Salinity Reference Solution - Hanna vs Neptune

[Gho5t]

So I was double checking my Apex salinity probe today and my Hanna salinity tester. I first calibrated the Hanna against its reference packet of 35ppt, then I calibrated my Apex probe against the Neptune 53,000 calibration solution. Both tested properly against their respective solutions at 35ppt. However when I tested my tank they were different, so I tested the Neptune calibration solution with the Hanna and it came up as 33.1ppt. This would explain the difference I’ve been seeing between my Apex and Hanna but this whole time I was assuming the Apex probe drifted. Now that I know the reference solutions don’t test the same I broke out my refractometer and it was closer to the Apex probe than the Hanna so I’m trusting that for now.

Has anyone else seen this difference with the reference solutions? Is there a reason they don’t line up at 35ppt? Which one should I trust considering they are both accurate to their respective reference?

 

[Acros]

Milwaukee salinity solution is always +0.02 or -0.02 depending on the bath. The refractometer is spot on tho.

 

[Randy Holmes-Farley]

So I was double checking my Apex salinity probe today and my Hanna salinity tester. I first calibrated the Hanna against its reference packet of 35ppt, then I calibrated my Apex probe against the Neptune 53,000 calibration solution. Both tested properly against their respective solutions at 35ppt. However when I tested my tank they were different, so I tested the Neptune calibration solution with the Hanna and it came up as 33.1ppt. This would explain the difference I’ve been seeing between my Apex and Hanna but this whole time I was assuming the Apex probe drifted. Now that I know the reference solutions don’t test the same I broke out my refractometer and it was closer to the Apex probe than the Hanna so I’m trusting that for now.

Has anyone else seen this difference with the reference solutions? Is there a reason they don’t line up at 35ppt? Which one should I trust considering they are both accurate to their respective reference?

One of the complications with such a cross check is that the temp probe for the APEX also needs to be in the salinity fluid, or it needs to be at exactly the same temp as the tank.

 

[Gho5t]

I did float the fluid in the sump next to the probes for an hour and then tested while it was still half submerged in the sump‘s water. I did not put the temp probe in the solution with the probe as I figured the hour long float would good to get it to the right temp.

 

[Randy Holmes-Farley]

I did float the fluid in the sump next to the probes for an hour and then tested while it was still half submerged in the sump‘s water. I did not put the temp probe in the solution with the probe as I figured the hour long float would good to get it to the right temp.

OK, that should have been good enough. maybe one or both of the fluids is off.

 

[Gho5t]

Yea that's what I'm thinking. I'm assuming at this point the Hanna is off since my refractometer was close to my Apex reading. Being off by 2ppt is a lot which made me nervous because I had been assuming the Apex was wrong for the last couple of months since it tends to fluctuate if micro-bubbles accumulate. Currently Hanna is reading 34 and the Apex is 36.5 and the refractometer looks like 35.5 so I'm just going to let it ride. lol

 

[jjencek]

One more think to check is to mix a known amount of salt at known salinity.

 

[Randy Holmes-Farley]

One more think to check is to mix a known amount of salt at known salinity.

That is the basis of my DIY recipes for salinity standards:

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

 

[Gho5t]

Thanks! I'll definitely give that a go and see where things end up.

 

[Jay Hemdal]

That is the basis of my DIY recipes for salinity standards:

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

Hi Randy,

I'm having a terrible time standardizing all of my facility's hydrometers and salinity meters. Seems like we have a "Tower of Babel" issue - floating hydrometers, swing arm hydrometers, optical refractometers, digital optical refracs and then digital conductivity meters - they just don't "play nice" with one another.

I know that we are close enough in salinity that the animals will be fine, but I buy around $70k worth of salt a year. If we mix it up even 5% hotter than it needs to be, I've thrown away $3500.

I have a Tropic Marin German glass hydrometer that I've used as my default standard for specific gravity for 20 years or so. My worry is that if that is wrong, everything bench marked from it will be incorrect as well.

I have had issues with commercial salinity standards in the past, they often seem to standardize low for me. So - I decided to make my own standard - I used a more anhydrous sea salt (Omega Sea) and I desiccated it a bit and then weighed out 35.00 grams in 1000.0 ml of RODI water. This also read low. I then read your NaCl recipe using 36.5 g salt in 963.5 ml RODI. I did that, but used the desiccated sea salt (my concern is that NaCl alone might not read properly with the optical refracs. This gave me better results, but I'm concerned that using synthetic sea salt is an issue. Thoughts?

Thanks,

Jay

 

[Randy Holmes-Farley]

I have had issues with commercial salinity standards in the past, they often seem to standardize low for me. So - I decided to make my own standard - I used a more anhydrous sea salt (Omega Sea) and I desiccated it a bit and then weighed out 35.00 grams in 1000.0 ml of RODI water. This also read low. I then read your NaCl recipe using 36.5 g salt in 963.5 ml RODI. I did that, but used the desiccated sea salt (my concern is that NaCl alone might not read properly with the optical refracs. This gave me better results, but I'm concerned that using synthetic sea salt is an issue. Thoughts?

The dried salt likely read low because it is quite hard to remove all the water from magnesium and calcium salts.

If you heat it high enough to do that, you cause other changes, such as carbonate being converted into oxides with loss of CO2.

This is why scientists mostly gave up on the method long ago.

 

[Jay Hemdal]

The dried salt likely read low because it is quite hard to remove all the water from magnesium and calcium salts.

If you heat it high enough to do that, you cause other changes, such as carbonate being converted into oxides with loss of CO2.

This is why scientists mostly gave up on the method long ago.

Yes, when I heated it too high, I formed some insoluble material, I threw that batch out.
I guess I’m just wondering why not use sea salt like I did, won’t that give more accurate results for optical refracts than just NaCl?
Jay

 

[Randy Holmes-Farley]

Yes, when I heated it too high, I formed some insoluble material, I threw that batch out.
I guess I’m just wondering why not use sea salt like I did, won’t that give more accurate results for optical refracts than just NaCl?
Jay

For a standard, a seawater mimic, or actual seawater, is a perfect way to go because if it is really 35 ppt, it should read 35 ppt on any type of device. You just need to know how to make it and to ensure it is 35 ppt. That last bit requires some other way to know the salinity of it because you cannot get it from the ingredients due to variable moisture in the solids.

Making a standard out of something else (say, NaCl) is no less accurate, and may be more so, since salts like NaCl or KCl do not absorb much moisture and can be thoroughly dried if you want. Using them you can make a standard that is highly accurate, but device specific, like 35 ppt seawater equivalent for a conductivity meter (but it will not give 35 ppt when used on a refractometer).

The benefits of these specific standards is so high, that the practical salinity scale is based on 35 PSU is defined as a seawater solution having the same conductivity as a KCL solution that is 32.4356 g of KCl at 15 degrees C in a 1 kg solution.