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Apex Salinity probe versus refractometer?

So it is not stray current in the sump.
I put the salinity probe in a small plastic container of tank water, and it did not change value in the slightest.

It is also not temperature.
I moved the probe from the main Apex unit to the salinity unit, and the change was tiny.

Still more tests to go.
 
I've used an apex salinity probe to automatically control my reef for 3 years. I simply pump 0.02% 4 times per day to waste(50 ml per min), and use float switches to replace water with either full strength saltwater, or rodi water.

I validate salinity with a refractometer once a month. The probe has never drifted. It is in a dark, medium flow, zero bubble area in the sump. Occasionally I get a stomatalla crawl in and the reading will jump from 35 to 50. Simply put relevant checks in apex and once the snail crawls out, control resumes normally. I've never had a problem with this system, and my water parameters are extremely stable as a result.


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PRETTY INTERESTING RESULTS

So some final testing.
RF1 = Marine Depot Refractometer
RF2 = DD Refractometer
APX = Apex Salinity Monitor

Pinpoint 35 reference solution:
RF1 = RF2 = APX = 35 ppt. All match

Aqua Craft 35 reference solution:
RF1 = RF2 = APX = 35 ppt. All match.

Freshly made salt water, carefully mixed:
RF1 = RF2 = APX = 35 ppt. All match.

DISPLAY TANK WATER:
RF1 = RF2 = 35 ppt
APX = 39 ppt. WAY OFF!!

So everything matches great except real tank water.

I was careful matching temp, so unlikely to be that.
I put tank water in a bucket, and had same result, so not stray voltage.

FYI: ORP = 330

Clearly "something" in the main tank is affecting conductivity.
 
Oops: I also dose magnesium, although not very much of course.
BRS 2 part:
Magnesium Sulfate + Magnesium Chloride.

Interestingly, Sulfate has a higher conductivity. I wonder if that is building up??
 
Interesting!

Sounds like you have a couple of more experiments to try.
Dropping a little GFO into a cup full of freshly mixed water and let it sit for a while, and measure.
Try mixing up some new water w/ a little extra magnesium sulfate in addition to the salt, and measure.

And it could get even more fun if you have a hydrometer lying around, to see what that says ;) Regardless, I bet you're close to figuring it out.

Also, is there a pH difference between the tank and the new salt water? Thinking about it, I would imagine that could have an effect on conductivity.
 
Corallus said:
Interesting!

Sounds like you have a couple of more experiments to try.
Dropping a little GFO into a cup full of freshly mixed water and let it sit for a while, and measure.
Try mixing up some new water w/ a little extra magnesium sulfate in addition to the salt, and measure.

And it could get even more fun if you have a hydrometer lying around, to see what that says ;) Regardless, I bet you're close to figuring it out.

Also, is there a pH difference between the tank and the new salt water? Thinking about it, I would imagine that could have an effect on conductivity.
Click to expand...
Tried GFO, Magnesium, and even Potassium. No effect.
Just dumped some in a cup of mixed SW, no measuring, but concentrations should have
been way higher than normal tank.
PH is the same. About 8.1
 
Hey rygh, before you calibrated the probe, how long did you let the probe soak in water? When I first got my conductivity probe I got really wacky readings for a couple weeks. Apparently that is normal. I let it sit in the probe mount in my sump for two weeks. After I re-calibrated with a fresh solution pack a couple weeks down the road, it behaved normally. Just a thought.

Seems weird, conductivity probes are a pretty proven thing. But so are refractometers.
 
Tskiller said:
Hey rygh, before you calibrated the probe, how long did you let the probe soak in water? When I first got my conductivity probe I got really wacky readings for a couple weeks. Apparently that is normal. I let it sit in the probe mount in my sump for two weeks. After I re-calibrated with a fresh solution pack a couple weeks down the road, it behaved normally. Just a thought.

Seems weird, conductivity probes are a pretty proven thing. But so are refractometers.
Click to expand...
I calibrated immediately, but the various re-calibrations and tests were done over a few weeks after.
I may try repeating everything.
 
I used a refractometer for 3-4 years, but got tired of having to calibrate it each which includes making sure the temp. of the water and room temp. are about equal. About a year ago I bought a Hanna electronic meter and has made testing the water much easier. I can test the water mix and the water in my 3 aquariums in less than 10 min. The meter seems very accurate based on what the what the directions on the salt mix state. The down side is that is not cheap, but in this hobby what is, and the salinity of the water is very important.
 
I would be interested to see if letting tank water sit for a day before testing affects the results. A difference is that your tank has life with all byproduct in it.


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@rygh Mark,

Just curious is the conductivity probe and the wire routed anywhere close to other power cords or electrical motors on its way to the PM2?

Not sure if you said this already but are you using a temp probe on the PM2?
 
Last edited:
jonmos75 said:
@rygh Mark,

Just curious is the conductivity probe and the wire routed anywhere close to other power cords or electrical motors on its way to the PM2?

Not sure if you said this already but are you using a temp probe on the PM2?
Click to expand...

Near cords, but moving things didn't do anything.

I have the temp probe in main unit, but moving it had no effect.
 
Very interesting! Can't wait to hear what's causing this. The GFO and sulfates both sound like good guesses to me.

Not to derail but does anyone else use a digital refractometer? I've been using one for years and absolutely love it.
 
pixelpixi said:
Very interesting! Can't wait to hear what's causing this. The GFO and sulfates both sound like good guesses to me.

Not to derail but does anyone else use a digital refractometer? I've been using one for years and absolutely love it.
Click to expand...

Im using the green milwaukee digital refractomer. Very easy to use. For me its usually 0.001 higher than advertised which is not a big deal for me. If it reads 1.027 its actually 1.026 when compared to the standard.

I like how easy/quick easy it is to read. Good precision.
 
pixelpixi said:
Very interesting! Can't wait to hear what's causing this. The GFO and sulfates both sound like good guesses to me.

Not to derail but does anyone else use a digital refractometer? I've been using one for years and absolutely love it.
Click to expand...
I used them when I was volunteering at Neptune Aquatics and I really liked it. However, I never got around to buying one but they do take up a bit of space.
 
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