I installed the KH Director with a Doser 2.1 (Slave) two days ago, following all the directions in detail: calibrated pH probe (twice), calibrated the three pumps (sample water at speed setting 3, reagent at speed setting 0 and waste water at speed setting 3). I then vented the reagent tube and flushed the measurement cell according to the instructions. I set the unit to use 80ml water samples and input the length of the water sample tube (both from the sump to the doser and from the doser to the Director, total 116cm). At the moment I am not using the Director for KH control as I wanted to ensure correct KH measurement first. The Director is connected to my Profilux 4.

The first manual measurement returned a result of KH 6.6. I compared it to a Hanna KH checker with returned a value of 6.8. Using the Director I added a manual dose of Carbonate to raise the KH by 0.5. An hour later the Director measurement was 6.7.

I also increased the standard dosing of Carbonate by about 15%.

Last night the Hanna checker showed a KH of 7.1 while the Director was at 6.6. I added another manual dose of Carbonate.

This morning the Director measured 6.7.

I decided to check both the Director and the Hanna against the GHL KH 7.5 Reference. The Hanna showed a KH of 7.5. The Director showed a KH of 6.7.

In all of the tests subsequent to the first one, I kept an eye on the "actual pH" reading of the Director and the the pH would descend constantly during the test, which I take to be the normal behaviour.

I then vented the reagent tube, flushed the measurement cell and completely flushed the measurement cell.

Another manual measurement and the KH Director gave a reading of 6.6. During the test the "actual pH" measurement started at 7.41 and descended to 4.35.

So for some reason the KH Director is not able to give readings other than 6.6 and 6.7, no matter what the sample liquid is at...

The reagent tube (the "stiffer" tube that came with the Director) is about 25cm long and goes straight down from the doser pump to the bottom of the reagent bottle, no curves or loops. The vent tube is not kinked or obstructed.

Any ideas what the problem could be?