Salinity differences between ICP and Home

[Miami Reef]

I have a Tropic Marin Hydrometer, verified with DIY 35ppt standards.

When I send the results, I usually check my salinity with my hydrometer. I’m always around 1.0264; however, the ICP result (ATI or Oceamo) usually comes back at 33PSU.

I’ve seen other people with similar observations. Is there an explanation?

Off-topic question: If I have freshly made RO/DI that initially reads 0ppt, is it possible for the salinity to rise just from contact with indoor air?


Thank you!

 

[steveweast]

I have the same issue. My results from both Oceanmo and Fauna Marin always come back at 33.5-34.0 ppt. I measure my salinity from not one....but two....TM Precision hydrometers at 77 F....which the hydrometers were calculated to. I also corroborate those two with a Hanna Optical sensor. All of these show that my submittals are at 1.0264....yet ICP disagrees.

 

[Miami Reef]

I use a Salinity Correction Calculator when my temperature is different than 77F to get accurate salinity results: https://www.hamzasreef.com/Contents/Calculators/SalinityCorrection.php

However, I still have differences between ICP and home testing.

My latest Oceamo test came back as 33.3PSU.

 

[DanyL]

I’ve noticed the same with both Fauna and Oceamo.

Following.

 

[Randy Holmes-Farley Verified Community Expert]

I have a Tropic Marin Hydrometer, verified with DIY 35ppt standards.

When I send the results, I usually check my salinity with my hydrometer. I’m always around 1.0264; however, the ICP result (ATI or Oceamo) usually comes back at 33PSU.

I’ve seen other people with similar observations. Is there an explanation?

Off-topic question: If I have freshly made RO/DI that initially reads 0ppt, is it possible for the salinity to rise just from contact with indoor air?


Thank you!

Salinity won’t rise unless quite a bit of salty dust enters it (unlike tds, which is the same thing but a unit of measure far lower.

 

[Randy Holmes-Farley Verified Community Expert]

I’m not actually sure what method they use to get salinity.

Christoph, do you get it from the icp or another way?

@Christoph

 

[Dan_P]

I have a Tropic Marin Hydrometer, verified with DIY 35ppt standards.

When I send the results, I usually check my salinity with my hydrometer. I’m always around 1.0264; however, the ICP result (ATI or Oceamo) usually comes back at 33PSU.

I’ve seen other people with similar observations. Is there an explanation?

Off-topic question: If I have freshly made RO/DI that initially reads 0ppt, is it possible for the salinity to rise just from contact with indoor air?


Thank you!

Is the vendor measuring or calculating the salinity?

Salinity or TDS increase after exposing to air?

 

[Reefahholic]

Mine comes out very close to ICP with my TM hydrometer. I try to keep it at 1.0264, but I sell a lot of frags so I’m constantly trying to top that bag water off with NSW, but sometimes I get lazy and I’ll be at like 1.0256 or a little lower.

 

[Christoph]

I’m not actually sure what method they use to get salinity.

Christoph, do you get it from the icp or another way?

@Christoph

Hi Randy,

we measure salinity using a lab grade conductivity meter (which is temperature compensated). We found this to be the most reliable way.

One potential reasons i see in differing salinity measurements is that salinity is not a parameter that can be measured directly - only indirectly by other physical parameters.

- refraktive index using a refractometer
- density using a hydrometer
- conductivity using a conductivity meter
- ... im sure i forgot something here

In theory all those measurements line up at the same result with very stanardized seawater. When for example magnesium varies (which has quite a huge span in reef tanks), those physical parameters will be affected differently, so there will be some salinity variation in between those measurement techniques. However i did not quantify this effect yet.

Best regards,
Christoph

 

[TangerineSpeedo]

Off-topic question: If I have freshly made RO/DI that initially reads 0ppt, is it possible for the salinity to rise just from contact with indoor air?

I believe the answer could be yes. There is a lot of salt in the air in Miami ( IMO ) horrible for cars and electronics. RODI is a magnet for anything because you took everything out of it. So yes, it is plausible.

 

[gbroadbridge]

I have a Tropic Marin Hydrometer, verified with DIY 35ppt standards.

When I send the results, I usually check my salinity with my hydrometer. I’m always around 1.0264; however, the ICP result (ATI or Oceamo) usually comes back at 33PSU.

