Hanna vs Refractometer salinity discrepancy

[Daftendire]

Greetings,

Just recently bought a Hanna salinity tester (conductivity) for my reef that has been running for about 2 months now. Was previously using BRS refractometer and calibration fluid all calibrated at 35 ppt.

However I'm getting conflicting readings. I calibrated the Hanna using the included fluid pouch, 35 ppt. Upon testing the tank and reserve water it gives me a reading of 31 ppt. Now I'm assuming my calibration fluid was low since I set this tank up, despite all coral and fish doing fine.

To verify, I created a 35 ppt salt solution using Randy's formula. Upon testing the home made fluid with the Hanna and the refractometer they both agree at 35 ppt.

Reef water and separate reserve water: Hanna=31ppt, Refracto=35ppt
Home made salt solution: Hanna=35ppt, Refracto=35ppt

All coral (zoas, acan, green slimer, monti cap, anacropora), 2 clowns and 25 gallon reefcleaners crew all growing and looking healthy. I did introduce 4 astrea snails from tidal gardens a few days ago that died within 2 days, I read a thread somewhere stating low salinity will kill snails.

I'm at a loss with these measurements. Please advise.

 

[Suohhen]

This is a common issue with that device. Look at the response to the top question on Amazon.

 

[Suohhen]

Sorry accidently hit post. Here it is, plus the top reviews all complain about inconsistency.

 

[reefinatl]

Keep in mind, these are two different methods of determining salinity. Both are inferential methods. One is optical while the other is an electrical measurement.

Impurities may affect one measurement method and not necessarily the other. In one case, a particular impurity may cause light to refract differently but it won’t have any influence on the conductivity. Example; let’s say you dumped some sugar in the sample. It will most definitely influence the refractometer, but the conductivity will hardly change at all. Conversely if you added some acid in the sample, the refractometer may not change much at all or might go lower while the conductivity goes higher.

Different salt mixes can also have an influence on the values. You might notice that one salt mixes to a certain specific gravity and spot on with thecexpected refractometer reading, but mix up a batch of a different brand of salt, and the conductivity might be higher or lower than the the expected refractometer’s equivalent reading. This shouldn’t be a huge difference, but it should be understood.

What’s important is that you get consistent readings with a fresh batch of salt and by doing regular water changes, you eliminate any impurities that cause optical or electrical interferences.

From another thread.

 

[Daftendire]

Thanks for the responses guys. I was thrown off since they both measured the same on the home made salt solution. But now I see that the hanna checker will be influenced by different impurities or in this case actual tank water.

 

[Suohhen]

From another thread.

True but I don't reach the same conclusion as the person you quoted at all. I can easily know the salinity of my salt mix with the most basic device on the planet, a measuring cup. However I have seen numerous threads with this issue whereby the Hanna is .003ppt lower than the refracto which is incredibly hazardous because anyone who uses it to set up their display salinity is going to be at 1.028-29 if they don't know about this limitation.
Hanna is a trusted brand and every time I have seen this come up as an issue it is clear that the user is trusting this device. I know new things have this problem so I certainly don't blame Hanna but it should be clear to all us in the know that this device has it's limitations.

 

[homer1475]

My hanna matched my refracto to the T. Been many threads about the hanna not reading correctly. It seems it's a cr*p shoot. You either get one thats good, or one thats off.

FWIW, if you know it's off by .003, then just adjust your makeup water to compensate(IE if the hanna says 1.022, then you know it's actually 1.025).

I personally would send it back to hanna for a replacement. This is a known issue, and hanna WAS replacing defective units.

 

[Randy Holmes-Farley Verified Community Expert]

I'm not sure why you see this issue.

Which of my standards did you make? Any given NaCl standard matching 35 ppt seawater will not read the same on a refractometer and a conductivity meter.

I'd also note that there are pretty wide uncertainties associated with these two devices. The Hanna only claims +/- 1 ppt and the refractometer is likely similar or maybe even a bit wider.

 

[Rickybobby]

There’s Also a massive difference in temperature between the two readings you took

 

[Daftendire]

I used 3.63g salt into 96.35g water. I (perhaps mistakenly) adjusted this mixture until the hanna checker measured 35ppt, then checked with the refractometer which also reported 35 ppt. Was this adjustment a critical error?

