Why do my Salinity measurements vary by 0.002 sg between refractometer and conductivity testers?

[Tim Olson]

I have 3 devices I use to measure salinity, a refractometer, a Hanna HI98319 salinity tester and a Pinpoint Salinity Monitor. I have acquired these 3 devices in my quest to find the most accurate salinity measurement.

My problem is that the Hanna and Pinpoint testers measure 0.02 sg less than my refractometer. For example, the Hanna/Pinpoint will measure 1.025 and the refractometer measures 1.027 sg.

All three devices have been calibrated multiple times, over the years, with their respective manufacturer calibration fluids. So I'm baffled as to why they're so far off from each other.

Any insights would be greatly appreciated.

 

[Retro Reefer]

From what I have been reading this is common for the Hanna checkers. I have the same issues, my calibrated refractometer will read 1.026 and my calibrated Hanna reads 1.025.. the margin of accuracy is +/- 0.001 SG so I’m the ballpark of Hanna’s specifications.

 

[W1ngz]

Hobby grade equipment. Honestly, I'd trust the simplest of the three and go with the refractometer.
You could also just take an average of the three.

 

[Tim Olson]

From what I have been reading this is common for the Hanna checkers. I have the same issues, my calibrated refractometer will read 1.026 and my calibrated Hanna reads 1.025.. the margin of accuracy is +/- 0.001 SG so I’m the ballpark of Hanna’s specifications.

Thanks ... It's frustrating that Hanna can't make the base reading more accurate, like by just adding adding 0.001 to the base reading or some other way. I know these are hobby grade testers, but it should would be nice if they could figure it out.

 

[Tim Olson]

Hobby grade equipment. Honestly, I'd trust the simplest of the three and go with the refractometer.
You could also just take an average of the three.

That sound like a good strategy. I think I'll use the refractometer for now, at least, since the LFS I go to uses a refractometer and it gives consistent readings with my refractometer.

 

[Tim Olson]

FYI, I just decided to get a Hydrometer as a 4th tool to check salinity. It will be interesting to see how the readings compare. I ended up ordering LaMotte Hydrometer that has been calibrated against an NIST certified standard ... http://www.lamotte.com/en/education/sampling-equipment/3-0025.html. If nothing else, it will be another toy to use for the tank.

Also, if anyone is interested, here's where I purchased it from ... https://www.agriculturesolutions.com/lamotte-hydrometer-1-0000-to-1-0700-sg-with-jar-3-0025.

 

[Retro Reefer]

I’m not sweating the differences in readings. What’s more important is consistency and the Hanna has been rock solid for me going on about a year now.. I love the ease and simplicity of the Hanna and can check all my tanks in a matter of minutes, my refractometer I have to calibrate before each use and is effected by temp.

 

[Cory]

This made me nuts for a long time too. I have the hanna digital refractometer reads 1.025, a sybon handheld refractometer reads 1.026 and my last and most trusted is a tropic marine floating glass hydrometer and scientific alcohol thermometer which reads 1.0265. Im pretty certain its 1.026 ish.

 

[biophilia]

I've checked my freshly-calibrated Hanna salinity tester against a freshly calibrated lab-grade Thermo Fisher Orion Star conductivity meter and found that the Hanna consistently reads just shy of 1ppt low -- which is within the +/- 1ppt accuracy stated in the user manual and so good enough for me. It's hard to find anything more accurate than that with hobby grade testing equipment anyway.

 

[Tim Olson]

I'm glad to see I'm not alone

I'm looking forward to getting my Hydrometer to see how it compares. Also, BTW, I use a Traceable Hi-Accuracy Dual Traceable Thermometer, which comes with an NIST calibration certificate. So hopefully this will give me the most accurate test yet. We'll see.

FYI, here's the NIST Traceability calibration process graphic.

 

[Tim Olson]

I've checked my freshly-calibrated Hanna salinity tester against a freshly calibrated lab-grade Thermo Fisher Orion Star conductivity meter and found that the Hanna consistently reads just shy of 1ppt low -- which is within the +/- 1ppt accuracy stated in the user manual and so good enough for me. It's hard to find anything more accurate than that with hobby grade testing equipment anyway.

