Test Kits VS Checkers. Lookin For Input

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I have elos, salifert and hanna alk and phosphate checker. I found that alk is a bit high, it is always 0.6 higher than both elos and salifert e.g. salifert will read 7.4 dkh but the hanna will read 8.0(with the conversion either time 0.056 or divide 0.056), hanna does have consistant reading if I might add. all I do is just minus 0.6dkh from the readings.