Salinity Calibration: Hydrometer vs. Apex Probe vs. Refractometer

demeyer2 · 03/17/2017, 09:30 AM

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.

crawlerman · 03/17/2017, 09:40 AM

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.

demeyer2 · 03/17/2017, 09:43 AM

Quote:

Originally Posted by crawlerman

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.

Great to know. I purchased this and I'll re-calibrate once I have it.

bobbravo2 · 03/17/2017, 10:22 AM

Quote:

Originally Posted by crawlerman

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.

This x1000. I trust my ATC expanded range refractometer with 35ppt calibration over the Apex, any day.

radecatur · 03/17/2017, 11:13 AM

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.

henryle · 03/17/2017, 11:58 AM

Quote:

Originally Posted by radecatur

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.

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

fishchef · 03/17/2017, 12:06 PM

So simple its scary.

f3honda4me · 03/17/2017, 05:50 PM

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.

elescher · 03/17/2017, 05:59 PM

Is there any reason one can't bottle up some tank water that you know is 35ppm and use that for calibrating?

radecatur · 03/17/2017, 07:20 PM

I'm not educated in the subject. but in my opinion, 35ppt is still 53.065uS
Maybe someone can give a better detailed explanation.

bif24701 · 03/18/2017, 08:13 AM

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.

ca1ore · 03/18/2017, 09:00 AM

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.

d-man · 03/18/2017, 09:49 AM

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

The Kahnasaur · 03/18/2017, 07:53 PM

Refractometer calibrated to 35 ppt will definitely be the most accurate.

demeyer2 · 03/28/2017, 04:47 PM

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!

bnumair · 03/28/2017, 04:53 PM

Quote:

Originally Posted by bobbravo2

This x1000. I trust my ATC expanded range refractometer with 35ppt calibration over the Apex, any day.

cant agree more.

03/17/2017, 09:30 AM	#1
demeyer2 Registered Member Join Date: Nov 2005 Location: Sausalito, CA Posts: 455	Salinity Calibration: Hydrometer vs. Apex Probe vs. Refractometer 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.

03/17/2017, 11:13 AM	#5
radecatur Registered Member Join Date: Aug 2003 Location: Brandenburg KY Posts: 183	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. __________________ -Rich Trigger 36, Cone S Q2, Bio-Pellets, Apex 2016, AWC, 3-part dosing, Gyre 3K, ATI 6X54, Jebao PP8's Birds nest, Duncan, Alveopora, Stylophora, Hydnophora, acropora, GSP, Hammer, montipora Current Tank Info: 75G Mixed

03/17/2017, 07:20 PM	#10
radecatur Registered Member Join Date: Aug 2003 Location: Brandenburg KY Posts: 183	I'm not educated in the subject. but in my opinion, 35ppt is still 53.065uS Maybe someone can give a better detailed explanation. __________________ -Rich Trigger 36, Cone S Q2, Bio-Pellets, Apex 2016, AWC, 3-part dosing, Gyre 3K, ATI 6X54, Jebao PP8's Birds nest, Duncan, Alveopora, Stylophora, Hydnophora, acropora, GSP, Hammer, montipora Current Tank Info: 75G Mixed

03/18/2017, 08:13 AM	#11
bif24701 Registered Member Join Date: Jan 2011 Location: Florida, FWB Posts: 3,389	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. __________________ 180 Mixed Reef SRO-5000 Skimmer Neptune APEX Gold Kessil AP700/ MP60+6105 Kalk+2 part/ Cheato Fuge Current Tank Info: 180 SPS Dominant

03/18/2017, 09:00 AM	#12
ca1ore Grizzled & Cynical Join Date: Apr 2006 Location: Stamford, CT Posts: 17,319	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. __________________ Simon Got back into the hobby ..... planned to keep it simple ..... yeah, right ..... clearly I need a new plan! Pet peeve: anemones host clowns; clowns do not host anemones! Current Tank Info: 450 Reef; 120 refugium; 60 Frag Tank, 30 Introduction tank; multiple QTs

03/17/2017, 09:40 AM	#2
crawlerman Registered Member Join Date: Mar 2006 Location: Pittsburgh, PA Posts: 503	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.

03/17/2017, 12:06 PM	#7
fishchef Registered Member Join Date: Feb 2009 Location: St. Louis, MO Posts: 1,222	So simple its scary.

03/17/2017, 05:50 PM	#8
f3honda4me Registered Member Join Date: Dec 2011 Location: Wisconsin Posts: 1,218	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.

03/17/2017, 05:59 PM	#9
elescher Registered Member Join Date: Nov 2011 Posts: 44	Is there any reason one can't bottle up some tank water that you know is 35ppm and use that for calibrating?

03/18/2017, 09:49 AM	#13
d-man Registered Member Join Date: Nov 2009 Location: San Diego, ca Posts: 2,732	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

03/18/2017, 07:53 PM	#14
The Kahnasaur Registered Member Join Date: Sep 2014 Location: New Jersey Posts: 53	Refractometer calibrated to 35 ppt will definitely be the most accurate.

03/28/2017, 04:47 PM	#15
demeyer2 Registered Member Join Date: Nov 2005 Location: Sausalito, CA Posts: 455	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!