Testing Salinity - Why is this so difficult?

[James Fox]

Disclaimer - I am probably doing something wrong here, I have not had a salt tank in over 30 years. Hopefully it's something simple
I bought a Red Sea Reefer 350 with the lights, then got a Neptune Apex system with the salinity, pH, ORP and temp probes (with WAV pumps, all that). I set the tank up, put a few inches of aragonite in the bottom, filled it with water, and added reef salt slowly, monitoring the level using the salinity probe that comes with the Apex system. I did the auto-calibration on the probe prior.
After a while I noticed that I was adding waaay too much salt, and the probe never registered anything over 25.
I grabbed my daughter's specific gravity meter, and the salt was off the charts - so I dumped a bunch and filled it up with R/O until her sg meter looked like things were back in line.
Apex probe was still sticking around 25, so I did the manual recalibration. It was a bit higher, maybe 28, but nowhere near it. I have about a week before my live rock arrives, so no big deal, I ordered a refractometer online and picked up another different branded sg meter at the LFS.
A few days later my refractometer arrives, as well as more calibration fluid - I checked 4 different things, and the readings are all over the map.
Refractometer is showing about 1.028 or 37, Fluval SG meter shows 1.021 or 28.5, Aquarium Products SG meter shows 1.029 or 39, and the Neptune Apex probe shows 32.4, but it has been going up every day slowly.
So... I have no idea what it really is. I did calibrate the refractometer and have calibrated the Apex probe a few times now (mostly manual calibration), and I do soak the calibration solution in the water, but I don't know which thing to trust.

 

[James Fox]

One other thing to mention, my pH was showing very low and I had to do a few calibrations to get it in the ballpark of what my water test kit showed... so I added an entire container of baking soda to raise the pH. It's been going up slowly each day, but by no means is near where I want it.
Also, since adding the baking soda I have a very stubborn deposit around the rim of the glass :/

 

[OriginalUserName]

Try validating them against RODI water as well. You can't calibrate with it, but it will give you a gut check. Run some water to a local LFS and have them test too. Those hydrometers are a complete tossup. They can be "accurate" but I've owned 3 of the same type that would read very differently from each other.

I'm a new apex user myself and the advise I've read is to treat their probe as a rough estimate to monitor things, not as something super accurate.

 

[Frogg]

I hear ya brother. No refactometer. But I have a hydrometer and a thermo/salt bobber and they read different. But mostly the hydrometer, I have to check multiple times after rinsing and knocking bubbles out to get a reading thats close to the bobber, which I hope is truer. And I find that i have to wait a few hours after mixing with slight agitation to get a good mix and a more reliable reading. I know others will use pumps and stuff, but Im just working with buckets.

 

[AdamNC]

Trust in a calibrated refractometer over all else. Throw out the swing arms and the Apex takes about a week to really settle in.

 

[Waters]

As others stated, calibrate the refractometer and go with that. I find the Apex probe to not be that accurate.....I use it to watch for swings in the number rather than the number itself.

 

[James Fox]

Yeah I just recalibrated the Apex again and now it reads at 28. These probes are the main reason I went with this Apex system, disappointing to know I bought into their eco-system and the probes don't even work as advertised.

 

[James Fox]

woops double post

 

[Chuk]

One thing to note is that those swing arm salinity meters don't have temperature compensation but the refractometer typically does if you bought a decent one. That can account for some of the variability you are seeing. you need to temperature correct your swing arm hydrometers to compare them to the refractometer.
I haven't tried the salinity probe on the Apex yet but I know based on some of my work experience that the salinity probe cables can be very susceptible to AC electrical interference. Make sure the probe cable is well away form power cables and if you can't get it away from them you may need to shield the cable with a ground to get reliable readings.

 

[James Fox]

Unfortunately my refractometer doesn't compensate for temp. Just doing some more testing (I have 100lbs of live rock arriving tomorrow, want to get this right) - so the Apex probe in calibration fluid is reading at 35.4 @ ~76F or 24C. Label says the calibration fluid is supposed to be 35 @ 20C. I put it in my refractometer, reads close to the same, maybe 36. So, keep in mind, the salinity probe was reading 28 in my sump, so I pulled some water out of the sump and put it in the refractometer, and it reads 35. Put the probe back in the water, untangled it from the other probe cords and now it reads 34.1 (temp is supposedly 77F in the tank)!
So... Like Chuk said, the cables seem to be very susceptible to interference?

 

[Waters]

Unfortunately my refractometer doesn't compensate for temp. Just doing some more testing (I have 100lbs of live rock arriving tomorrow, want to get this right) - so the Apex probe in calibration fluid is reading at 35.4 @ ~76F or 24C. Label says the calibration fluid is supposed to be 35 @ 20C. I put it in my refractometer, reads close to the same, maybe 36. So, keep in mind, the salinity probe was reading 28 in my sump, so I pulled some water out of the sump and put it in the refractometer, and it reads 35. Put the probe back in the water, untangled it from the other probe cords and now it reads 34.1 (temp is supposedly 77F in the tank)!
So... Like Chuk said, the cables seem to be very susceptible to interference?

The salinity probe, much more than the other probes, can be affected by interference. That being said, I have not noticed that......mine is tied together with everything else and the readings are no different than they were before, when it was kept separated.