fine tuning water sample tubing volume for accuracy

Hi,

I've been getting inconsistent readings compared to hanna alkalinity checker, and i'm trying to rule out any possible inproper setups i may have done. The following are the steps i've taken to ensure everything is dones correctly.

1) pump calibrated using 5ml graduated cylinder,
2) using flex pvc tube on the reagent doser pump and minimizing its length to reagent container to only 4 cm,
3) pump to khd tube length according to instruction 16cm(water sample), 17cm(reagent), 17cm(waste)
4) making sure water sample intake point in the sump is free of debris and installing an inline filter provided
5) calibrate ph probe using ph 4 and ph 7 solution as provided and rinse with rodi water and dry it in between each solution (ADC1 at ph4=51014, ADC2 at ph7=30830)

am I missing any other points I should be aware of ? assuming I did all the above correctly, the only point I'm not too confident about is in regards to calculating the water sample tube length. mainly because there are subtle details that are vague. such as,

6) I have installed a inline filter. this is said to add 7ml to the total volume. the inline filter has two barbed connecting ends where tubing will slip onto. these two ends meaure 1 cm. i'm guessing the volume 7ml includes the volume in these two 1 cm ends ? so when you slip on the tubing onto the ends part of the tubing will overlap onto the barbed ends thereby holding it in place. this overlapping (approximately .5 cm each end) must be subtracted right ?

7) also the section of the tube that is underneath the water level in the sump, for me about 6cm. this must also be subtracted right ?

8) finally, how much will these differences in the calculation of water sample tube length (~7cm for me) affect the accuracy of the final result ? If I was to adjust the tube length calculation by around 7cm, would this greatly change the dkh reading ? or is this too small of a change to make a difference and I should focus finding errors elsewhere ?

9) can the moderator please explain the mechanics inside the khd and how the water sample traverses through them and how and why the water sample tube volume come into play ?


sorry for the lengthy questions, but I'm quite discouraged and did not expect this much inconsistency. As I weren't expecting khd to add even more work and maintenance when the reason i got it in the first place is to decrease maintainance.

here are my khd vs hanna dkh readings

khd vs hanna
14.7 vs -
6.6 vs -
6.6 vs 7.4
6.8 vs 7.5
7.6 vs 7.9



Thank you very much for reading and helping.