I agree that the test strips are using but since the AquaRite will measure the salt level in pool, why have a separate test kit?
The Hayward AquaRite has a good internal algorithm for reporting salinity level that is close to real-world, but it, in and of itself, is still a proxy method of measuring salinity. All SWG's that report salinity are actually reporting TDS and, to be even more specific, they are reporting the electrical conductivity (EC) of the water passing through the cell. So, the basic algorithm is this -
1. Measure voltage, current and temperature
2. Calculate the conductivity of the water in the cell
3. Use a series of correction and scaling factors to output salinity based on factory calibrated models.
Haywards do that algorithm well, Pentair's not so well and I have no feel for Jandy. In the above method, there are interferences. Chloride (Cl-) is only one of several ions in solution that can conduct electricity. Solution conductivity is also affected by pH and temperature. So there's lots of "other factors" that can vary and cause the conductivity to change which have nothing to do with actual salt levels. Also, once a salt cell starts to build up scale on the plates or if the coating becomes damaged, the reported salinity levels will be far off from the actual value. In fact, that is one way to diagnose the health of an SWG - if the reported salinity levels fall but the actual levels remain the same, you either have scale on the plates or a damaged coating.
On the contrary, the K-1766 is a chemical test for chloride ion based on Mohr's Method (also known as the argentometric method). It is a titration of chloride ions,
and only chloride ions, using silver nitrate. The reaction is mostly independent of pH in the range of pH one finds a pool to be in and is not interfered with by any other ion in solution (unless you happen to have bromide ions in solution or cyanide anions). So the K-1766 measures
the exact salt level and reports that with an tolerance that's based on the test volume. So, the standard test uses a 10mL water sample and gives +/-200ppm. However, all you need to do is use a 25mL water sample and the tolerance becomes +/- 80ppm. A 50mL water sample will have a tolerance of +/- 40ppm. That is both exact (measures chloride only) AND more accurate than any other method available.