As far as temperature difference, the hanna checker and refractometer are temp compensating. The hanna will read 1.023 on both my tank water in the 70s vs storage water in the 60s. Refractometer measures both @1.026.

 

[Randy Holmes-Farley Verified Community Expert]

To read 35 ppt seawater equivalent on a conductivity device like the Hanna, you need to make a 3.29 weight percent NaCl solution:

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

To make a 3.29 weight percent sodium chloride solution, dissolve 1 teaspoon (6.20 grams) of Morton's Iodized Salt in 182 mL (182 g) of fresh water (making a total volume of about 184 mL after dissolution of the salt). This solution can be scaled up as desired.

For a refractometer you need 3.65 weight percent to match 35 ppt seawater::

This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified fresh water. That amount roughly corresponds to ¼ cup (73.1 g) of Morton's Iodized Salt dissolved into 2 liters (2000 g) of water (giving very slightly more than 2 L of total volume).

 

[Daftendire]

Thank you for the info Randy. I will have to rerun my test as I was using kirkland pure seasalt instead of morton's.

 

[Randy Holmes-Farley Verified Community Expert]

Thank you for the info Randy. I will have to rerun my test as I was using kirkland pure seasalt instead of morton's.

Just a note on that. "sea salt" is mostly a marketing thing. It is far and away mostly sodium chloride. It may have more trace elements than other salts, but it is nothing close to seawater composition.

 

[Butcher333]

@kohibaman said. Keep in mind, these are two different methods of determining salinity. Both are inferential methods. One is optical while the other is an electrical measurement.

Impurities may affect one measurement method and not necessarily the other. In one case, a particular impurity may cause light to refract differently but it won’t have any influence on the conductivity. Example; let’s say you dumped some sugar in the sample. It will most definitely influence the refractometer, but the conductivity will hardly change at all. Conversely if you added some acid in the sample, the refractometer may not change much at all or might go lower while the conductivity goes higher.

Different salt mixes can also have an influence on the values. You might notice that one salt mixes to a certain specific gravity and spot on with the expected refractometer reading, but mix up a batch of a different brand of salt, and the conductivity might be higher or lower than the the expected refractometer’s equivalent reading. This shouldn’t be a huge difference, but it should be understood.

What’s important is that you get consistent readings with a fresh batch of salt and by doing regular water changes, you eliminate any impurities that cause optical or electrical interferences.

From another thread.

Thanks for the responses guys. I was thrown off since they both measured the same on the home made salt solution. But now I see that the hanna checker will be influenced by different impurities or in this case actual tank water.

I'm not sure why you see this issue.

Which of my standards did you make? Any given NaCl standard matching 35 ppt seawater will not read the same on a refractometer and a conductivity meter.

I'd also note that there are pretty wide uncertainties associated with these two devices. The Hanna only claims +/- 1 ppt and the refractometer is likely similar or maybe even a bit wider.

To read 35 ppt seawater equivalent on a conductivity device like the Hanna, you need to make a 3.29 weight percent NaCl solution:

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

To make a 3.29 weight percent sodium chloride solution, dissolve 1 teaspoon (6.20 grams) of Morton's Iodized Salt in 182 mL (182 g) of fresh water (making a total volume of about 184 mL after dissolution of the salt). This solution can be scaled up as desired.

For a refractometer you need 3.65 weight percent to match 35 ppt seawater::

This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified fresh water. That amount roughly corresponds to ¼ cup (73.1 g) of Morton's Iodized Salt dissolved into 2 liters (2000 g) of water (giving very slightly more than 2 L of total volume).

Just a note on that. "sea salt" is mostly a marketing thing. It is far and away mostly sodium chloride. It may have more trace elements than other salts, but it is nothing close to seawater composition.

