If your KHD measurements are consistently around 5.9dkH, I would trust those readings more than the other kits.
Hanna tests have the tendency to read higher values in general. That is why
it shows higher than both the KHD and Salifert kit. Keep in mind also that the KHD performs tests using laboratory standards. A more comparable lab-grade test kit to compare to would be ones made by LaMotte.
With traditional hobby-grade test kits, other variables must be considered. Some which are more susceptible to human-error.
For example:
- Water sample collected: Some folks under or over estimate their water sample size which results in skewed test results.
- Personal perception of liquid "color change": What is considered the ending-color change to one person, may not be interpreted the same by another. An extra drop more may be added and that alters the test result.
KHD minimizes these possibilities of human-error by using pumps which can precisely measure-out consistent water sample size and reagent added. With the KHD, provided the pump has been properly calibrated, if you want to use an 80ml water sample, the KHD will use an 80ml sample every time. Whereas traditional kits rely on our eyes to notice a full color change, the KHD registers the ALK reading with a pH probe. The probe alone is going to be more accurate at reading the correct KH value than relying on our eyes to notice when a full color change has occurred.
One must also consider that the degree of KHD accuracy will depend heavily on correct pump calibration, probe calibration, water sample size, correctly measured sample tube and reagent tube length. The KHD measurement accuracy is +/- 0.1dkH - 0.2dkH; all depending on calibration results and water sample size.
Nonetheless, if you strongly feel the other test results are more credible than the KHD, you have the option to adjust the measurement end-results in the KHD settings page.
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