Perplexing conductivity issue

[rygh]

I recently purchased an Apex salinity probe for monitoring auto-water-exchange.
Calibrated it, put it in my tank, and reading was 39.3 ppt. (1.0296)
Confused, I did quite a few re-calibrations and testing versus refractometers after that.
I am stumped.
It seems something in the display tank water is throwing off conductivity.

For results below:
RF1 = Marine Depot Refractometer, calibrated using Pinpoint 35.
RF2 = D-D Refractometer, calibrated using Pinpoint 35.
APX = Apex Salinity Monitor, calibrated using Pinpoint 35.

Pinpoint 35 reference solution:
RF1 = RF2 = APX = 35 ppt. All match, obviously, since this was calibration fluid.

Aqua Craft 35 reference solution:
RF1 = RF2 = APX = 35 ppt. All match. Apex was technically 35.2

Freshly made salt water, carefully mixed:
RF1 = RF2 = APX = 35 ppt. All match. Apex was 35.3

DISPLAY TANK WATER:
RF1 = RF2 = 35 ppt
APX = 39.1 ppt. <=== OFF!!

So everything matches great except real tank water.

I originally used the Apex supplied 53 ms calibration fluid, but ran out. (Hence the 39.3 -> 39.1 mismatch). Basically the same.
I tried to keep temperature the same when calibrating and testing. Likely imperfectly though.
I tried tank water in a bucket, to eliminate stray voltage issues, and it had no effect.
I dose Baking soda and Calcium Chloride. Along with small amounts of magnesium sulfate + magnesium chloride.
I run Carbon + GFO. Water changes < 10% per month.
Water temp = 77.5
ORP = 330
I re-calibrated Apex to tank water, and it only varied about +/- 0.3 over the last week.

Any ideas?
Looking at other sites, it seems I am not the only one with conductivity issues.

 

[Randy Holmes-Farley]

Temperature is critical in conductivity.

When you calibrate, is the Apex temp sensor in the calibration fluid (or it is exactly the same temp as the tank)?

 

[rygh]

Temperature is critical in conductivity.

When you calibrate, is the Apex temp sensor in the calibration fluid (or it is exactly the same temp as the tank)?

To calibrate, I put a test tube in the sump with the solution in it. Zip-tied to a pipe.
So calibration solution should have been at the same temperature as sump water.
I also tried moving the Apex temp probe from main unit to salinity module, but that made no difference.

That said, the freshly made water was not the same temp. Probably 70 or so.
But that still matched at 35.

---

Someone else mentioned I should wait a few weeks and try it all over .. Probes can give funny readings for a while.

 

[rygh]

Apex temp probe remained in sump throughout tests.
I also tried moving the Apex temp probe from main unit to salinity module, but that made no difference.

To calibrate, I put a test tube in the sump with the solution in it. Zip-tied to a pipe.
So calibration solution should have been at the same temperature as sump water.


That said, the freshly made water was not the same temp. Probably 70 or so.
But that still matched at 35.

---

Someone else mentioned I should wait a few weeks and try it all over .. Probes can give funny readings for a while.

 

[Randy Holmes-Farley]

To calibrate, I put a test tube in the sump with the solution in it. Zip-tied to a pipe.
So calibration solution should have been at the same temperature as sump water.
I also tried moving the Apex temp probe from main unit to salinity module, but that made no difference.

That said, the freshly made water was not the same temp. Probably 70 or so.
But that still matched at 35.

---

Someone else mentioned I should wait a few weeks and try it all over .. Probes can give funny readings for a while.

No. An ORP probe can take time to equilibrate, but not a conductivity probe. It reads almost instantly.

 

[Randy Holmes-Farley]

So if you have the Pinpoint solution at tank temp, and pull the probe from the 39 ppt tank solution and put it into the 53 mS/cm solution without changing anything, what does it read?

 

[Cory]

I had an apex once, while the whole system is awesome, conductivity sucked.

The reason to me was unknown as to why it isnt so good, but in my research these are the things that effect am accurate reading:

1) air bubbles
If any air is stuck on or in the probe durring measurement in tank or dirring calibration it reads falsely.

2) electrical current (stray current)
If a reading is taken in a tank with pumps and other electronics submerged in the water, chances are that there is some electrical current messing with the reading. Solution is to measure tank water outside in a cup.

3) improper calibration. Like Randy said temperature is important.

4) improperly made calibration standard. If the company supplies you with a calibration fluid thats expired, or made with impurities or wrong quantity/ concentration, then the calibration is useless. Solution is make your own fluid, ask Randy for that one.

5) electrical wires nearby the probes wires. I never experienced this but apparently it can cause interference.

6) damaged probe. The glass bulb is easily damaged even by bumps caused by shipping. You may habe a faulty probe.

7) faulty temp probe. Test your probe in ice water, it should read 0.

8) not using a dedicated temp probe with the concuticuty moduel. You should have a temp probe specificallu for the conductivity meter.

9) not using a temp probe durring calibration. This must be done.

10) improperly calibrated temp probe. Two probes i had were 3f off!

After all this, and you still want to keep it, great, but if not id suggest a lamotte nist certified floating glass hydrometer. Way less complicated imo. Ill be getting one soon.