Salinity testing observations

Theolox · Jan 28, 2023

In the past, I didn't have a RODI unit and just bought all my water so I took it on faith (probably a bad idea) that it was the right salinity level. Now that I am a using RODI and making my own water, the need for accuracy is there.

I have been struggling a bit with trying to figure out what my salinity is. I have a Hanna HI938319 tester and a Milwaukee Instruments MA887 tester. The Milwaukee has been showing consistently higher than the Hanna for the same water sample. I dug up my manuals and also watched some videos and feel like I got them both calibrated this morning. I used some freshly made water and the Hanna measured it at 1.025 while the Milwaukee measured at 1.027. I have no doubt that the Milwaukee is a better piece of equipment, but it says in the manual that it up to +/_.002 tolerance so that seems to check out. I am a little disappointed that it's off, but at least now I have a baseline and I can just subtract .002 from whatever reading I get.

For those who would suggest a refractometer, I have one and I've tried to use it, but after several eye surgeries, I just can't seem to focus it well enough to get a good reading so I'll have to stick with the digitals.

Just my observations. Happy reefing!

TheBear78 · Jan 28, 2023

I have the Hanna HI938319 tester as well.
I get my water (both RO and mixed salt) from my LFS who makes it at 1.025. My Hanna always measures this water at 1.023.
I don't rely on it for an actual salinity figure but use it for consistency and to compare what I've been given with the water in my tank prior to carrying out water changes. As long as they both read within 0.001 I'm good to go.

Big G · Jan 28, 2023

Choose one and track for consistency. You read my mind ^^^ Best of luck.

TangerineSpeedo · Jan 28, 2023

The Hanna is a conductivity meter and the Milwaukee is a refractometer. Two different things.
The Hanna measures the Sodium Chloride in your water and the Milwaukee measures the light passing through the sodium chloride and everything else in NSW so there will be a difference. The best thing to do so you will not pull your hair out is to make up your own NSW standard or purchase one for your refractometer. Calibrate that, make up some water that matches that. Then measure that water with your Hanna after you calibrate your hanna with the provided 35ppt.
So let's say you calibrated your Milwaukee to 35ppt, and your new water measures 35ppt with the Milwaukee. You then test with your Hanna and it reads 33.5 ppt. Then you know when you want to quickly test your water with the Hanna it should read around 33.5.