KHD measuring 6.1 - Hanna measuring 8.4

[TCoach]

So, I was able to setup my new to me KHD yesterday. It came with no hoses, so I had to use my own out of dosing lines I already had on hand. The tank supply, waste, and emergency waste are all 3mm lines. The reagent line is 4mm. Once I had every thing plumbed up, I ran some tests. It is coming in at 6.1DKH vs 8.4 on my checker.

One caveat is the pH probe had been left out of moisture for a few months. However, I soaked it in tap water for +24 hours and it calibrated without any issues.

Any suggestions on what to change to get it reading closer to the Hanna checker?

Thanks,
-Chris

 

[Gaël]

Hello,

Is the hose for the reagent made of PVC or silicone? Don't use silicone, normally use the PVC-FLEX supplied with the unit.
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:

KH Director tips and troubleshooting

www.aquariumcomputer.com

Achieving Alkalinity Stability with the KH Director

www.aquariumcomputer.com

 

[Lasse]

IME - when using dosing pumps like GHL:s - it is very important to use relative hard pvc tubes on the suction side because otherwise (if using soft silicon tubes) it risk to implode the tube and alter the amount of pumped liquid.

Sincerely Lasse