So you think you know your salinity...

[Munch]

So I had an optical refractometer, and trusted it.

Then I picked up a digital Hanna refractometer, and trusted it.

I then picked up calibration fluid, and tested each.

When I calibrated my optical refractometer with calibration fluid, and then tested the water on the digital unit, both had different results. Sure they are supposed to be precise within .001

I ditched the optical, thinking it may have been dropped, damaged, etc.

So I picked up a second Digital Hanna refractometer.

I then grabbed a cheapo float salinity checker for sanity check. While it's important to knock off the tiniest of bubbles, it came up with various results as well.

Interesting results (sorry for the crappy pics, I wanted something fast)

Pic 1 is tank water on Hanna's and float, 1.024 1.027 and 1.022

Pic 2 is tank water again on Hanna's and float, 1.024, 1.027 and 1.024

Pic 3 is 1.026 calibration fluid on each of the Hanna's 1.028 and 1.030 - don't have enough to waste on the float.

Bottom line, you're probably not where you think you are. You need to establish a baseline, and check it on a regular basis.

The Hanna's produce repeatable results, BUT, they may be off consistently. Your swing arm might look like its producing your expected results, when infact its not.

If anyone is near Parker, and wants to test this out, let me know. I'd love to compare this equipment against what you have.

 

[CRW Reef]

So..... which one are you trusting the most as accurate?

 

[Munch]

I honestly think I have 1 digital Hanna thats accurate. This is assuming of course my calibration fluid is not what I think it is as well, as both Hanna's resulted in higher readings. The float can sometimes hit it, but it's hit or miss.

This is actually why I want to measure against a few more samples, and try to confirm it all.

 

[djkms]

I have the digital Hanna Refractometer as well and get similar results. The problem with the Hanna digital is you calibrate it with distilled water (I use 0 TDS RO/DI) and there is no way to calibrate it at 35ppt. I also use the pinpoint solution to check against and the Hanna is usually reading 1-2 points higher. I just adjust in my head from the reference solution every time I test.

Nothings perfect I am afraid. I just hope the reference solution is accurate!!!!

 

[jahmic]

What were the results with the optical refractometer?

I make DIY calibration fluid and haven't had any issues with my optical refractometer giving inaccurate results...I think after about 6 months it needed to be recalibrated as it read 1.027 rather than 1.026 with the standard solution.

 

[Munch]

Yup, I think that's the critical point, if the Hanna's read consistently higher or lower, you can adjust. I'm guessing accurate baselines are hard to come by.

If your 1.026 is reading 1.024 on the Hanna - that's fine, add .002 to your readings. But if your 1.026 baseline is off, then you will be off accordingly.

 

[Munch]

I ditched it, when I shoulda kept it, so I can't comment on how it would be in this specific test. As I recall, I could take 3 readings of salt water in a row, and it would be off each time. It was an ATC model, but honestly I never got consistent results, so I went the way of digital.

The surprising info was two Hann's reading salt water, and calibration fluid differently, each by .002 and .004 - against the 1.026 calibration fluid

So if I calibrate off it, my water could possibly be 1.028 or 1.030 if I am actually shooting for 1.026 - scary.

 

[Munch]

I also mixed up a batch of homemade calibration fluid, following the coke 2 liter method, and my initial readings against that were also high (I get 1.027 and 1.028 on my Hannas), so I then questioned whether the 2 cups salt were firmly packed, etc... My home solution mighta been 1.024 or 1.026 without knowing it.

I think in the end, everything is close, within .004 of one another. But to me that's a huge difference when typically you'd pick one method and "trust" it.

 

[jahmic]

I definitely agree, .004 could be a huge difference. I try to be as accurate as possible after having issues with low salinity early on in the hobby...my refractometer was off by .004 because I calibrated with RO and I lost an acan with a few days of adding it to the tank.

