Alexander1312
Supporting Member
Still I was, a mystery we are not able to solve here. All good Randy.
Refractometer Troubleshooting - Vee Gee STX 3
- Thread starter Alexander1312
- Start date
Alexander1312
Supporting Member
Haha, yes, and this is where belief comes into playAlso, 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.
. I believe that the Fauna Marin Reference solution is as accurate as possible. Unopened the solution is supposed to last for 2-3 years, opened it is supposed to be stable for quite a while too, but I do actually need to check how long really. I do shake it before using for feeling better about it . Water testing sometimes seems more an art than a science, strangely.
RandyC
Supporting Member
Huh? What's the mystery that's not solved? If you calibrate the refractometer with 35 PSU calibration fluid, it will not be accurate when trying to read RO/DI water. It's not a mystery.
Here, maybe Gemini can explain it better for you.
1. The "Perfect Slope" Fallacy
Most hobbyist refractometers are designed for brine (sodium chloride) rather than seawater. While they are very similar, they refract light at slightly different angles.
When you calibrate, you are setting a single "anchor point."
• If you calibrate at 0 (RO/DI): The device is accurate at zero, but because the internal scale might have a slight tilt or "slope" error, it becomes increasingly inaccurate as you move toward 35 ppt.
• If you calibrate at 35 ppt: You have ensured the device is perfectly accurate at the most critical level for your tank. However, any slight manufacturing deviation in the lens or scale means that as you move back down to zero, the error "stretches," causing RO/DI to appear as a negative value or slightly off-center.
2. Refractive Index: Seawater vs. Brine
Seawater contains a complex mix of magnesium, calcium, and sulfates, whereas calibration fluid is typically a specific refractive index match for 35 ppt seawater.
• Refractometers measure how much light bends as it passes through the liquid.
• The physical properties of pure water (RO/DI) are different enough from 35 ppt saltwater that a low-cost prism may not be able to track the transition linearly across the entire scale.
3. Why 35 ppt is the "Right" Wrong Answer
In chemistry and reef-keeping, you always want to calibrate as close to your target measurement as possible.
• Scenario A: You calibrate with RO/DI (0). Your 35 ppt tank might actually be 33 or 38 ppt because of slope error.
• Scenario B: You calibrate with 35 ppt fluid. Your RO/DI reads -2 ppt.
Scenario B is safer. You don't actually care if your RO/DI reads perfectly at zero, because you aren't trying to maintain a "zero ppt" aquarium. You care that your tank is exactly 35 ppt.
Tips for Accuracy
• Temperature Matters: Even with Automatic Temperature Compensation (ATC), let the fluid sit on the prism for 30 seconds before reading so the fluid and the metal reach the same temperature.
• The "Dark" Calibration: If your RO/DI is significantly off (e.g., more than 2–3 points), your refractometer's prism might be dirty or the calibration screw might be loose.
• Trust the Fluid: Always trust a quality 35 ppt calibration standard over RO/DI for hobbyist-grade equipment. RO/DI is only a reliable calibration point for high-end, laboratory-grade digital refractometers (like a Milwaukee or Hanna).