I’ve seen other people with similar observations. Is there an explanation?

Off-topic question: If I have freshly made RO/DI that initially reads 0ppt, is it possible for the salinity to rise just from contact with indoor air?


Thank you!

I deliberately ignore any salinity number given by ICP vendors as I know that my measurement is correct given I make my own reference solutions :)

FWIW, I don't know how my local ICP lab measures salinity but it always reads higher than it should by about 1psu.

 

[Randy Holmes-Farley Verified Community Expert]

Hi Randy,

we measure salinity using a lab grade conductivity meter (which is temperature compensated). We found this to be the most reliable way.

One potential reasons i see in differing salinity measurements is that salinity is not a parameter that can be measured directly - only indirectly by other physical parameters.

- refraktive index using a refractometer
- density using a hydrometer
- conductivity using a conductivity meter
- ... im sure i forgot something here

In theory all those measurements line up at the same result with very stanardized seawater. When for example magnesium varies (which has quite a huge span in reef tanks), those physical parameters will be affected differently, so there will be some salinity variation in between those measurement techniques. However i did not quantify this effect yet.

Best regards,
Christoph

Thanks, Christoph!

 

[Pod_01]

Just another set of data, I am not after the absolute values just relative so my TM Hydrometer usually shows 34-35. FM ICP lab is 34-35 as well.
I also use GHL conductivity probe that I calibrate based on my TM Hydrometer and when clean it is in the 34.5-34.6 range. Sometimes it will drop to 33 and back up.

In my early days I used to chase salinity and in hindsight I am quite sure it was all user error.
Once you add salt to water and if you use RODI water for top off there is not an easy mechanism to increase it or decrease salinity and 2PSU is a lot of salt.
When you measure there are few errors, temperature compensation is one.
Another is actual water volume in the tank. Did you measure just before ATO dumped RODI water in the tank or right after. These days I keep a line in my sump return chamber and I try to take measurements when the water level is at that line. This seems to have made things lot more stable.

Also as mentioned another source may be the actual water chemistry and the device used.

Good luck,

 

[Miami Reef]

Thanks, Christoph!

Having the same magnesium level and testing with different measurement tools will lead to different salinity results??

I just want to make sense of this.

 

[Miami Reef]

I will say that when I use my Milwaukee refractometer set to PSU, I get equal results to the Oceamo.

 

[ingchr1]

As a data point, my last Fauna Marin ICP closely matched how I read my TM hydrometer.

FM = 34.2 PSU (converts to 1.0258 SG)
TM = 34.6 PSU (converted from 1.0261 SG)

The ICP actually matched my uncorrected Apera salinity tester reading. I use my TM hydrometer to apply a correction to the Apera. For example, if the Apera reads 0.5 PSU lower than the TM hydrometer I will apply +0.5 to the Apera. Once in a while, I will calibrate the Apera to match the TM hydrometer if it starts to get too far out.

Note that there is nothing too controlled with my measurements, but the alignment appears to be within any margin of error to not be concerned with the differences.

 

[Randy Holmes-Farley Verified Community Expert]

Having the same magnesium level and testing with different measurement tools will lead to different salinity results??

I just want to make sense of this.

What he is saying is that all of these tools may give different readings if the mix of ions present greatly deviates from normal seawater, even if they all agree exactly in normal seawater, because there are some differences in how, say, elevated sulfate and lowered chloride like you have, impact refractive index, conductivity, and density.

Typically these effects are too small to be important (I ran through some calculations in one article), but if the deviation of ion balance is significant enough, it may be detectable. Your sulfate chloride deviation was fairly large and might explain some salinity differences by method.

 

[Miami Reef]

Oh. Awesome. That makes sense. Thank you guys very much.

 

[H.E. Pennypacker]

I haven't noticed it differ much using ATI. I am usually around 1.0258. Measured 1.0256 making samples and last ATI stated 1.0257.

I use a 500mL polypropylene graduated cylinder with my TM hydrometer.

 

[REEFRIED!]

I have had the same issues. Both ICP tests I sent is this year (ATI) came back low on salinity. I calibrate my refractometer about 4-6 times a year. I can’t explain why they get a lower reading than I do. Maybe they use other parameters to determine salinity?