Salinity question

[DavidA]

My Hanna checker indicates the tank as 1.024 while the tropic marin registers 1.0264 even after calibrating the Hanna checker. I followed Randy's DIY recipes for two separate calibration fluids-hydrometer for the Tropic Marin and conductivity for the Hanna. I used a coffee scale and both devices were precisely on target with their calibration fluids (everything done at 77 degrees). The Hanna registered its DIY fluid at 1.026 while the Tropiic Marin registered 1.0264. Yet when I used each device to test my tank water, I still got 1.024 for the Hanna and 1.0264 for Tropic Marin. I also used the Hanna in Hanna calibration fluid and it read on target at 1.026. A difference of .002 seems significant. What can explain that each device reads its benchmark DIY fluid perfectly while there is a .002 discrepancy when sampling the same water?

 

[exnisstech]

I've had two of the Hanna salinty testers and both read 0.002 low when calibrated using Hanna calibration fluid. I used a TM hydrometer and calibrated refractometer to verify.
I was using them and just adding 0.002 to my result which wouid probably be fine but I want a tool that reads correctly so I trashed them

 

[DivingTheWorld]

If you’re referring to the Tropic Marin High Precision Hydrometer, I’d use that as the benchmark.

 

[Randy Holmes-Farley]

Did you allow for temp issues when using the hydrometer, especially with the standard?

What temp was the Hanna diy standard? Maybe it does not temp correct well.

 

[DavidA]

All measurements done at 77 degrees

 

[exnisstech]

If you’re referring to the Tropic Marin High Precision Hydrometer, I’d use that as the benchmark.

I've actually started doing that. I mix my fresh saltwater to 1.025 at 77 degrees tested with the hydrometer and then use that water to check the calibration of the refractometers. I mix weekly so refractometers are calibrated weekly.

 

[DavidA]

Correction. The hydrometer standard was 77. The Hanna was room temp-about 70. The Hanna has built in temp adjustment but you are right-maybe the temp adjustment is off. I will repeat the Hanna standard at 77 tomorrow

 

[Randy Holmes-Farley]

Correction. The hydrometer standard was 77. The Hanna was room temp-about 70. The Hanna has built in temp adjustment but you are right-maybe the temp adjustment is off. I will repeat the Hanna standard at 77 tomorrow

An interesting test would be Hanna in same fluid at different temps and see how well it corrects.

 

[DavidA]

Good idea. I will report back tomorrow

 

[John Neuzil]

My hanna has always read lower than my TM floating hydrometer. I just mentally make the correction to match the TM>

 

[Randy Holmes-Farley]

My hanna has always read lower than my TM floating hydrometer. I just mentally make the correction to match the TM>

Lots of folks say this, and I cannot really understand it, unless it’s the Hanna standard itself that is off.

 

[promotheus2070]

Following

 

[blackholereef]

I have a Hanna Salinity Tester and a Sybon refractometer. The Hanna tester I added a about a year ago and have ended up using both methods to measure my tank water and the Hanna only when mixing salt. The Hanna's digital display is much easier to read than the refractometer. The two don't give the same results when measuring my tank. Typically, there is a 0.5 to 1 ppt (sorry I use ppt instead of specific gravity) difference between the two methods. The Hanna is consistently lower than the refractometer. I calibrate with the Hanna standard for the tester and Randy's recipe for the refractometer standard.
My goal salinity for my tank is 35 ppt and I now want to see 34.5 with the Hanna (example mixing salt or measuring tank water) and 35 with the refractometer.
According to the Hanna owner's manual the accuracy of the tester is plus or minus 1.0 ppt. I don't have the owner's manual anymore for the Sybon so don't know the accuracy reported by the manufacturer.
My ICP came back from Reef-Labs yesterday from a sample sent in last Saturday (10/25) and their result is 34.9. My last two tests for salinity on tank water were 10/21 (34.6 with Hanna 35 with Sybon) and 10/31 (34.5 with Hanna and 35 with Sybon).

