Salinity Errors - who do I believe

[Richard Newman]

So I've run across an interesting issue. While performing my weekly water tests, my salinity was up to 36ppt/1.027sg. I run an ATO and until this week, my salinity has been consistent at 35ppt/1.026sg. I have this refractometer. And I am using the Aqua Craft calibration solution. I used to calibrate prior to each test, but I never had to make any adjustments. Now I just spot check the calibration. So after this reading I triple checked my calibration and made sure everything was clean of any potential salt creep. Still, my water tested at 36ppt. Well, I buy my saltwater from the local LFS, maybe they had an issue. So I checked the water I just bought (yes, I keep saltwater for up to a month...different discussion...). The LFS water was reading 40ppt/1.030sg. YIKES! What the heck. Well, this would account for the increase in my tank. So I took my water back to have them verify. They initially read it at 38ppt, but did a calibration. They then read the water at 35ppt. So, what now...is my refractometer not working, is my control solution bad... Needless to say lots of research and reading ensued. I can't say that I really learned anything about my situation.

I was out browsing our local selection of LFS. The wife wanted to browse so we went out. Found out later that she and my kids were birthday shopping for me...nice. I got a nice hammer and some type of other frag that I'll have to post to another group to ID. Since we were out I figured I'd take some of my water and my control solution. Trust, but verify. The store where I bought my tank uses a Milwaukee refractometer. They test both my water and my control solution. My water read 37ppt/1.028sg and my control solution was bouncing between 35ppt and 36ppt.

So yes, I have a problem where I need to slowly reduce the salinity. And now I need to figure if I want to mix my own saltwater. Mixing my own would definitely be cheaper. And I could mix it only when I need it. My tank is an IM Fusion 40, 31ish gallons of actual water, so my water changes are 5 gal at a time. Normally every other week.

Any similar experiences, any solutions? Can I trust my refractometer?

Thanks much!


My build thread

 

[Dkeller_nc]

One issue with inexpensive "look through" refractometers is that the temperature compensation may not work all that well. It's just a bi-metal strip inside the unit that moves the scale according to temperature.

I'd suggest putting the refractometer and the calibration solution inside the cabinet of your tank (ideally resting on the top of the sump) for about 30 minutes. Then calibrate the refractometer and measure your tank and your change water from the LFS again. My guess is that the reason you came up with something quite different than the LFS for the purchased water is that it was likely around 21 deg C in storage in your home, and the ATC of your refractometer isn't terribly accurate. When the LFS tested your samples, they may well have had a refractometer with a bit better temperature compensation.

If you'd like to dispense with the uncertainty of a manual-read refractometer with questionable temperature compensation, you might consider one of these.

 

[Richard Newman]

Hi dkeller_nc. Thanks for the reply. Yeah, you get what you pay for in some respects. However, until recently I never got much of a discrepancy. The second LFS I went to that did the double check used a Milwaukee digital refractometer. Their device was more in line with my readings and what I was seeing. I should have also taken some of the store water for alternate testing as well. But at the time I was more concerned with the accuracy of my device and control solution.

I do keep my refractometer and control solution in my tank cabinet so they stay at the house temp. Summer time here in the Tampa, FL area my house is at 77F which is close to my current water temp of 79F. I do let the control solution / water sit on the device a bit (directions say 45 seconds) before taking a reading. Though I haven't noticed any differences between when the sample is placed on and a minute or two later.

The LFS I get my water is a big RedSea distributer and they use the RedSea refractometer. I did do a little research on that device and it seems to be calibrated a little differently than most other refractometers. Hence, my confusion and post. Needless to say if the measuring sticks are not the same, how can we achieve any accuracy. Though I guess more important than accuracy is that the environment is kept at a consistent level.

Cheers!

 

[Dkeller_nc]

Don't keep your measurement instruments inside your aquarium cabinet on a permanent basis; this also goes for test kits. The heat, humidity and salt will destroy digital thermometers, electronic and manual refractometers, pH meters and similar instruments over time. Test kits will degrade faster than they otherwise would, though they're probably a bit more resilient to the environment than test equipment.

There is a way to determine the specific gravity of your seawater in a no ifs-ands-buts-or-maybes manner. It involves getting a scale with readability/accuracy down to about 0.1g and a capacity of at least 2 kg and a 1 Liter class A volumetric flask. Both items are available off of Amazon. You tare the scale with the empty volumetric flask, fill it to the scored mark on the neck (be sure that you know how to correctly read the meniscus), and re-weigh it. The specific gravity is the weight of the contents of the flask divided by the equal weight of water. For this purpose, you can simply use 1000 g for the weight of water. While the specific gravity of water/seawater does vary a bit with temperature, it varies a lot less than the refractive index of water/seawater with temperature, so for this purpose you can ignore temperature.

Most of us aren't OCD enough to go to these lengths, as you're right - stability is the most important aspect, though one would certainly want to be within the range of 1.023 - 1.029 or so.