was reading much lower than my refractometer so I ordered a fresh bottle of calibration solution to check my refractometer and to add some redundancy I made a batch of my own solution which matched the commercial solution exactly so I was confident in results.. turns out the Hanna is in fact reading consistently low but as long as I know the discrepancy it’s not a issue as I just compensate for it when testing. The Hanna makes checking salinity so much easier than my refractometer or hydrometer.

For 35 ppt solution mix 1.55 grams iodized salt with 40.25 grams of distilled water.

 

[Ardeus]

I own both a digital (Milwaukee) and a refractometer and I can can I always trust the refractometer with calibration solution over the digital. The digital always reads high and would rarely give me consistent readings, even back to back.

Thanks. Until I understand what's going on, I will trust the refractometer.

 

[Ardeus]

I

That's crazy... tg hus is where the old school comes into play. I have a hydrometer and a refractometer. Every once in a while, once every 2 months, I calibrate my hydrometer and compare with my calibrated refractometer to make sure my refractometer is on point.

I never expected to be thinking about getting an hydrometer after 13 years with a reef tank, but having a 3rd equipment makes sense.

 

[wangspeed]

You are measuring the wrong thing. Your digital meter is not a refractometer. It is measuring conductivity. You cannot use a seawater conductivity standard of 50,000 uS/cm in a refractometer, nor can you use a typical refractometer calibration fluid as an electro conductivity standard. You need to calibrate each with its own appropriate fluid and then check it against a known seawater salinity.

 

[Ardeus]

You need fresh calibration fluid of 35ppt.
I’m not too sure about this table salt thing but I wouldn’t trust it.
If your new digital meter came with soulution, it most likely is 35 ppt
I would trust that first BUT the solution could be bad too.
If you ordered the digital meter online, I would contact them for some more solution or buy a bottle of 35 ppt. solution n

No one sells callibration fluids in Portugal. You can only get it with new equipments.

I found the information on how to make your own here http://reefkeeping.com/issues/2004-06/rhf/index.php

 

[Ardeus]

You are measuring the wrong thing. Your digital meter is not a refractometer. It is measuring conductivity. You cannot use a seawater conductivity standard of 50,000 uS/cm in a refractometer, nor can you use a typical refractometer calibration fluid as an electro conductivity standard. You need to calibrate each with its own appropriate fluid and then check it against a known seawater salinity.

Ah, that explains it. Thanks a lot. I can't use the liquid of the digital meter on the refractometer. And I can't use the home recipe I found here for the digital meter http://reefkeeping.com/issues/2004-06/rhf/index.php

 

[4n2xtacy]

You are measuring the wrong thing. Your digital meter is not a refractometer. It is measuring conductivity. You cannot use a seawater conductivity standard of 50,000 uS/cm in a refractometer, nor can you use a typical refractometer calibration fluid as an electro conductivity standard. You need to calibrate each with its own appropriate fluid and then check it against a known seawater salinity.

That does make a lot of sense... thanks[emoji108]

 

[Marksreef]

You are measuring the wrong thing. Your digital meter is not a refractometer. It is measuring conductivity. You cannot use a seawater conductivity standard of 50,000 uS/cm in a refractometer, nor can you use a typical refractometer calibration fluid as an electro conductivity standard. You need to calibrate each with its own appropriate fluid and then check it against a known seawater salinity.

Nice catch.

 

[Abdullah Al Faruq]

Mr. Ardeus, any update on what's going on now with your refractometers ? Did you find your problem solved?

 

[Ardeus]

No and in fact I am about to try something out.

I took my refractometer, which I callibrated with rodi water to a fish store and I compared with theirs and it was spot on.

I tried the digital meter after callibrating with the appropriate fluid and it was .005 off.

I am going to make a batch of water at 30 ppt and callibrate it with this water and then try it on my tank. I will report how it went in a couple of hours.

 

[ZoWhat]

You need fresh calibration fluid of 35ppt.

Theres the golden answer. Dont calibrate with 1.000 solution. Dont calibrate with homemade ingredients. Buy a labgrade solution specifically made to test SG

Calibrating at 1.000 is like calibrating a speedometer at a 2mph speed and expect it to be exactly accurate at 65mph. Sooooo much slope btwn 1.000 and 1.025 bc testing is NOT completely straightline linear


.

 

[Water Dog]

That homemade iodized salt and distilled water calibration solution formula from ReefKeeping magazine is from Dr. Randy Holmes-Farley, so I would have absolutely no issues with using it to calibrate my instruments.

 

[Ardeus]

The instructions for this digital meter do not mention 35 ppt but 30 ppt for calibration. The liquid they sent also says 30 ppt on the bottle. Maybe they are wrong?

I know most refractometers shouldn't be calibrated with rodi water (although D-D specifically advises to calibrate their refractometer with rodi water), but it was what I had at hand and at the store it was spot on.

 

[Ardeus]

I made a batch of water at approximately 1.0225 (measured with the refractometer) and used it to callibrate the salinity meter. Now the refractometer and the digital meter agree when I test the tank water.

 

[ZoWhat]

That homemade iodized salt and distilled water calibration solution formula from ReefKeeping magazine is from Dr. Randy Holmes-Farley, so I would have absolutely no issues with using it to calibrate my instruments.

Wouldnt using Morton's Salt Crystals still be in tiny small hardened crystalline deposits and not fully dissolved in the water as a liquid?

I'm no where near being Chemist but I do question salts crystals floating around suspended in water versus micron salt particles in the same water using a refined salt powder.....

Idk. When it comes to testing....question everything.....

 

[Water Dog]

Wouldnt using Morton's Salt Crystals still be in tiny small hardened crystalline deposits and not fully dissolved in the water as a liquid?

I'm no where near being Chemist but I do question salts crystals floating around suspended in water versus micron salt particles in the same water using a refined salt powder.....

Idk. When it comes to testing....question everything.....

I’ve made Randy’s calibration solution before using my digital kitchen scale and to my eyes, it appeared to be fully dissolved into solution. In fact, I’d take it a step further and say I trust Randy’s DIY solution over the Aquacraft solution sold by BRS as I have experienced inconsistent results from different bottles of the Aquacraft calibration fluid before. As with everything in this hobby YMMV.