 

[EnterName]

Correction. The hydrometer standard was 77. The Hanna was room temp-about 70. The Hanna has built in temp adjustment but you are right-maybe the temp adjustment is off. I will repeat the Hanna standard at 77 tomorrow

An interesting test would be Hanna in same fluid at different temps and see how well it corrects.

It should be easy to see if it is actually the temperature compensation, as the Hanna salinity meter also shows the temperature when taking measurements.
If it shows the correct temperature, the correction should be fine as well, as it is done mathematically. It should be based on "International one-atmosphere equation of state of seawater" [Millero et al, 1981] and the practical salinity scale [PSS-78]. In this case there is only a 0.0036kg/m³ standard error (for the actual density approximation), so I wouldn't expect errors above 0.005 in the specific gravity readings shown by the salinity meter when the temperature is measured correctly.

If you are right about the temperature correction being off, the measured temperature would need to differ by like 10°C/18°F to get from 1.024 to 1.0264 through temperature differences.

 

[DavidA]

An interesting test would be Hanna in same fluid at different temps and see how well it corrects.

Randy, your diagnosis is correct! Mystery solved. I believe the issue is how Hanna adjusts the value based on temp. This morning I calibrated the Hanna meter with the Hanna calibration fluid like I always do- I used the 35 PPT (1.026) at room temperature (68 degrees). It read the DIY solution at 1.026 at 68 degrees correctly. I then used a different Hanna calibration sachet at various temps and found the following: 1.024 at 93 degrees, 1.025 at 74.3 degrees and 1.026 at 66 degrees- a whopping .002 difference dipping in the same calibration solution but at different temps.

Now it gets really interesting. I checked my tank (temp 78) with the Hanna which had been calibrated this morning to a calibration solution at 68 degrees. It measured 1.024 (recall the TM hydrometer was 1.026). I then re-calibrated the Hanna with a calibration sachet at the same temp as the tank. After re--calibration, the tank was 1.025. (minutes after the value of 1.024 with the Hanna calibrated at 68 degrees). I am OK with a difference of .001 between the Hanna and the hydrometer. That is within the error limit of the Hanna. I was not OK with an a delta of .002

Going forward, I will calibrate the Hanna at tank temp rather than room temp and periodically spot check the values with the TM hydrometer. I like the convenience of the Hanna coupled with the reliability of the TM hydrometer.

 

[EnterName]

An interesting test would be Hanna in same fluid at different temps and see how well it corrects.

Randy, your diagnosis is correct! Mystery solved. I believe the issue is how Hanna adjusts the value based on temp. This morning I calibrated the Hanna meter with the Hanna calibration fluid like I always do- I used the 35 PPT (1.026) at room temperature (68 degrees). It read the DIY solution at 1.026 at 68 degrees correctly. I then used a different Hanna calibration sachet at various temps and found the following: 1.024 at 93 degrees, 1.025 at 74.3 degrees and 1.026 at 66 degrees- a whopping .002 difference dipping in the same calibration solution but at different temps.

Now it gets really interesting. I checked my tank (temp 78) with the Hanna which had been calibrated this morning to a calibration solution at 68 degrees. It measured 1.024 (recall the TM hydrometer was 1.026). I then re-calibrated the Hanna with a calibration sachet at the same temp as the tank. After re--calibration, the tank was 1.025. (minutes after the value of 1.024 with the Hanna calibrated at 68 degrees). I am OK with a difference of .001 between the Hanna and the hydrometer. That is within the error limit of the Hanna. I was not OK with an a delta of .002

Going forward, I will calibrate the Hanna at tank temp rather than room temp and periodically spot check the values with the TM hydrometer. I like the convenience of the Hanna coupled with the reliability of the TM hydrometer.

The Salinity meter should display the measured temperature as well. Did you check if the shown temperature is correct?

The temperature correction is done mathematically so I'm really confused how it can be wrong when the temperature measurement is correct.

I will make some tests today with my Hanna salinity meter... Maybe they forgot to use a different formula for the temperature correction of the calibration solution, which would explain why it needs a specific temperature during calibration to work.

