Salinity level in saltwater tank

reefiniteasy · Jun 6, 2020

I’ve learned that it’s best to have two methods of measuring salinity so one device can check the other. Makes life easier and stress free. LOL

Billdogg · Jun 6, 2020

Alex Sadowsky said:
I’ve learned that it’s best to have two methods of measuring salinity so one device can check the other. Makes life easier and stress free. LOL

I couldn't agree more! For my day to day testing, I have 3 swing-arm hydrometers that I've had for over 30 years that work just fine.

But wait! Aren't they notoriously inaccurate/prone to error? Sure - if you don't treat them nicely. What I did when I finally broke down and bought a refractometer was to test and calibrate the 3 of them so that I know where my target value actually is. On one of them, the reading was ~.03 low, another was ~.02 high, and the third was pretty much dead on. I used a paint marker to indicate where 1.026 really was on all three of them and have been happily using them ever since, safe with the knowledge that they are, indeed, just as accurate as any refractometer, and that with proper use, cannot fall out of calibration.

How do I use them, you might ask? I dip one of them into whichever system I am at, pour it back into the tank, and refill. I then tap it once or twice on the top trim of the tank to dislodge any tiny bubbles that tend to cling to the indicator arm. Allow it to settle and DONE! pour the water back into the tank and set it there, upside down, to drain/dry, ready for the next time. No rinse in rodi, no other cleaning at all. If there is any salt residue left, it gets easily removed with the first fill when I test the next time.

Randy Holmes-Farley · Jun 6, 2020

Billdogg said:
But wait! Aren't they notoriously inaccurate/prone to error? Sure - if you don't treat them nicely.

They can be inaccurate even when used perfectly. I tested several for an article years ago. Some were good and some not. Curiously, the temperature correction ability was quite good, but some were just off.

Chemistry and the Aquarium: Specific Gravity: Oh How Complicated!

This month, Randy explains and reviews specific gravity.

reefs.com

from it:

So how do these hydrometers measure up? In my tank the water
was measured to be S=35 ± 0.5 by conductivity. Using the Deep Six
swing arm hydrometer I got readings of S=32.5 ± 0.5 at 81 °F and
S=32 ± 0.5 at 68 °F. Using the SeaTest I got S=34.5 ± 0.5 at 81
°F and S=34 ± 0.5 at 68 °F.

For the standard type Tropic Marin hydrometer, I got a 77 °F/
77 °F specific gravity of about 1.0265 ± 0.0003 (Figure 4), which
compares well to the expected value of 1.0264.

Hetts · Jun 6, 2020

Billdogg said:
^^^What Randy said!!!^^^

FWIW - you should calibrate your refractometer using a solution that is at 35ppt (1.026) and NOT RODI water. I use Pinpoint calibration fluid. It's inexpensive and if properly stored will last a very long time. Whenever you calibrate an instrument you will get the most accurate end result using a known source that is as close as possible to your desired end point as possible.

Also - no more mixing salt directly in the tank! Once you have living creatures in there adding dry salt directly to the water can really irritate most of them. That's the reason about 99% of us have a dedicated mixing barrel somewhere around the house! Mine is a 55g barrel that I've used for 30+ years now. It takes ~ 1 50g bag of IO or IORC + 6 cups to = 1.025 for me.

The people at my LFS said it was the wrong refractometer and wasn’t one used for water salinity and yes I know about the thing about not mixing salt into tank. I’ve got 8 5 gallon buckets for that. The only reason I put a little salt in tank at the time was because I have no livestock in there atm