When he talks about a "combined effect", he is saying that a) my water is too soft to get accurate readings and b) I am seeing the 0.15 slide- to- slide variation.
I think you should email them yourself and ask if you will be getting accurate pH readings in your tank of 3 KH.
Alternatively, don't you have a pH pen you can use to compare the readings, if you still don't believe me?
Also:
Hi Sacha,
Thanks for your E-mail
Can you let us know the following so that we will be able to help to diagnose what is happening please?
The KH of your aquarium
What other test methods you are using to validate the pH.
We have not come across what could be described as a dud slide before. They chemistry inside should always react to a pH that is greater than 6.4. The slide works by changing colour dependent on the pH. A pH of 6.4 will be yellow, and the pH higher than 8 will appear more blue. If the slide is bright yellow, then it is likely that your pH is around 6.4. You could test the slide in some de-chlorinated tap water (assuming that this has a higher pH) if it changes colour from bright yellow, then the slide is working.
Best regards,
seneye support
6.4 is the lowest pH the unit can register.
6-7 is the lowest KH in which the slide can measure pH.