Refractometer Troubleshooting - Vee Gee STX 3

Alexander1312

Alexander1312

Supporting Member
BARcard
Caution, this could be a rabbit hole conversation.

OK, so I bought this highly recommended refractometer to FINALLY have my salinity readings matching ICP results, i.e., FM ICP results.

After being correctly reminded to calibrate the refractometer out of the box, I started measuring salinity between my two tanks and the fresh saltwater mixes.

Unfortunately, I am getting far off readings after a couple of uses - and I remember this refractometer was supposed to keep calibration longer.

For those who have been using this successfully, and having ICP results matching the device’s readings, can you please share how to achieve this?

What I do:
  1. I use distilled water (not RODI), and align the blue scale with the bottom of the 0 bar (at least two drops of distilled water).
  2. I also tried aligning the blue scale with the top of 0 bar, and I am not sure which one would be better, as the instructions seem to say the middle of the 0 bar. However, I get either the bottom, or top, which means it reads either slightly less or slightly more I assume, and I recognize this does not matter too much.
  3. For the calibration to work, I had to unscrew the little screw on the side, to allow the calibration ring to move.
  4. After calibration, I am cleaning both the prism and the daylight plate, and tighten the screw so that the calibration ring would not move.
  5. I have been experimenting with how much I tighten the screw as it states in the manual that if too tight, it might move the calibration.
  6. I then add two drops at least of the saltwater from either tank.
  7. Salinity seems to be broadly within expectations.
  8. I did read to wait a little after each calibration - this is actually an important point - Milwaukee recommends waiting 1 minute for the auto temp compensation (ATC) to take effect if temps between the device and the water, Vee Gee does not state a certain timeframe. However, given the small amount of water that is measured, the timeframe is essential as if too long, evaporation might occur, I believe, increasing salinity. Not waiting long enough, ATC might not yet be effective, showing a potential lower than actual salinity.
  9. When I then measure another tank, the salinity readings seem to be more out of range, mostly higher than expected - both tanks are run the same way with similar water changes, same ATO, same dosing systems. I do recognize though that the main tank receives proportionately more dosing (major and minor traces) than the smaller tank, which should impact the salinity.
  10. When I then go back and check the calibration again with distilled water (after cleaning the prism and daylight plate), the calibration is above 0. When I calibrate the device again to 0 and read again, salinity is closer to expectation. I also check this with the Milwaukee and the results are pretty much identical, but only if the Vee Gee is calibrated EVERY single time.
  11. The only explanation I have is that I do not tighten the calibration ring enough, so I tried this too, and check again with the distilled water, and it would show then higher than 0 results if too tight :(.
Needless to say, I am frustrated, as I do not get consistent readings with this unless I calibrate before every reading.


Instructions: https://cdn.shopify.com/s/files/1/0050/6907/8621/files/VEE_GEE_Model_STX-3_Manual.pdf?v=1583444019

IMG_3008.jpeg
 
I've had the same issue with refractometers and digital testers, I switched to the tropic marin high precision hydrometer, I bought two and they're consistent, I clean them with RODI before storage, I test in a 500ml graduated cylinder and my tank is at 77-78° F, so I don't need to compensate for temperature.
 
Yeah, I said this in the tank journal as well. If you have a scale you can make your own 35PSU solution too

Temperature also matters considerably. So if you were holding the refractometer for example and it got warmer, then the bimetallic strip is going to flex and cause a different reading
 
You can also check calibration by making your own calibration solution:
I checked the calibration on my VeeGee STX-3 last week and it was inline (used both the RODI method as well as the DIY 35 ppt solution). I use gram and milligram scales to measure out the water and salt.
The DIY solution showed a very slightly lower salinity than expected (ie, the blue line a hair lower than 35 ppt) but I didn’t further adjust since I will wait until I run my next ICP. I also didn’t measure the water as precisely as I normally do but was likely 98% - 99% within spec. I have not had to adjust the calibration of the VeeGee since I purchased it 2 years ago. It was calibrated straight out of the box. Make sure the eye piece is in focus and you are looking at a bright light source so you can see the line clearly.

