jccaclimber
Supporting Member
This came up in another thread, so I figured I would write this up.
What we want when taking measurements is something that is both sufficiently accurate and sufficiently precise for our needs. I'm not going to go into accuracy vs. precision right now, but let's assume that our refractometers are sufficiently precise. In the chart below the X axis is the salinity (as specific gravity) of the water. Not what our device tells us, what it actually is. The Y axis is the value our instrument is telling us, which may or may not be correct.
The blue line is what we get if our instrument is perfect. At any point on the line the reported value perfectly matches the actual value. We tend to pretend this is how the world works, even if it is rarely the case.
Next we have the red line. The linearity, slope in this case, is correct, but it is incorrectly calibrated by a 0.002 offset. As a result when we test 1.025 SG water it reads 1.027 and when we test 1.026 SG water it reads 1.028. This device could be better, if only it was calibrated correctly. Because the slope is correct, it doesn't matter where we calibrate it.
After this we have the gray line. The gray line has a 1% error in its slope. It was calibrated with RODI water (or tap, it's close enough). It reads perfectly at 1.000 where we calibrated it, but with 1.025 water it reads 1.0275 and at 1.026 it reads 1.0286.
Finally we have the yellow line. It also has the 1% slope error, but we calibrated it at 1.025. As a result, while it is incorrect reading RODI water at 0.9975, it is spot on at 1.025. By 1.026 it is still very close, reading 1.0261.
You might next ask "How do I get my refractometer to read correctly at 1.000 and 1.025 SG?" The answer is either you get it dialed in at one of those two points and it's already correct at the other, or there is nothing practical you can do. Our refractometers only have one adjustment on them, which is the offset. The slope is what it is, and you then have to move that line up and down until it's somewhere you are comfortable with.
Better than this would be a 2 point calibration. In that you are able to set both ends of the line, and thus get good accuracy at all of the points in between. This is why pH probes are calibrated with 2 references. That process lets you get things correct across a range, not just at one point. In the real world this gets a bit more complicated because the error in the instrument isn't often a straight line like I drew above. It's probably curved, maybe not in an obvious way, and likely changes with time, temperature, and other factors as well. No matter what though, you will still get the best results when you calibrate as close to your measurement value as practical.
The other big assumption this is making is that our calibration fluid is correct. I've found this to not always be the case. A couple years ago I bought 3 bottles of calibration fluid from a well known supplier. One for me, and two for friends. I found an almost 0.002 SG difference between the three bottles, which arrived together. One of these days I'll make my own solution, or find a way to verify some. Until then I actually use tap or RODI water because I know it to be consistent. In the case of my specific refractometer, when I calibrated it at 1.000 with RODI water it read between the three bottles of calibration solution at 1.025. I am assuming, although without actually knowing, that the slope does not change over time. I would not assume this to be true of other refractometers, so while tap water is likely just fine for many of us, there are probably a few members out there where it produces more error than we would like at 1.025 SG, or wherever you choose to run your system.
What, you use tap water not RODI? If I have RODI nearby I'll use RODI. If I'm in front of a sink I'll use tap. Periodically I'll calibrate with one, then immediately check with the other. I've yet to find a difference between the two.
What we want when taking measurements is something that is both sufficiently accurate and sufficiently precise for our needs. I'm not going to go into accuracy vs. precision right now, but let's assume that our refractometers are sufficiently precise. In the chart below the X axis is the salinity (as specific gravity) of the water. Not what our device tells us, what it actually is. The Y axis is the value our instrument is telling us, which may or may not be correct.
The blue line is what we get if our instrument is perfect. At any point on the line the reported value perfectly matches the actual value. We tend to pretend this is how the world works, even if it is rarely the case.
Next we have the red line. The linearity, slope in this case, is correct, but it is incorrectly calibrated by a 0.002 offset. As a result when we test 1.025 SG water it reads 1.027 and when we test 1.026 SG water it reads 1.028. This device could be better, if only it was calibrated correctly. Because the slope is correct, it doesn't matter where we calibrate it.
After this we have the gray line. The gray line has a 1% error in its slope. It was calibrated with RODI water (or tap, it's close enough). It reads perfectly at 1.000 where we calibrated it, but with 1.025 water it reads 1.0275 and at 1.026 it reads 1.0286.
Finally we have the yellow line. It also has the 1% slope error, but we calibrated it at 1.025. As a result, while it is incorrect reading RODI water at 0.9975, it is spot on at 1.025. By 1.026 it is still very close, reading 1.0261.
You might next ask "How do I get my refractometer to read correctly at 1.000 and 1.025 SG?" The answer is either you get it dialed in at one of those two points and it's already correct at the other, or there is nothing practical you can do. Our refractometers only have one adjustment on them, which is the offset. The slope is what it is, and you then have to move that line up and down until it's somewhere you are comfortable with.
Better than this would be a 2 point calibration. In that you are able to set both ends of the line, and thus get good accuracy at all of the points in between. This is why pH probes are calibrated with 2 references. That process lets you get things correct across a range, not just at one point. In the real world this gets a bit more complicated because the error in the instrument isn't often a straight line like I drew above. It's probably curved, maybe not in an obvious way, and likely changes with time, temperature, and other factors as well. No matter what though, you will still get the best results when you calibrate as close to your measurement value as practical.
The other big assumption this is making is that our calibration fluid is correct. I've found this to not always be the case. A couple years ago I bought 3 bottles of calibration fluid from a well known supplier. One for me, and two for friends. I found an almost 0.002 SG difference between the three bottles, which arrived together. One of these days I'll make my own solution, or find a way to verify some. Until then I actually use tap or RODI water because I know it to be consistent. In the case of my specific refractometer, when I calibrated it at 1.000 with RODI water it read between the three bottles of calibration solution at 1.025. I am assuming, although without actually knowing, that the slope does not change over time. I would not assume this to be true of other refractometers, so while tap water is likely just fine for many of us, there are probably a few members out there where it produces more error than we would like at 1.025 SG, or wherever you choose to run your system.
What, you use tap water not RODI? If I have RODI nearby I'll use RODI. If I'm in front of a sink I'll use tap. Periodically I'll calibrate with one, then immediately check with the other. I've yet to find a difference between the two.
