High Quality Refractometer Recommendations?

Aspect · Mar 16, 2026

Mebbid said:
Is there much temperature fluctuation in the house? A change in temperature will change the reading.

I thought this at first but I gave it 5 minutes to equalize and reading didn't change

Aspect · Mar 16, 2026

slingfox said:
Sounds odd. I haven’t had to make any adjustments to my VeeGee since I got it a little over two years ago. Just checked its calibration today and still no adjustment needed.

Yeah no it definitely is odd, a refractometer of this quality should hold calibration. I used new 35 ppt brightwell aquatics solution a day apart and got a ppt drift upwards. Even if the solution isn't accurate the calibration should hold.

Mebbid · Mar 16, 2026

Aspect said:
I thought this at first but I gave it 5 minutes to equalize and reading didn't change

Im talking more a change in the temperature of the refractometer itself. The water sample will adjust rather quickly to whatever temp the refractometer is.

slingfox · Mar 16, 2026

Aspect said:
Yeah no it definitely is odd, a refractometer of this quality should hold calibration. I used new 35 ppt brightwell aquatics solution a day apart and got a ppt drift upwards. Even if the solution isn't accurate the calibration should hold.

The only idea that comes to mind is to doublecheck that the set screw on the calibration ring is snug. If it is not the calibration ring can move. You should created the DIY calibration solution posted by Randy Holmes-Farley. I purchased three different off-the-shelf calibration solutions and they all gave slightly different readings which was frustrating. Better to prepare your own.

diverjm · Mar 16, 2026

Aspect said:
Yeah no it definitely is odd, a refractometer of this quality should hold calibration. I used new 35 ppt brightwell aquatics solution a day apart and got a ppt drift upwards. Even if the solution isn't accurate the calibration should hold.

Doesn’t the manual state to use Distilled/RODI for calibration? I’d try testing with this as I also have had mine for over 3 years and calibration is dead on as I just checked mine.

diverjm · Mar 16, 2026

Also wonder if that calibration solution settles in the bottle and can change the reading?

Aspect · Mar 16, 2026

diverjm said:
Doesn’t the manual state to use Distilled/RODI for calibration? I’d try testing with this as I also have had mine for over 3 years and calibration is dead on as I just checked mine.

Yes it does but it won't be accurate if used for reef tank application. Needs to be calibrated with 35 ppt solution

diverjm · Mar 16, 2026

Aspect said:
Yes it does but it won't be accurate if used for reef tank application. Needs to be calibrated with 35 ppt solution

I don’t understand how the STX-3 wouldn’t be accurate for reef application as it is designed for salinity testing?

Aspect · Mar 16, 2026

diverjm said:
I don’t understand how the STX-3 wouldn’t be accurate for reef application as it is designed for salinity testing?

It won't be accurate if calibrated with rodi water for reef tank application

diverjm · Mar 16, 2026

Aspect said:
It won't be accurate if calibrated with rodi water for reef tank application

I believe the RODI calibration sets the unit to true zero without any salinity present. All I can say is I just did an ICP test and my salinity matched exactly to what the refractometer reads.

RobertK · Mar 16, 2026

slingfox said:
The only idea that comes to mind is to doublecheck that the set screw on the calibration ring is snug. If it is not the calibration ring can move. You should created the DIY calibration solution posted by Randy Holmes-Farley. I purchased three different off-the-shelf calibration solutions and they all gave slightly different readings which was frustrating. Better to prepare your own.

I recently bought the VeeGee as well. The other thing about the calibration that some folks have noticed is that if you tighten the set screw too much it will turn the calibration ring a bit, and I did see that mine tends to do that as well. So tighten it carefully and not too much.

Randy Holmes-Farley · Mar 16, 2026

diverjm said:
Also wonder if that calibration solution settles in the bottle and can change the reading?

No. Calibration standards neither go bad (except by evaporation) nor ever settle.

