Salinity Calibration: Hydrometer vs. Apex Probe vs. Refractometer

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demeyer2

New member
I've been in the hobby for 10+ years, but only recently got back into the game. I bought a Red Sea Reefer Nano about a month ago and I'm having some troubles with what salinity numbers to trust and I'm hoping the community can help me.

When I set up my tank, I was using a refractometer to get the salinity right. While my tank was cycling, I was contemplating what controller to get and I ended up going for the Apex since I wanted the wifi model + all the probes. When I set the Apex up, the salinity was close to what my refractometer stated so I decided to trust the Apex and use that for my main point of reference for my tank's salinity.

My tank cycled (I'll do a proper tank thread soon) and my 1 fish / cleaner shrimp looked happy as can be so I decided to add some coral. The coral (SPS + zoas) were fine for the first week but they started to look agitated the second week after I got them. I also noticed that my salinity kept rising against reason (my top off is RO/DI and I skim wet so if anything my salinity should have gone down). Yesterday I decided to clean all the probes and do water tests. After cleaning, my Apex probe went from 36ppt to 30ppt which resulted in a proper freak out of me. I readied a salt heavy water change and while that water was heating/circulating, I went out to the store to grab a floating hydrometer for another point of reference. It showed my tank was at 1.020 so the freak out was warranted.

I've slowly been rising the salinity back up but I'm baffled how I can have three readings so far apart. I've re-calibrated the refractometer off of room temperature RODI water multiple times and my current readings at 80 degrees water temp are:

Hydrometer 1.022
Apex Salinity Probe 1.024 (35ppt @ 80 degrees)
Refractometer 1.027

I'm going to re-calibrate the Apex probe tomorrow after it has been dry for 3 hours, but can anyone help me explain these differences and which one to trust? Given the Apex has been volatile, I'm leaning towards trusting the least expensive tool (floating hydrometer) given there's no calibration or moving pieces.
 
You need to calibrate your refractometer with calibration fluid, not RODI. It's like $7 a bottle. Calibrating to zero isn't very helpful, the fluid is 35PPT. Once calibrated using 35ppt fluid you know it's right.
 
crawlerman said:
You need to calibrate your refractometer with calibration fluid, not RODI. It's like $7 a bottle. Calibrating to zero isn't very helpful, the fluid is 35PPT. Once calibrated using 35ppt fluid you know it's right.
Click to expand...



This x1000. I trust my ATC expanded range refractometer with 35ppt calibration over the Apex, any day.
 
To add to this, my apex was generally about .4 parts off. I wound up using the refractometer, getting my tank where i wanted it, then calibrating the apex off the tank water. Now both the refractometer and the apex are within .1 and attribute that to the 10 minute polling window and acceptable drift.
 
radecatur said:
To add to this, my apex was generally about .4 parts off. I wound up using the refractometer, getting my tank where i wanted it, then calibrating the apex off the tank water. Now both the refractometer and the apex are within .1 and attribute that to the 10 minute polling window and acceptable drift.
Click to expand...



I've been using my refractometer with calibration fluid but didn't think to calibrate my apex with my tank water after checking with the refractometer. Very clever

Yes, I understand I should calibrate with proper calibration fluids for the probe to be accurate but I'm only using the probe to see swings not accurately measure the level.


Sent from my iPhone using Tapatalk
 
The apex and refractometer won't give you the same reading most times because they don't measure salinity the same way. One uses refraction the other conductivity. That said the purpose of the apex is to show you stability in your salinity. It doesn't matter if it says 35 or 31 so long as it consistently says it.

I use a Milwaukee refractometer to test or mix and apex probe to monitor the tank.
 
I'm not educated in the subject. but in my opinion, 35ppt is still 53.065uS
Maybe someone can give a better detailed explanation.
 
I use my APEX to check for swings or for anything unusual. I like it to be with in 34-35 so that my APEX programming works well. As long as I know that its good when I do a water change verified with my refractometer. I will get an alert say if something goes wrong with my ATO/Kalk. That's really all, honestly not that important. I use a timed pump for my ATO/Kalk so there is low change for something to go really wrong. Every if it stopped working it would take a week to become a real problem with my overall volume.
 
Apex salinity probe is quite accurate as long as you calibrate it regularly, otherwise just like a refractometer it will drift over time. I use both as redundant measurement means.
 
Hydrometer for me. Love the simplicity to it. I just check it multiple times and see what it comes to most. I use NSW water though and don't mix anything other than ato rodi
 
Just an update. The refractometer solution definitely helped, but I found I basically had to recalibrate every day so that was annoying. The Milwaukee digital refractometer was probably my favorite purchase for my tank though. That thing works like a dream! With that + my handheld refractometer I have a strong grasp on my salinity now. Thanks everyone!
 
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