Refractometers Vs Conductivity Probes/Readers/Etc

[Cohibaman]

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.

 

[AquamanE]

I trust Milwaukee.

 

[Derrick Picker]

I agree with what has been said. I just am wondering which one is right.

 

[Cohibaman]

I agree with what has been said. I just am wondering which one is right.

The answer, as frustrating as it sounds, might just be, both are right. You might have to compromise on both to get a consistent reading between the two.

Was this a fresh batch of salt?

 

[Derrick Picker]

No. It’s the display. That’s why I am concerned and trying to figure this all out.

 

[Cohibaman]

Mix up a fresh batch, aerate, and check the conductivity vs refractometer. I have a sneaking suspicion they’ll be closer than your tank water.

Are you doing regular water changes?

Any dosing that might change cond vs refract index?

One last thing, do the two methods diverge more/less in the morning before tank lights turn on vs evening just before they turn off?

 

[aqua_code]

4-5 years ago I had a one of the big Milwaukee green units. Eventually it got some "leveling" error and I didn't want to pay $50 to fix it. It seemed nice but really fragile. Now I just have a bunch of calibration solutions and refractometers.

This way I always have a good reference and am checking directly. I use the Neptune probe as well but it gets miscalibrated over time and is more of a quick check to see if anything looks majorly off. I'm looking forward to the mindstream salinity monitor.

If you are worried about accuracy buy a backup calibration solution and compare it to your current one on a refractometer. I usually try to check several calibration solutions at 1.025.

 

[ZoWhat]

If you take a standard 16oz mason jar and fill it with sugar granules..... and just one of those granules were RED..... that single red granule would represent 1-part-per-million

Why are you stressed over 1-part-per-thousand?

Just curious bc unless you have professional lab equipment, you're no getting a specific # but a # with prob a +/- 5% error range

If my refractometer reads 1.024 but in reality its 1.026 or 1.022.... I'm ok with that. My fish nor my coral gives a "snikey" over a .002 difference in SG

Dear lord, go put your feet up and drink a beer .....LOL


.

 

[Hanna Instruments]

So I noticed today while doing some testing on my salinity the following. Please note that all items here were calibrated before testing.

Conductivity/Readers
Hanna Marine Salinity meter - 33.5PPT
Apex Neptune Conductivity Probe - 33.5PPT
Pinpoint Salinity Monitor - 33.5PPT

Refractometers
BRS Manual - 35PPT
Milwaukee Digital Refractometer - 35PPT.

What do I trust!! It is off by a clear 1PPT. Which one is actually correct. I realize either number is fine but I would still like to know. Do refractometers read higher than a conductivity probe?

What gives? Any ideas?

When comparing conductivity to refractometery to measure salinity, the consensus among the scientific community favors conductivity. This is because there is non-conductive material in your sample which can impact the refractive index of seawater but not the actual salt concentration. For example, if we add sugar to artificial seawater, we will see that our salinity value will increase but we have not changed the concentration of salt in the water. If we measured the salinity of that sample with our HI98319 conductivity meter you’ll notice the value is largely unchanged. It is common to have inflated values with a refractometer due to the large number of materials which will affect the density of that water outside of the dissolved salt values. For example, anti-caking agent in salt mixes, organic waste, sugars, potential non-ionic contaminants or uneaten fish food can increase the values produced on a refractometer but this would be less likely to occur on a conductivity meter like the HI98319. Added bonuses of using conductivity to measure salinity are that you no longer have a light interference, and the temperature compensation is of your direct sample and not as influenced by ambient atmosphere near the surface of a prism.

