Ah, ok... verrrrry interesting. Thanks for the synopsis, fkshiu.
Reference fluids of 35ppt seawater are a brilliant idea. I really wish most testers we have would have this available. I know Seachem testkits used to have reference solutions but I stopped using Seachem when they changed their reagents so that it the colour changes were so faint it was impossible to use.
The only tester (electronic or otherwise) that I have any degree of confidence in is my Pinpoint NO3 meter, and that's because you need to calibrate at the endpoints of the measuring scale for every test. You can use the calibration fluids as the reference. Reference solutions ought to be an industry standard, that way if you have any doubt about a reading, you can at least still verify your testing. Oh well, [/rant]