In fact, no dissolved chemical can ever settle out of (or rise to the top of) water. The thermal motion alone is enough to keep water solutions homogeneous.

Some refractometers are designed properly for seawater and are always called true seawater refractometers. If those are made perfectly, remain perfect, and are calibrated perfectly, then RO/DI is adequate.

Many hobby refractometers are not designed for seawater, but are repurposed brine refractometers. No matter what the instructions say, if they are made perfectly, remain perfect, and are perfectly calibrated with RO/DI, then will be inaccurate for seawater for well understood reasons (in short, the refractive index vs salinity profile for brine is different than for seawater).

In both cases, and whether it was made perfectly or not, and regardless of whether it remains perfect, calibration with an accurate 35 ppt standard will always give the right salinity at or near 35 ppt.

Randy Holmes-Farley · Mar 16, 2026

diverjm said:
I believe the RODI calibration sets the unit to true zero without any salinity present. All I can say is I just did an ICP test and my salinity matched exactly to what the refractometer reads.

If you used a nontrue seawater refractometer and got the salinity exactly correct, then there was a lucky summation of different errors that made it accurate (or your criteria for accuracy is wide enough that you did not see it).

slingfox · Mar 16, 2026

Randy Holmes-Farley said:
If you used a nontrue seawater refractometer and got the salinity exactly correct, then there was a lucky summation of different errors that made it accurate (or your criteria for accuracy is wide enough that you did not see it).

The VeeGee STX-3 is marketing as a salinity refractometer. It has also lined up with every ICP test I’ve done in the last 2 years. I have never had to adjust calibration since I got it. The way I found out my old refractometer was not working is my ICPs were coming back with a very different reading. That sent me down the rabbit hole of buying several different instruments and off the shelf calibration solutions. The VeeGee solved my issues at the end of the day—at least for now.

diverjm · Mar 16, 2026

Randy Holmes-Farley said:
If you used a nontrue seawater refractometer and got the salinity exactly correct, then there was a lucky summation of different errors that made it accurate (or your criteria for accuracy is wide enough that you did not see it).

I understand. But wouldn’t you agree the VeeGee STX-3 is a true seawater refractometer?

diverjm · Mar 16, 2026

Just read the manual and it states calibration must be done in a controlled environment of 68 degrees for both the refractometer and the distilled water it suggests using, otherwise it can alter the calibration. OP, was the calibration performed in this manner? Maybe the temp was different between the two days?

Randy Holmes-Farley · Mar 16, 2026

diverjm said:
I understand. But wouldn’t you agree the VeeGee STX-3 is a true seawater refractometer?

I do not see it making any such claim, and thus I would expect it is not.

https://www.veegee.com/products/handheld-salinity-refractometer?_pos=2&_sid=5513d9973&_ss=r

Randy Holmes-Farley · Mar 16, 2026

diverjm said:
Just read the manual and it states calibration must be done in a controlled environment of 68 degrees for both the refractometer and the distilled water it suggests using, otherwise it can alter the calibration. OP, was the calibration performed in this manner? Maybe the temp was different between the two days?

Despite some manufacturers making that claim, it makes no sense to require that if it is an ATC refractometer as it claims.

Think of it this way: it mechanically adjusts the reading for any temp in the 10-30 deg C range (its claim). Thus, the calibration to zero is also automatically adjusted for any temp in this range. You actually cannot stop it from making that correction either in calibration or measurement.

fryman · Mar 16, 2026

I have used a variety of conductivity probes, hydrometers and refractometers. I find most of them to be adequate but don't trust the manual refractometers anymore because they don't hold calibration. I'm using a milwaukee digital refractometer most often nowadays because it's very easy to use and holds calibration a long time. But I believe a good calibrated conductivity probe is more accurate.

I only ever mix saltwater in 5 gallon buckets and have a cup that's the right size to hit 34ppt with one scoop. I check every bucket but in my experience if it reads off it's usually the instrument.