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The probe should be stored in water if it won't be used for more than 2 days. Otherwise it has a chance of drying out completely if it sits inside an inactive KHD measurement cell.

The KHD must lay horizontally with the probe also laying in the same way. The pH probe from the P4 is NOT the same kind used by the KHD and would therefore not be a suitable alternative.

It is possible the probe has reached end of life and will need replacing. Being that this was a second hand unit, there's no telling how long it sat in the box before being put to use. This also...

How old is the probe?
If it's showing slower reaction than what you remember seeing before, it's probably time to re-calibrate.

As per the filter manual, from sump to doser.

Stock GHL tubing will fit over the GHL filter. It will fit very snug, but it will fit.
Dipping the end of the tube in hot water will give it enough flex to connect the filter.

You can press...

Those numbers look OK.

Any reason why you are not using the water sample filter?
Without a filter installed, one cannot rule out the possibility of sediment getting into the sample which is...

What settings did you enter into the dosing pumps?
Speed? Flow rate?

What number did you enter into the KHD water sample volume?
Are you in front of the device during a measurement? Do the pumps...