What to do next? KHD not giving correct measurement.

I'm a little lost on what to do next.

I have multiple machines for measuring KH in my system and KHD is one of them. I also do some manual tests and I always "secure" the different options with reference solutions. Both my own mix and brands.

I'm having trouble with the KHD giving way to low value. The error is confirmed many times over with different ref. solutions against all my machines.

I thought I'd found the problem when I re-calibrated the pumps connected to the KHD since they all were a little of.

Intake: 1,5 ml off,
Reagent: 0,2 ml off,
Out: 4 ml off,

But when recalibrating again it seems like the intake was ok since it gave me another / the old volume again. I re-did the calibration multiple times and all values entered in the app are "up to date".

The probe is calibrated multiple times and shows spot on result when placed in ref. sol.

Test chamber rinsed multiple times.

Internal hose rinsed multiple times.

Checked sample volum and filter. They are consistent and fine.

Im going to open the machine again and have a look at the electrode again - might be some issue/dirt on it again but nog likely since I re-calibrated last week and this week as well.

What else can I check for?

Opened up and checked electrode/probe - nothing wrong.

Re-calibrated sample pump again and got a totally different value than before. Earlier 36 ml now 38 ml.

Update:

So this morning I did the following.

Changed internal pump tubes, rollers and housing on all pumps.

Calibrated all pumps.

Checked calibration.

Filled the entire line in in the following order filter - hose - tubing - and small hose between doser/KHD and entered the value inside the app.

Calibrated electrode.

Still getting way to low value.

I can only think of to things it could be..

1. Something wrong with the reagent.
2. volume inside the machine not calculated since I measured the volume by hand.

Should I add more volum due to the fact that there's line/hose inside the machine and if so how much?

changed reagent. No change in result.

What else could this be?

Hello,

A few points to check:

-> Pump calibration is very important:
Use a precision balance (2 decimal places).
Calibrate with fresh water (except for the reagent pump, which can be calibrated with reagent, as its density is close to that of fresh water).
Pour the pump into a container placed on the balance (1g = 1ml if using fresh water).
Perform several calibrations in succession and check that the value is almost constant (choose the mean value). If the value varies greatly, it's time to replace the dosing pump head (it's a wearing part).
Please note that the reagent pump head may need to be replaced if worn, despite stable flow, as it may no longer function correctly at very low speeds during titration.
-> To determine the volume of the sample tube, fill the entire sample tube with fresh water and empty it onto the precision balance. This is more reliable than using the calculator.
-> Check pH probe calibration by dipping the pH probe in pH4 and checking that 4.0 is displayed in the diagnostics area at the bottom of the KHD page. Do the same at pH7 (standards must still be valid and uncontaminated).
-> The filter must be clean and free from dirt.
-> Use GHL FLEX PVC tubing.
-> Use the shortest tubes possible.
-> The sampling point must be located before and away from the KH addition.
-> Fittings must be tight, no air bubbles must pass or they will falsify the test.
-> The KHD must lie flat (horizontally and vertically).
-> The rear "VENT" outlet must be open to the air and must NEVER be able to suck in any liquid. Water must never come out of this outlet. Only air must circulate here.
-> Doser and KHD must be supplied with 24V (not 12V).
-> At the end of the test, the sample tube must be completely emptied, and the pump must stop when the tube has been emptied. Not before or after.
-> The reagent bottle must not allow evaporation and must not be contaminated or the measurement will be wrong.
-> The greater the sample volume, the greater the precision (at the expense of water and reagent consumption and test time).

See also these dedicated articles in our knowledge base:

https://www.aquariumcomputer.com/kno...oubleshooting/

https://www.aquariumcomputer.com/kno...e-kh-director/

Gaël