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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

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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.
 
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