PH bug?

I bought a brandnew electrode a month ago, did a recalibration, measured my KH with drop test.

PH 6.7
KH 4

The problem:
The CO2 drop checker i bought (also new) indicates i have < 15ppm co2 which is impossible based on the above readings.

So i did a retest with:
- different co2 checker fluid
- different KH test
- recalibrated electrode

I retested all things 4 times.

Same results. Can you please tell me if there might be a problem with the PH calculation formula you use? Any other idea's?

I had to lower my PH to 6.5 / KH 4 to get 25ppm green color, which according to the co2 table should indicate i have a CO2 of 40(!)

What did the pH-sensor show when you put it into the calibration fluids after calbration? Have the pH-readings been correct?

we had this discussion - drop test against industrial pH-probe - several times

-> if the pH-electrode shows the correct values in the calibration fluids then the probe and the ProfiLux are working 100% perfect and the drop test produces the wrong results
there is no dedicated pH calculation formula - this is a simple linear 2 point calibration which has been working perfectly for about 10 years now