Which DIY calibration solution did you previously use? I assume the Hanna salinity meter is based on a KCl solution according to the practical salinity scale (PSS-78) standard. This could mean calibrating it at different temperatures with NaCl or seawater doesn't work as the temperature correction is different for these solutions.

 

[TangerineSpeedo]

Things of note;
With the Hanna, you should alway calibrate it at tank temp.
I have two TM precision hydrometers and they read differently. Value difference of .0007. So if one reads 1.026 the other will read 1.0267. I think it is interesting that people look at the TM as the gold standard, when the maximum deviation is .001 which is quite significant.
As far as using the Milwaukee digital refractometer, I have found that ambient temperatures affect it greatly, even though it has temp compensation. Since my household is not temperature controlled, it can vary from low 60's F to mid 80's F, I tend to not use that very much.
I am sorry, but I can't remember which reefer who works in the scientific community. But they referred to the differences in a standard of measurements as "noise". Which I think is the most accurate description of what happens in your brain trying to make sense of "WHY"
For the best peace of mind;
Pick a method and have a back up with a expected deviation for when you come across a "this just doesn’t seem right moment".
Throttle back slightly. I run my tank @34.5 PSU because it is a safe zone within error.
Also by picking one method, it does not have to be the same for everything. I use the TM hydrometer for mixing salt and my Hanna for the tank. Which technically using a Hanna for the tank is probably the worst thing for accuracy, because of other biologicals in the tank that can affect the reading. But I don’t need the hanna to be accurate as much as consistent.
Happy Reefing

 

[DavidA]

The Salinity meter should display the measured temperature as well. Did you check if the shown temperature is correct?

The temperature correction is done mathematically so I'm really confused how it can be wrong when the temperature measurement is correct.

I will make some tests today with my Hanna salinity meter... Maybe they forgot to use a different formula for the temperature correction of the calibration solution, which would explain why it needs a specific temperature during calibration to work.

Which DIY calibration solution did you previously use? I assume the Hanna salinity meter is based on a KCl solution according to the practical salinity scale (PSS-78) standard. This could mean calibrating it at different temperatures with NaCl or seawater doesn't work as the temperature correction is different for these solutions.

I used a hanna sachet (kcl per google AI) to perform the temp tests. I used Randy’s DIY recipes https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/ to check that the Hanna and hydrometer were performing to standard. The thermometer on my Hanna records a nearly identical temp for a my tank compared to the Apex probe.

 

[DavidA]

It should be easy to see if it is actually the temperature compensation, as the Hanna salinity meter also shows the temperature when taking measurements.
If it shows the correct temperature, the correction should be fine as well, as it is done mathematically. It should be based on "International one-atmosphere equation of state of seawater" [Millero et al, 1981] and the practical salinity scale [PSS-78]. In this case there is only a 0.0036kg/m³ standard error (for the actual density approximation), so I wouldn't expect errors above 0.005 in the specific gravity readings shown by the salinity meter when the temperature is measured correctly.

If you are right about the temperature correction being off, the measured temperature would need to differ by like 10°C/18°F to get from 1.024 to 1.0264 through temperature differences.

Fascinating! Recall that yesterday I checked a Hanna calibration sachet (potassium cloride) at different temps and the values varied from 1.024 at 93 degrees to 1.026 at 66 degrees. This morning I tested tank water with the Hanna and it was 1.025 at 68, 74 and 88. We can conclude from the two sets of observations (yesterday with KCL sachet and today with tank water) that the Hanna temp adjustment algorithm works for predominantly based saline saltwater but does not work for KCL calibration fluid.

This probably explains why so many hobbyists on R2R find that the Hanna is too low by .002. Like me, these hobbyists probably calibrated with a room temp packet of the Hanna KCL solution. I think the solution is easy- calibrate the Hanna at tank temp and spot check occasionally with hydrometer.

 

[DavidA]

I can’t vouch for Google AI but it agrees that the formula to adjust specific gravity for seawater will not work for KCL

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