Here is my salinity on the ICP I ran in November 2025.

IMG_7079.jpeg
 
way back in the day. OG's use to say always calibrate with new calibration solution and calibrate each time you use it. if you wanted to be correct. they would say ro/di or distilled water is in correct cuz thats the base line not the target line.
 
jaysway said:
I've had the same issue with refractometers and digital testers, I switched to the tropic marin high precision hydrometer, I bought two and they're consistent, I clean them with RODI before storage, I test in a 500ml graduated cylinder and my tank is at 77-78° F, so I don't need to compensate for temperature.
Click to expand...
Unfortunately, it is a myth that these are accurate, they are not. Even if apps for temp is considered or you are using the correct temp (mine is the same temp, I clean them meticously, I use a 500 ml cylinder, and I am reading +1.3 PSU more than FM ICP).
 
Alexander1312 said:
Unfortunately, it is a myth that these are accurate, they are not. Even if apps for temp is considered or you are using the correct temp (mine is the same temp, I clean them meticously, I use a 500 ml cylinder, and I am reading +1.3 PSU more than FM ICP).
Click to expand...
Shoot the club just needs it's own a rolling icp lab in a semi truck trailer, or a cleared out charter bus. Problem solved.

We just add it to barcode lmao.
 
I calibrated the Vee Gee against the FM calibration fluid [https://premiumaquatics.com/collect...2&_sid=2efc0e926&_ss=r&variant=47329634975986]

Unfortunately, when I do this, and test against distilled water, the distilled water measure shows -1 PSU.

So either one is correct, calibrating against distilled or 35 PSU fluid, but not both. It seems more intuitive to me to calibrate against the 35 PSU fluid, but their instructions state distilled water, most likely due to simplicity of the process, not due to greater accuracy.

And yes, their fluid can be off too, and I can try making my own (and probably will when I have time), but I know several folks who tested the accuracy of these FM fluids (which are all checked with their ICP testing procedure) against various conductivity meter readings and concluded that these are accurate.

The salinity rabbit hole is annoying to say the least, and it might just be easier to find a device which happens (by chance) to be accurate out of the box.

Oh, and fun fact @CaseyP - when I use this fluid and test on the Milwaukee (with a 30 second delay), it shows 37 PSU….
 
Alexander1312 said:
I calibrated the Vee Gee against the FM calibration fluid [https://premiumaquatics.com/collect...2&_sid=2efc0e926&_ss=r&variant=47329634975986]

Unfortunately, when I do this, and test against distilled water, the distilled water measure shows -1 PSU.

So either one is correct, calibrating against distilled or 35 PSU fluid, but not both. It seems more intuitive to me to calibrate against the 35 PSU fluid, but their instructions state distilled water, most likely due to simplicity of the process, not due to greater accuracy.

And yes, their fluid can be off too, and I can try making my own (and probably will when I have time), but I know several folks who tested the accuracy of these FM fluids (which are all checked with their ICP testing procedure) against various conductivity meter readings and concluded that these are accurate.

The salinity rabbit hole is annoying to say the least, and it might just be easier to find a device which happens (by chance) to be accurate out of the box.

Oh, and fun fact @CaseyP - when I use this fluid and test on the Milwaukee (with a 30 second delay), it shows 37 PSU….
Click to expand...

You're spending too many brain cells on this. Refractometers do not measure salinity directly. It's the index of refraction they measure and then translate to a scale that is non-linear. Refractive index change for 0-5 ppt is not the same as 30-35 ppt. This "issue" is probably worse with less expensive refractometers.

Calibrate to the range you want to measure. Use 35ppt calibration solution.

Edit: Your Milwaukee refractometer is working well within normal. Your calibration solution could be off slightly or it's at 35 PSU and it's just reporting within it's error range.

