Refractive Index vs. Brix Digital Refractometer Accuracy Specification.

Question:

Please explain why your Palm Abbe refractometer lists accuracy as 0.0001 nD AND as 0.1 % Brix when an interval of 0.1 % Brix is almost exactly 0.0002 nD. Am I right in thinking the Brix % should be changed to 0.05 or the nD to 0.0002, which? I use a refractometer to indicate the concentration of firefighting foam solution, by comparison with calibration samples and most other digital hand held refractometers on the market list accuracy as 0.0002 nD or 0.1 % Brix. Some popular firefighting foams have quite low RI, which results in having typical solution RIs of 1.3330 - 1.3338 nD, and if the accuracy claimed is twice as good as your competitors, the error on readings could be halved. Note: FPEs working with foam systems often have to report on RIs as low as 1.3332.

Answer:

    You caught us! The Palm Abbe digital refractometer actually has better Brix precision than we claim, and here’s why: The actual precision of the Palm Abbe refractometer is +/- 0.0001 nD20. Where “nD” represents the refractive index measured at the sodium D-line (589.3 nm) and “20” represents 20 °C, the reference temperature for the measurement. There is a non-linear relationship between Brix and refractive index. The average change in refractive index for a corresponding 0.1% change in Brix is 0.0002 nD20. The average change in refractive index for a corresponding 0.05% change in Brix is 0.0001 nD20. So why do we claim a precision of +/-0.1 Brix when clearly it should be +/-0.05 Brix? The reason has to do with the Palm Abbe refractometer display resolution. Since the Palm Abbe refractometer only displays Brix in 0.1 increments, in a practical sense, it does not really matter that it is capable of +/-0.05 Brix precision.

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