Ok, this is a new build. Only weeks old, no livestock yet. Brand new Apex system. I'm brand new to Apex too although I've been in the hobby many years.
So I went through several days of communicating with Neptune Systems through a support ticket to get the salinity probe calibrated (there was a technical reason why I couldn't calibrate the probe and they really helped me out). Once calibrated I asked the question "Why does the Neptune Systems 53,000 uS/cm calibration fluid not work on refractometers (as they had indicated to me during the support ticket exchange)? Out of curiosity, I tested the 53,000 uS/cm fluid on my refractometer and it read 25 ppt not the 35 ppt I was expecting (which does indeed confirm their statement that you can't use that fluid on refractometers)." This was Neptune Systems Response:
Refractometers use a brine calibration solution typically calibrate them. Some use RO water to calibrate the refractometer. I would check with the manufacture of your refractometer. Also below is information about our probe and other measuring devices and how they will not read exactly.
The Apex probe measures the conductivity of the water, the refractometer measures the change in the speed of light when it goes from air to water, a hydrometer measures how dense the water is. These are 3 different things you are measuring, none of them is a count of the particles of stuff in a million particles of water. All 3 will give different readings if you dissolve different things, 35.0 ppt of sodium chloride will read different than sea water with 35.0 ppt of stuff in a refractometer. Our cond probe will not read number to number identically with milwakee's calibration or any other third party tools for checking this measurement. It can read +/- 2ppt different then the other measurement devices. If you calibrated the Cond probe properly when placed In the 53mS solution it should read around 34.8~35.2. If it did read that after you calibrated it than you successfully calibrated the probe. That's what's important. That it read 34.8~35.2 in the calibration solution from Neptune systems.Can you put the probe back in the 53mS solution and what does it read? If it does not read within that range then calibrate the Cond probe again. If it did, than you're done.
After that, if you drop it in to a tank or any environment it will read what the conductivity is of that tank. It's not going to be the same as your refractometer, hydrometer, or anything. Our probe reads the conductivity. It's not the same.
To clarify, the conductivity probe functions by measuring the displacement of NaCl ions in the water. The Conductivity probe is not a refractometer. It's not going to read like your refractometer. This is common. It's a conductive sensing probe. Your refractometer is designed to detect specific gravity/density of all the salts in your water like KCl, MgCl, not just NaCl, but the conductivity probe will give you a closer look at the the true salinity (ppt) reading in your aquarium.
Typically most refractometers are not calibrated for Seawater, but a brine solution, thus they are reading the SG/salinity of many other salts in your water other than NaCl. So they will read slightly differently than a conductivity probe which only measures your actual NaCl concentration. Here is ant article on Reef Builders illustrating this discrepancy (http://reefbuilders.com/2014/05/14/r...rium-salinity/). So it is not uncommon to see this difference between most refratometers and salinity probes.
If you follow the link above you will see that not all Refractometers are considered equal. So I bought the Red Sea Refractometer (referenced in the link) and after calibrating it to RODI water (as their instructions guided) I measured my water. This is what I got:
- Aquarium = 31-32 ppt
- Salt Water Reservoir (for auto water changes) = 30 ppt
Now compare this to the graphs from my Apex:
2018-09-05 - Temp-pH-Salinity Graphs.jpg
Note: The salinity probe changes with changes in temperature. So, I know why the salinity probe changes.
So, here we are at the question of this post.....which device do I believe when deciding if I need to add salt or not?
1) The Apex probe says don't add more salt, but
2) The Red Sea Refractometer says I need to add more salt.
I understand why they could be different but which one is right or should be trusted first?
By the way, my old Refractometer that is not calibrated to Seawater or aquarium temperature as my new Red Sea Refractometer is, measures the aquarium and salt water reservoir at 35 ppt.
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