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Refractometer Troubleshooting - Vee Gee STX 3

Alexander1312

Alexander1312

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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.
 
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