Since we're sort of on the subject, FWIW...I make my DIY calibration fluid by weight and use 0 TDS RO/DI water mixed with natural sea salt (with no additives). I usually will mix 5 small batches of about 400-500 mls each. I measure all 5 batches with my refractometer, then toss the 2 outlying solutions that read high and low and mix the remaining 3 together. The solutions usually end up being fairly accurate and I end up with a couple that are low/high by about .001 compared to the other 3. I can't say it's ALOT more accurate than just mixing one batch and going with it, but you do get the chance to catch any mistakes you may have made with measuring/weighing things out by taking the average across 5 samples and tossing the 2 outlying samples.

 

[Munch]

On your DYI, what recipe do you follow?

I opted to try the coke experiment as I couldn't easily measure known quantities of water and salt.

 

[that0neguy1126]

I think the key is stability. Regardless of weather you are at 1.025 or 1.026. as long as you are not fluctuating all over the place, I think you are fine.

I would be curious to try your meters though, I am down in castle rock, but work near parker.

 

[Munch]

Mines been stable, but my initial though was, what's the real number I'm at.

I need to ship back one Hanna on Wednesday of next week at the latest. I'm here all day Sat-Tuesday. If you wanna bring some water, I can meet you here, or someplace easy and we can test.

 

[jahmic]

On your DYI, what recipe do you follow?

I opted to try the coke experiment as I couldn't easily measure known quantities of water and salt.

I couldn't find the direct link...but I use Randy's recipe that I think appeared on reefkeeping.com I'm pretty sure that's where the coke bottle recipe appeared too, correct? I just used his weighted measurements and adjusted them for a smaller batch.

 

[that0neguy1126]

I am actually headed out of town for the week =( so probably wouldnt be up near you until the 28th

 

[DyM]

http://reefbuilders.com/2010/07/12/dd-seawater-refractometer-scaled-calibrated-seawater-saltwater/

Not what your talking about but this is interesting nonetheless.

 

[Munch]

I wanted to provide an update, which I do find quite interesting. I did order a new ATC optical refractometer, and a second set of 1.026 calibration fluid 53 mS/cm with refractive index of 1.3394.

I believe the issue with using a Hanna is the fact it is calibrated with RO/DI water 0 TDS.

When I calibrate my optical ATC unit (with either calibration fluid), it does reads both solutions at 1.026, which is good. When I drop each of these calibration fluids onto the Hanna, the Hanna reads both at 1.028.

So the bottom line is the Hanna unit will always (consistently it seems) read your salinity .002 high, as it was calibrated with RO/DI 0 TDS water.

I'm honestly shocked you cannot calibrate the Hanna with calibration fluid, seems like a serious flaw in their design.

$200 later, I know what my salinity is

This to me stresses the importance of calibrating your refractometer at 1.026, using RO/DI water will distort your measurement as you get close to 1.026!

 

[Cherub]

I wanted to provide an update, which I do find quite interesting. I did order a new ATC optical refractometer, and a second set of 1.026 calibration fluid 53 mS/cm with refractive index of 1.3394.

I believe the issue with using a Hanna is the fact it is calibrated with RO/DI water 0 TDS.

When I calibrate my optical ATC unit (with either calibration fluid), it does reads both solutions at 1.026, which is good. When I drop each of these calibration fluids onto the Hanna, the Hanna reads both at 1.028.

So the bottom line is the Hanna unit will always (consistently it seems) read your salinity .002 high, as it was calibrated with RO/DI 0 TDS water.

I'm honestly shocked you cannot calibrate the Hanna with calibration fluid, seems like a serious flaw in their design.

$200 later, I know what my salinity is

This to me stresses the importance of calibrating your refractometer at 1.026, using RO/DI water will distort your measurement as you get close to 1.026!

Thanks for the heads up. I've been using ro/di lol

 

[Ghosty]

I have one of those cheapie ($60?) eBay/Amazon Refractometers, just arrived. I'll have to test it against some reliable calib. fluid and report back. For now I just use the floater. Tested it against Elite Reef's saltwater RO water (1.025) they sell and seems pretty accurate.

 

[Suzanne Lynn]

Im bot cool enough to have the digital ones so I just soak my plastic one in vinegar. seems to work all right for fish, but I'd maybe invest in better equip if i get corals.