CSI, Pool Math vs kit's slide rule

[Saltyhog]

Why such a large difference? My parameters

pH 7.6

FC 5.0

CC 0

Alk 90

Cal 380

CYA 80

water temp 60°

The stupid slide rule says +0.05 and Pool math says -0.32. That's a pretty big difference. Definitely going with the PM numbers but wondering where the error might be in the slide rules calculation?

 

[JamesW]

The slide rule uses "Adjusted TA", which is the total alkalinity minus cyanurate alkalinity and borate alkalinity.

Typically, you subtract about 1/3 of the cya reading from the TA to get adjusted TA.

The book that comes with the test kit explains the adjusted TA calculation.

PoolMath automatically makes the calculations when you enter all of the readings.