AccuracyPSU
PPT
S.G.
Temp		± 2
± 2
± 0.002
± 0.3°C / ± 0.5°F
 
Last edited:
RandyC said:
You're spending too many brain cells on this. Refractometers do not measure salinity directly. It's the index of refraction they measure and then translate to a scale that is non-linear. Refractive index change for 0-5 ppt is not the same as 30-35 ppt. This "issue" is probably worse with less expensive refractometers.

Calibrate to the range you want to measure. Use 35ppt calibration solution.

Edit: Your Milwaukee refractometer is working well within normal. Your calibration solution could be off slightly or it's at 35 PSU and it's just reporting within it's error range.

AccuracyPSU
PPT
S.G.
Temp		± 2
± 2
± 0.002
± 0.3°C / ± 0.5°F
Click to expand...
I am aware of this but thank you for providing this context.

Also, I disagree as I am of the opinion that ‘we’ are not putting enough effort into getting salinity right, and have therefore a less positive reputation in ICP labs which get water from various places around the world.
 
As a FM fanboy, I was able to talk to Claude S about the salinity issue, mainly because I was using his reference solution and wanted to confirm its proper application.

These are the takeaways, some are quite similar to some of the comments made here:
  1. There are too many cheap refractometers in use (in the US) which result in salinity measures often far off from targets. Reasonable quality refractometers are not cheap [which is why we probably like the Vee Gee].
  2. Many of the calibration solutions are wrong as they are based on Natriumchlorid (only) vs seawater which will not provide correct results.
  3. Not using a proper reference solution can result in up to 3 PSU differences, specifically if calibrated with distilled water - which is not the way to calibrate the refractometer.
  4. The best way to measure is to use the 35 PSU reference solution to make sure the device is calibrated, then add the sample, then check against the reference solution again (this is how I have been doing it at the moment). The Vee Gee seems to hold the calibration so this additional step might not be needed but I will do it for the next few weeks.
  5. The specific gravity scale of (most) refractometers should not be used, only the PSU scale.
Will see what the next ICP says, but with a good reference solution, the Vee Gee might yet be the best device to measure salinity :).
 
Alexander1312 said:
I am aware of this but thank you for providing this context.

Also, I disagree as I am of the opinion that ‘we’ are not putting enough effort into getting salinity right, and have therefore a less positive reputation in ICP labs which get water from various places around the world.
Click to expand...

If you're aware of this, then you shouldn't have been surprised that the refractometer measured at -1 PSU for ro/di when calibrated with 35 PSU calibration solution. And we can definitely disagree about chasing salinity. The ocean ranges from 32 to 37 PSU and isn't constant. Trying to have a perfect 35 PSU versus 34 or 36, will likely show little to no gain, but if there is any research or papers that state otherwise, I'd be happy to read about it.
 
Last edited:
I went down the salinity rabbit hole once. Bought a ton of testers.
At some point, all testing sucks, and you are chasing a dragon.

Now, chasing that kind of dragon can be good for you, you will learn and experince all kinds of things - except for finding a superior way to test! :)
 
Thales said:
I went down the salinity rabbit hole once. Bought a ton of testers.
At some point, all testing sucks, and you are chasing a dragon.

Now, chasing that kind of dragon can be good for you, you will learn and experince all kinds of things - except for finding a superior way to test! :)
Click to expand...
Agreed. I am at a point where if I find a convenient way to test salinity which brings me consistently <0.5 PSU to my target, I am content.
 
Also, how do you know if your 35 psu solution is 35 psu, and how do you know it maintains that over time?


You don't, unless you mix it and use it right away. :)



Testing suuuuuuuukkkkkkkkkkkkssssssssss!


Well it doesn't really suck, it is pretty great. But wanting it to be 'right' and 'match' and be anything but ball park is a wish for kit that does not exist, and may never exist.
 
You must log in or register to reply here.
Back
Top