That's awesome you were able to get an accurate reading with the Orion Star meter. I wish I had access to one.

 

[biophilia]

That's awesome you were able to get an accurate reading with the Orion Star meter. I wish I had access to one.

I volunteer at a large public aquarium and one of the things I do is water quality testing — so fortunately was able to do the comparison that way.

 

[jurgenph]

i think there is also the fact that both are testing something different.

J.

 

[sfin52]

It's interesting skip to the last 10 min or so.

 

[mtfish]

This gets back to accuracy versus precision. I don't know of anything we keep in our tanks that will care which one of those readings you use (if precision is good). Just be consistent with the instrument of choice. I really like conductivity meters the least as I believe there is more issues with precision (may not be the case with a high dollar one). I use a good refractometer and have no worries about salinity.

 

[AquamanE]

This gets back to accuracy versus precision. I don't know of anything we keep in our tanks that will care which one of those readings you use (if precision is good). Just be consistent with the instrument of choice. I really like conductivity meters the least as I believe there is more issues with precision (may not be the case with a high dollar one). I use a good refractometer and have no worries about salinity.

Agreed.

 

[Tim Olson]

It's interesting skip to the last 10 min or so.

Good video ... He shares the same frustrations I have, especially with the Milwaukee refractometer. I bought one about 3 years ago and used it for a few months, then sold it. I could never get consistent readings. It would vary +/- 0.001 sg, which is within spec, I guess. But I would take the same sample and press the button every minute or so and it would 1.024 and then read 1.026 or 1.025. At least my Hanna handheld tester is low, but gives consistent measurements.

I know the exact value is not that critical for home reef tanks, but at least it should be consistent over time.

BTW, as frustrated as I may sound, I really enjoy doing the testing and analysis.

 

[Tim Olson]

This gets back to accuracy versus precision. I don't know of anything we keep in our tanks that will care which one of those readings you use (if precision is good). Just be consistent with the instrument of choice. I really like conductivity meters the least as I believe there is more issues with precision (may not be the case with a high dollar one). I use a good refractometer and have no worries about salinity.

Totally agree on precision.

Also, for fun, I just ordered an ATI ICP-OES Complete Saltwater Water Test Kit, since it includes a salinity test. It will be interesting how it compares to my refractometer, Pinpoint and Hanna testers.

 

[Tim Olson]

FYI, this morning I measured my tank water's salinity with the 5 devices I now have. Also, all of them had been calibrated to their respective standards.

• Vee Gee STX-3 refractometer (newly acquired) calibrated to zero with RODI water (per instructions) also verified with Brightwell Refractometer Calibration Standard at 35 ppt - 1.0248 sg
• Hanna HI98319 Salinity Tester - 31.3 reading + 1.3 ppt adjustment = 32.6 ppt => 1.0246 sg. The 1.3 ppt adjustment was calculated comparing the reading with Randy Holmes-Farley 53 ms/cm standard I made.
• ATC refractometer, which was calibrated with Brightwell Refractometer Calibration Standard - 1.0250 sg
• NIST certified LaMotte Hydrometer with NIST certified Traceable thermometer - 1.0245 before temperature adjustment, 1.0268 sg (35.5 ppt) after temp adjustment
• Pinpoint Salinity Monitor, calibrated with a Randy Holmes-Farley 53 ms/cm standard I made - *1.0235 sg. Although, the monitor has been acting finicky lately, so this reading should not be counted. I'm thinking about getting a new one or replacing the probe to get repeatability again.

I'll also be comparing ratings soon, when I get the ATI ICP-OES Complete Saltwater Water Test Kit, which measures salinity.

The results were somewhat different than I expected, but it would seem that ~1.0250 sg is the actual salinity.

 

[Cory]

Did you just average them all? My problem with this is if you take 5 wrong readings the average is still wrong.

I trust the floating glass hydrometer more than the others. Fwiw my salinity was 28ppt with icp. I used marin labs. I asked how they measured it, with a refractometer. Really? I of course didnt believe it was 28ppt when 3 devices said 1.026 ish