Measurements fluctuating

Hi,
I have had a KHD since the last February and since then, I have not really succeeded to obtain stable measurements from one day to the other.
Here is the setup and what I did:
Firstly, I read carefully the installation instruction, used the inline filter, all tubes are under 35cm, the vent tube has the head out of the water, making sure the sample tube was not in a bubble area of the sump, etc...
I also used the reference solution to validate my setup and obtained 7.6-7.5dkh after 5 consecutive measurements as advised, so I guess my setup is ok right?
I also tried to move the sample tube to the next sump, away from the Alkalinity doser, and still got big fluctuations from one day to the other.
The KHD is doing a single measurement every day at exactly the same time so it should always be at the same point in the Alkalinity consumption cycle from the reef... I guess...
So far, I could never really rely on its measurements to set the Alkalinity dosing, I ratter check with two other tests (hanna and redsea) which are not giving exactly the same results but the results are
really stable from day to day and it is what I'm expecting as my system is over 500g and 7 years old with a lot of SPS and few LPS and all are healthy and colorful.
My readings from other tests fluctuate by about 0.2dkh every 2 weeks and I slightly modify the alkalinity daily dose at this time.
Here is an example of the KHD dkh readings sequence at the same time every day for 21 days:
9.5
11.9
9.4
11.5
11.5
11.3
11.3
11.7
10.1
10.0
10.1
10.2
9.8
9.4
9.4
9.2
9.2
10.2
11.0
10.1
10.6
11.1
...
During all this time, the alkalinity dosing amount/day was the same and my other test kits were reading stable results.
Any good advice is welcome!
Please don't hesitate
Best regards

Hi there,

What KH results do you get if you run 3 tests back-to-back?

Since the probe has been used since Feb 2022, that would make the probe about 6 months old. How many times has the probe been calibrated in that time?

Have you checked your water sample filter? Maybe it is getting clogged and is affecting the actual sample amounts being used for each test.

For 3 tests back to back, I obtained 10, 10, 10

The probe was calibrated twice during the first week, and since then, never. I Understand the calibration could offset all the measurements but not affect random results, from my understanding...

The Sample filter is white and clean just like when it was brand new.

Is it possible that the KHD is not adapted to perform only a single measurement per day? I mean could the remaining liquid in the chamber and the probe react somewhat for too long during 24 hours and affect the next reading sometime?
I tried going from 1 measurement/day to 4 measurements/day in the app settings to see if it could yield more steady results, so far it only been 3 days so I can't conclude but so far it is steady at around 9.7 which is good but sometime measurement are steady for 3 days but it still gets crazy after so I won't conclude too fast.

Since the results are consistent with itself, that means the KHD is working normally and providing results within factory tolerances. If you have 7.5dkH Reference solution, you can also test with that and fine tune the results using the adjustment tool in the KHD settings page.

The widely fluctuating results suggest that there may be an event or scheduled dose happening around the time of a scheduled KH test. If you'd like to test this, you take a cup of tank water and test out of that cup. As long as there are no air bubbles in the sample or reagent tube, you will likely see more consistent results.

Here are the results I had when testing in a 5 gallons sample of water for 9 days at a rate of 4 tests/day:
9.8
9.6
9.5
9.5
9.6
9.4
9.4
9.5
9.5
9.8
10.0
10.0
9.9
9.8
10.1
10.0
9.9
9.9
10.0
10.1
10.0
10.1
10.1
9.8
9.8
9.8
9.7
9.6
9.4
9.5
9.6
9.6
9.7
10.0
10.8
10.8
10.7
It was the same sample test from the first day to the last day, the tube in the sample was always well under the water level, no bubble in the reagent tube, etc...
This 10 to 10.8 is typically the kind of big margin I get once every two weeks or so but now it was from the same water sample.
Even before this big jump, it was going back and forth from 9.4 to 10.1, still from the same water sample. Is it expected behavior from the KHD?

Hi,

Is it new or second hand doser?
Can the heads of your dosing pumps be worn out? This is because it is possible that the precision of the dosages are affected and therefore the precision of the KH measurement.

Gaël

No, it was purchased with the KHD and the Profilux 4 in February. It's 2.1 slave doser.

Have you opened a support ticket?
If not, please do so. A support member can then provide one-on-one assistance.

I had this happen too..
But after repacing dosingpump heads, as in the maintenance set, the KHD is testing with its normal accuracy again..

I have this same issue. Were you able to figure out the cause of the inconsistent readings? Mine vary by as much as 3dKH between successive readings. What is the expected range of deviation between measurements of the same sample? I just did 3 back to back measurements, and got results of 6.6, 8.5, and 7.3.

Another thing I'm finding is that if you start a measurement manually, the results are not stored in the history. Is it supposed to work that way?

First thing I would check is rollers and tubes (dirty, broken, etc) next I would check the calibration of the pumps using a scale also sample water with a scale; next I would check with reference solution 3 tests back to back to back. All should be within range of 7.5 +-4 ( it think). As long as you get 3 tests the same you should of solved issue. At that point you can adjust the difference with +-. Hope this helps. You should also check if calibration is accurate do a dosing of 20ml of each pump using a scale it should read 20ml if not double check hoses, etc.