I believe that all of these comments and responses provide the answer I’ve been trying to explain about inconsistent readings.
You can calibrate your salinity probe with a conductivity solution which is not the composition of seawater or rather our salt mix and makeup.
You can calibrate a refractometer with a solution which is also not the composition of seawater or makeup of our salt mixes.
You/we are told we cannot calibrate our instruments with our tank water or salt mixes also. And after calibrating these devices with these other solutions are not getting consistent or accurate readings from our tanks.
Aren’t we supposed to be keeping a standard? Are we keeping our tanks at 35ppt, 1.026, or are we looking for a conductivity reading irregardless of these standards?
I’m going to insist that my conductivity probe provide readings which match my hydrometer and refractometer or that it is useless. Perhaps a conductivity probe should require inputs about our salt mixes ionic ratios and this data should be used to provide us with more useful information through readings.
Being able to get a predetermined reading from a known solution does not equate to this device providing an accurate reading in our tanks.

From “Apps Laboratories”
Sometimes it is useful to have some sort of comparison for values measured on a conductivity meter. Remember that conductivity of seawater is around 54000 µS/cm. This is approximately 35000 ppm TDS. Seawater has a high proportion of sodium chloride and this is around 28000 ppm.

Now the more complete answer.

Different salts in water have a different ability to conduct electricity. This is because of the differences in charge and size / weight and mobility of the different ions. This difference is quantified as a property called the specific conductance. The specific conductance is a value based on the theoretical conductivity of ions at very low concentrations. Although it is possible to calculate the conductivity for any electrolyte at any temperature and concentration, see Conductivity calculator, the exact contribution of individual ions is difficult to determine due to interactions between the ions. This means that it is difficult to work out what the conductivity of a particular salt mix should be, and hence it is difficult to establish the theoretical relationship between conductivity and TDS for a given mixture.

A simple experiment was carried out at Apps Laboratories. Three salt solutions all at the same concentration 0.01 mol/l were made up. There were sodium chloride NaCl, calcium chloride CaCl2 and sodium bicarbonate NaHCO3. Conductivity of the solutions was meaasured using the Hanna Instruments HI 9835 conductivity meter. See this meter at HI9835. The good thing about this meter is that it automatically compensates for temperature and reports the conductivity at a standard 25 degrees C. Here are the conductivities of each individual solution.

Salt (all 0.01 mol/l)	mg/l *	Conductivity µS/cm	TDS factor - mg/l / Cond
NaCl	584	1156	0.51
CaCl2	1110	2310	0.48
NaHCO3	840	865	0.97

* For none chemists this is how many mgms needed per litre to make up a 0.01 mol/l solution.

There is already a trend and it is explained like this: To get mg/l for a NaCL solution multiply the conductivity by 0.51 etcetera for each solution. Interestingly it seems that conductivity in µS/cm and mg/l are very close for NaHCO3. However in nature there are never pure salt solutions like these.

 

[Randy Holmes-Farley Verified Community Expert]

You/we are told we cannot calibrate our instruments with our tank water or salt mixes also.

Well, yes you definitely can do that. But to calibrate any device, you need to know what the salinity is in advance. So you need to know that some other way.

Aren’t we supposed to be keeping a standard? Are we keeping our tanks at 35ppt, 1.026, or are we looking for a conductivity reading irregardless of these standards.

Not sure what you mean, but there's no uncertainty in these issues. All aspects of device calibration and salinity standards and such are perfectly well established and understood facts (at least by informed scientists, not necessarily hobbyists) .

A conductivity probe typically needs calibration, or at least checking that it is accurate. It makes no difference how you do this if you verify that it is actually accurate. You could use real seawater at 35 ppt (or other salinity), or any solution with the conductivity of 35 ppt seawater at the temperature used.

I’m going to insist that my conductivity probe provide readings which match my hydrometer and refractometer or that it is useless.

The standards certainly do NOT need to match, since they may be device specific, but the actual salinity readings in seawater should match after proper calibration (within the uncertainty of the devices used, which for hobbyist devices can be surprisingly high).

Being able to get a predetermined reading from a known solution does not equate to this device providing an accurate reading in our tanks.

That is incorrect. If you calibrate properly, such as I have outlined in my articles, then it DOES equate to getting a proper reading in seawater. That is exactly how the procedures are designed and they are absolutely correct. Obviously, if you do it incorrectly, then you will get an incorrect reading in seawater.

