CN0359 conductivity strange problem of reading values

Hello...

We are using CN0359 for measuring conductivity. We have 2 calibration liquid . ( 1.4 mS solution and 12.9 mS solution )

We use 4 graphite electrode sensor from another company. The sensor works well with its control unit.

When we plug the sensor to CN0359 we calibrate the K value witch is 0.475

Our Ecx Voltage : 2 V

Freq : 100 Hz

Hold time : 10

Set time : 10

The strange problem is when we put the sensor in to solution and power the CN0359 the reading value is successful.

For example when we use 12.9 mS solution the reading is also around 12.9 with good accuracy.

Our problem is when we put out the sensor from the solution The EC is getting close to nS which is good also.

But when we put back it in to solution we are not getting 12.9 mS again from the reading.

When we put the sensor inside the solution and power off and on again the CN0359 the Ec come to 12.9 mS againg with no problem.

We observed the Gain Values from the ADC inputs off the EC inputs.

When we put back the sensor in to solution Vgain become to normal value but I gain is not turning to same value.

I gain stick in to area that there is no solution.

When we want to make good measure we need to put to sensor in to solution before we power on the CN0359.

Do you have a solution for us to fix this problem. 

Regards...