Quote Originally Posted by Vinny View Post
The trend depends on your specific tank and KH consumption patterns. Typically consumption increases just before the lights turn ON, but this is not always the case as every tank will have a different "norm". If you're doubting the KHD results, you can always test it with the 7.5 reference fluid 3 times back-to-back. If the results are consistent, the KHD is providing the correct values. It would also help to check the water sample filter as these do become clogged over time and when it does happen, it can restrict the actual water sample used for the test, therefore skewing the KH end results.
The test with the reference fluid are ok, low but that is adjusted with the correction factor.

The problem is probably that over a few days time, there's a little air bubble forming just at the outlet side of the reagent dosing pump.

A few days or maybe a week it produces values that probably are in range of the true value, without seeing a returning pattern. Which I would expect, because my KH dosing values are stable.
I recalibrated the pumps probably 5 times, never a big deviation.
Sample size is 100 ml.
The inline sample filter is ok. I presume that the measured sample tube value is also correct, because when a measurement is done and the sample tube is pushed empty by the sample dosing pump, it’s just getting emptied, maybe 2 bubbles of air are pushed out, before the pump stops.
The vent tube is empty and above the waste water level, same for the waste water tube.
I still have a problem that a little air bubble is formed, right at the outlet of the reagent pump. It takes a few days. So the most probably cause for my problem.
I don’t know why the air bubble is forming.
I replaced the PVC flex tubing with new PVC flex tubing, they are pushed to the end on the connection pieces of the dosing pumps.
The reagent bottle cap has an extra little hole, so under pressure is not possible.
The suction side flex PVC tube to the reagent pump is straight to the pump, no turns. Reagent tube Length is about 33 cm and fully submerged.
The room temperature is pretty stable, between 20 to 22°C.
I already used 2 bottles of GHL KH 7.5 reference. The KH director produces stable values, but not with in the expected 7.1 to 7.9 range.
The last set was 7.1, 6.9, 6.8, 6.8, 6.8. So this I can adjust whit the correction factor. But it should be in range because the PH probe is 2 months old, I bought the KHD, SA unit brand new.
Does someone has another idea why the little air bubble is forming, right at the outlet of the reagent dosing pump? I can’t see any “micro” bubbles in the suction side.