Perhaps a conductivity probe should require inputs about our salt mixes ionic ratios and this data should be used to provide us with more useful information through readings.

The ONLY needed information is the conductivity of seawater. One does NOT need to know anything about the composition any more than one needs to know the composition of air to measure its temperature with a thermometer.

I will agree that if you aquarium water is not fairly close to seawater, but rather is something else (say, only sodium chloride) you will get an incorrect reading from a device calibrated for seawater. But the normal deviations that reefers experience in reef tanks are not sufficient for this error to be significant.

 

[Randy Holmes-Farley Verified Community Expert]

I believe that all of these comments and responses provide the answer I’ve been trying to explain about inconsistent readings.
You can calibrate your salinity probe with a conductivity solution which is not the composition of seawater or rather our salt mix and makeup.
You can calibrate a refractometer with a solution which is also not the composition of seawater or makeup of our salt mixes.
You/we are told we cannot calibrate our instruments with our tank water or salt mixes also. And after calibrating these devices with these other solutions are not getting consistent or accurate readings from our tanks.
Aren’t we supposed to be keeping a standard? Are we keeping our tanks at 35ppt, 1.026, or are we looking for a conductivity reading irregardless of these standards?
I’m going to insist that my conductivity probe provide readings which match my hydrometer and refractometer or that it is useless. Perhaps a conductivity probe should require inputs about our salt mixes ionic ratios and this data should be used to provide us with more useful information through readings.
Being able to get a predetermined reading from a known solution does not equate to this device providing an accurate reading in our tanks.

From “Apps Laboratories”
Sometimes it is useful to have some sort of comparison for values measured on a conductivity meter. Remember that conductivity of seawater is around 54000 µS/cm. This is approximately 35000 ppm TDS. Seawater has a high proportion of sodium chloride and this is around 28000 ppm.

Now the more complete answer.

Different salts in water have a different ability to conduct electricity. This is because of the differences in charge and size / weight and mobility of the different ions. This difference is quantified as a property called the specific conductance. The specific conductance is a value based on the theoretical conductivity of ions at very low concentrations. Although it is possible to calculate the conductivity for any electrolyte at any temperature and concentration, see Conductivity calculator, the exact contribution of individual ions is difficult to determine due to interactions between the ions. This means that it is difficult to work out what the conductivity of a particular salt mix should be, and hence it is difficult to establish the theoretical relationship between conductivity and TDS for a given mixture.

A simple experiment was carried out at Apps Laboratories. Three salt solutions all at the same concentration 0.01 mol/l were made up. There were sodium chloride NaCl, calcium chloride CaCl2 and sodium bicarbonate NaHCO3. Conductivity of the solutions was meaasured using the Hanna Instruments HI 9835 conductivity meter. See this meter at HI9835. The good thing about this meter is that it automatically compensates for temperature and reports the conductivity at a standard 25 degrees C. Here are the conductivities of each individual solution.

Salt (all 0.01 mol/l)	mg/l *	Conductivity µS/cm	TDS factor - mg/l / Cond
NaCl	584	1156	0.51
CaCl2	1110	2310	0.48
NaHCO3	840	865	0.97

* For none chemists this is how many mgms needed per litre to make up a 0.01 mol/l solution.

There is already a trend and it is explained like this: To get mg/l for a NaCL solution multiply the conductivity by 0.51 etcetera for each solution. Interestingly it seems that conductivity in µS/cm and mg/l are very close for NaHCO3. However in nature there are never pure salt solutions like these.

These articles explain these issues in much more detail and are definitely accurate:

Refractometers And Salinity Measurement
http://www.reefedition.com/refractometers-salinity-measurement/

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

 

[DeputyDog95]

Maybe I missed it, but did the OP ever make up a batch of conductivity reference solution and check the Hanna meter again?

 

[arafas]

Hanna reads 32, refrac 35, apex 35. So hanna is crap I guess?

 

[Randy Holmes-Farley Verified Community Expert]

Hanna reads 32, refrac 35, apex 35. So hanna is crap I guess?

Maybe salinity is 33.5 lol

 

[arafas]

Hahaha

Maybe salinity is 33.5 lol