Hi,ty for replies,instructions on refractometers said to calibrate with distilled water.is 1034 much too high? Shall I do water change using just rodi water to bring it down?
Post the instruction sheet and I'll show you why this is a bad idea.
Most of the refracs that most of us have are saltwater refracs, not seawater refracs. The relationship between salinity and refractive index is different for saltwater and seawater. If you calibrate a saltwater refrac with RODI and then try to measure 35ppt seawater you'll get something around 36.5 and it looks like it is too high.
The manufacturer tells you in the instruction sheet to use RODI because that's the best the manufacturer can recommend. The manufacturer does not know what you intend to measure with their instrument, so they give you the closest thing they can to a calibration. But in ALL cases it is sub-optimal. No matter what you use a refractometer for, you should always try to calibrate it as closely to the thing you want to measure as possible. This is the nature of single point calibrations.
A calibration does two things, it sets the zero point and it also tells you the slope of the response. It would be much easier to think about a bathroom scale. We all know how to set the scale to zero with nothing on it, and that sets the zero point. But the slope determines how much the scale moves for each pound you add. Imagine we had a scale where the dial only moved one half a pound for every pound you put on it. The reading would always be exactly half the real weight even though you have zero set at the right place.
To really use a scale, you'd have to calibrate at two weights. You use one to set the zero, and a second weight to make sure that the divisions on the scale are right. The second weight sets the slope of the response. It makes sure that for every pound you add you actually move the scale by one pound.
Unfortunately with a refractometer we only get one point to calibrate. We can't adjust the scale. So there's never any guarantee that for every 1ppt the salinity increases that the reading on the refractometer actually increases by 1ppt. In many cases it doesn't and that can lead you to a false reading if the only point you know to be accurate is the 0 mark.
So with single point calibration the best we can do is to set the thing to read right at one point. We can only guarantee one point to be accurate.
Would you rather know that the 0 mark is accurate and 35ppt may be off? Or would you rather know that 35ppt was accurate and 0 might be a little off?
Now if the manufacturer knew you were going to measure seawater with this instrument, they would recommend you to calibrate it to something close to seawater. But lots of people buy refractometers for lots of different purposes from salt brine to beer. The manufacturer doesn't know which one of these uses you are going for, so they just say to zero it with water because that is as good as any other point they tell you to calibrate to if it isn't the point you're measuring to and because RO water is easy to come by while a calibrant may not be.