 

[tankstudy]

When comparing conductivity to refractometery to measure salinity, the consensus among the scientific community favors conductivity. This is because there is non-conductive material in your sample which can impact the refractive index of seawater but not the actual salt concentration. For example, if we add sugar to artificial seawater, we will see that our salinity value will increase but we have not changed the concentration of salt in the water. If we measured the salinity of that sample with our HI98319 conductivity meter you’ll notice the value is largely unchanged. It is common to have inflated values with a refractometer due to the large number of materials which will affect the density of that water outside of the dissolved salt values. For example, anti-caking agent in salt mixes, organic waste, sugars, potential non-ionic contaminants or uneaten fish food can increase the values produced on a refractometer but this would be less likely to occur on a conductivity meter like the HI98319. Added bonuses of using conductivity to measure salinity are that you no longer have a light interference, and the temperature compensation is of your direct sample and not as influenced by ambient atmosphere near the surface of a prism.

Sounds like I need to get a HI98319 conductivity meter. My milwaukee was reading their provided calibrated solutions relatively high this weekend. This is my 2nd unit in the past 6 years. How often does the meter need to be calibrated? The photos show only 2 calibrated solutions being provided, so I'm assuming not as often?

 

[bluprntguy]

Sounds like I need to get a HI98319 conductivity meter. My milwaukee was reading their provided calibrated solutions relatively high this weekend. This is my 2nd unit in the past 6 years. How often does the meter need to be calibrated? The photos show only 2 calibrated solutions being provided, so I'm assuming not as often?

I’ve calibrated my Hanna salinity meter once since I bought it 6 months ago. Every time I think to calibrate it again, I test it in 35 ppt solution first and it reads 35 or 34.9.

 

[Rick Mathew]

So when I get the 35PPT solution and re-calibrate it should be dead on with the Hanna I presume?

If you mean by "dead on" within the error limits of each instrument...Yep my BRS refractometer and Hanna Salinity meter are within +- .1 my Apex conductivity probe always reads .2-.3 lower...

 

[reeftasticllc]

Here is my take on the differences between conductivity verses digital refractometer readings and why I choose to use a digital refractometer or light refraction devices. Without getting to technical and for easier understanding, Seawater is, for the most part made up of mostly sodium chloride (NaCL) along with some other types of salt. For argumentative sake, yes you can add Sugar to your seawater and change the readings on your digital refractometer while your conductivity probe would show no affect but who would do that anyway? LOL. There are Pros and Cons to both types of units but the likely hood of the cons affecting your readings when using a digital refractometer are slim unless you like adding sugar to your saltwater! So follow me on this: Potassium is more electropositive than sodium, so the electronegativity difference between K and Cl is higher than that of Na and Cl. Conductivity is simply the measure of the flow of electricity through the water. For measuring seawater, special conductivity meters are used that measure the ratio of the conductivity of a sample of seawater ( NaCL) compared with a reference solution of KCl. Wait a minute, what? Why would you use a calibration solution made of KCI to calibrate your conductivity probe to read NaCL seawater? Here lies the answer to the on going questions!! KCI (potassium chloride) is established as an international calibration standard for conductivity measurement. Chances are, if you are calibrating your conductivity probe, you are calibrating using a KCI calibration solution. So now when you measure your sea water your readings will be slightly off because of the calibration solution difference. Another thing to keep in mind is, because conductivity meters measure total conductivity, they do not discriminate between the ions present in a solution. The presence of chlorides, sulfates, phosphates, nitrates and stray voltage could contribute to inaccurate readings using a conductivity probe. The presence of certain ions, like calcium and sulfates, can present another problem in that they have a higher conductivity than sodium and chloride ions and therefore can create a greater error. Most of these things that can contribute to higher readings are normally found in a saltwater aquariums, but not sugar! LOL I could go on and on. I personally use the Milwaukee MA887 digital Seawater refractometer and it has always been very accurate. I trust it over a conductivity probe as it was designed for measuring seawater where a conductivity probe wasn't. Remember when using a conductivity device for measuring synthetic seawater, all synthetic sea salt for aquariums are not made exactly a like. Some have more calcium, etc than others that will skew your conductivity readings where it wont with a digital refractometer. and PLEASE remember to calibrate your digital refractometer with distilled water NOT RODI water. You can buy a gallon jug at Walmart for 99 cents!