The original point I made was measuring closer to your target is better so buying the calibration solution is a good way to check your refractometer is reading what it is supposed to be reading. Panda went into this strange arguement about double checking both ends... blah... blah... blah. My point is that if the refractometer is set to 0 but has a faulty manufactoring it will read 0 just fine but may have an error at 1.025 of unknown proportion since you only measure it against one fluid at one value. If you measure it at 1.030 even if you are off, you won't be as off. If you measure it at 1.030 then 0 and both read correctly, you can be pretty much assured your refractometer will read anything between those fairly accurately at least more so than reading one number. A lot of people are ASSUMING that if 0 reads properly 1.025 will read properly as well... simply not true. You have no idea how far off the scale could be when taking only one measurement. The more calibrations you make espeically closer to the target the more accurate your measurement device is going to be.
Lets say the mirror is a little bent or the scale was printed incorrectly. You dial it to zero and when you compare your zero water it is fine, but given the example of the scale being off you could be reading 1.025 and your water could be 1.015. This probably isn't going to be a problem and virtually no chance with a high quality refractometer, but these cheap knock offs could easily suffer from a manufactoring defect. By dialing in your source so far below your target measurement you more easily allow for an incorrect reading. It isn't about a two point calibration. It is more about verifying the units scale is correct. Ten calibration readings is better than 5, 5 is better than 2 and 2 is better than 1. Also, calibrating to a known value very near or exactly where you want to keep your water is MUCH safer as every time you read it will be on the number you expect rather than a projected number from a scale.
A refracotometer that has a manufactoring defect could read 1.050 and 0 perfectly and read 1.030 wrong. If that is the case, you will never get it to read 1.050 and 0 correctly if you dial it in to correctly read 1.030 as the scale will be off at the ends. However if you have a refractometer that couldn't read 1.030 properly and 0 properly without recalibrating I think it is probably best that you use a new refractometer even if it will still work if calibrated at 1.030 fairly well.
Pretty much. It read 1.025 when the water was really 1.030. The first clue was my hydrometer wasn't lining up, but I wrote that off to it being a hydrometer. Then I read a post where someone bought a cheap refractometer, properly calibrated at 0 and had a near .01 variance so I decided to check mine out against a borrowed one. Sure enough the hydrometer and borrowed refractometer were lining up. Next I bought some calibration fluid and a new refractometer. I got a correct reading on the new refractometer as well as the borrowed one so the original one was off. I calibrated the off one at 1.030, but distilled water was then reading .006 off which I though was a bit weird but I didn't really spend much time trying to figure it out.
