KH Director reading is variable

[ss162400]

Hi all,

I am having problems with the reading from my KH director.

I have recalibrated the pumps multiple times, and have also recalibrated the ph probe multiple times.

Have also flushed unit, vented regent etc. The regent tube is well under 70cm.

The readings are consistently variable, Salifert reading is consistently 7.7;

When I finish the sample pump calibration, KH Director reading is 7.7, but once its readings changed into 8.3, after I recalibrated the sample pump, the reading returned to 7.7. Sometime its reading changed into 7.4 or more low. I need often recalibrated the sample pump, so that I get the right reading.

Basically I need to check the sample pump every three days so that I can guarantee that the reading is correct. What should I do?

By the way, I have 2 units GHL Doser 2.1, I changed the Dosers, but the issue still stand here.