When looking at water (briny or not) you have a problem with light penetrating the liquid, being slowed down, hitting the bottom of the tank and reflecting. If the liquid is deeper than about 15 cm, or if you paint the bottom with a non-reflective coating, this effect goes away.
Could you run the experiment again putting more than 15cm of water in the bucket?
Looking at your data, I'm going to guess you added some liquid to a bucket, then lifted your sensor and took measurement. So, the bottom might have an 'offset' effect. Both sensors over-range by about the same amount (except at 600 - that data point is an outlier.
I going to guess that maybe you did NOT do an offset calibration before you started. That might account for all your numbers being low.
Try doing an offest cal on your sensors - see what it tells you. If you get 20mm of offset, I'd say the numbers looks as good as they can. (Just update your data by the offset amount and have another look.)
I'm not sure I can explain the data point at 600 though. Experimental error.
Try it again with a deeper pool of water, and doing the calibration. Then we can re-visit the numbers.
- john
