CN0359 Conductivity Measurement Problem

Hello.

We have been using CN0359 for conductivity measurement. We are trying different types of EC probes and PT100.

Our problem is in the same container when you dip the EC probe and PT100 together the EC value is increasing after 2 seconds. Around 0.3mS.

We have a refferance EC meter from WTW and we are comparing our probe with it.

Our probe is 4 electrode cell.

Ex Volt : 3V

Ex Freq : 1 kHz

Cell Cont : 0.4868

Set up time : %10

Hold time : %1

If we dont put the pt100 in the same container with our EC probe.

The EC measurement is good and very stable. ( Temp of the water and the air is same so temp effect is not the problem here)

But when we put the PT100(stainless steel cased) in to container The EC value is directly increasing around 0.3mS in 2 seconds. And when put back from water its turning to normal.

Do we need speacial pt100 for this measurement ?

In few pt100 We recognized that the metal body of the pt100 has a volta on it. Some of the chinese brand pt100 which are very often in the all markets.

They have like 1V or 2V on their metal case. Maybe this is effecting the measurement.

After that we tried to use good brand pt100 which is italian brand SPT85.

The problem is same.

Can anybody give me the reason of this problem ?

Regards...

Parents
  • Any idea people ?

    Our electrode material is 316SS 

    The EC value is increasing after 2 seconds.

    But when we reset the power its turn to same EC value and after 2 seconds increasing again.

    Didnt understand why

  • 0
    •  Analog Employees 
    on Jan 15, 2020 12:25 AM in reply to raseyns

    Hi Raseyns,

    What type of RTD are you using?  is it a 2/3/4-wire RTD?

    To rule out the effect of the RTD probe in the measurement, can you dip the RTD probe on the different container which contain same water solution and has same temp with the other container?

    another thing you could try in your current setup is to ground the container from pin 5 of connector J5 of CN0359. 

     

    Cheers,

    Erbe 

  • Hello Erbe thanks for reply. We tried both off them. As you said when we connect the pin 5 with container or water. We use glass container so we just use a wire with pin 5 and water. So pin 5 directly goes to water. So when we connect the pin 5 with the EC solution, EC value is directly changed. So we re calibrated it. But after  6 hours EC value is start to increase again. Its depend on the EC sensor quality that some of them start to increase after 10 minutes some of them start to increase after many hours.

    But we want to build stationary EC meter so it should work 6 months wihout calibration.

    What we think is maybe we need to reset the hardware every 10 seconds. or can we do this reset by software ?

    Maybe it will work better with reseting it. Because when we reset it after 3 or 4 reset the EC value turn to true EC value.

    For example we have a solution 4,64 mS  we have a reference EC meter from WTW and other brands. So we always check the results.

    After one night both of EC meters are in the solution. When we come to morning. Our EC meter shows 5,05 mS

    But WTW is showing 4,68 mS .

    When we reset the power of CN0359 like 3 or 4 times. the EC values turns to 4,68mS which is true.

    But after this reset it start to increase again to 5 mS in short time like around 1 minute.

    We didnt figure out the reason

  • 0
    •  Analog Employees 
    on Jan 19, 2020 11:41 PM in reply to raseyns

    Hi Raseyns,

    It seems that EC probe needs to be properly coupled from the input of the CN0359 which has an amplifier that has an input bias current that is enough to load the EC probe and might be the source of this drift.  Capacitive coupling might be the solution which I would like you to try out.  

    Note that you can use a breadboard to make the following connections shown in the attached circuit.

    Here is what was suggested that should be added to your setup for proper coupling: 

    A 10 µF tantalum capacitor in series between the conductivity cell excitation and J5P1
    A 1 µF polycarbonate capacitor in series between conductivity cell sense output and and J5P2
    A 1 µF polycarbonate capacitor in series between conductivity cell sense output and J5P3
    A 1 MΩ resistor to ground on J5P2s
    A 1 MΩ resistor to ground on J5P3

    For more details, you can refer from this link in EZ which issue is almost similar from this. 

    Let me know if that helps.

    Cheers,

    Erbe 

Reply
  • 0
    •  Analog Employees 
    on Jan 19, 2020 11:41 PM in reply to raseyns

    Hi Raseyns,

    It seems that EC probe needs to be properly coupled from the input of the CN0359 which has an amplifier that has an input bias current that is enough to load the EC probe and might be the source of this drift.  Capacitive coupling might be the solution which I would like you to try out.  

    Note that you can use a breadboard to make the following connections shown in the attached circuit.

    Here is what was suggested that should be added to your setup for proper coupling: 

    A 10 µF tantalum capacitor in series between the conductivity cell excitation and J5P1
    A 1 µF polycarbonate capacitor in series between conductivity cell sense output and and J5P2
    A 1 µF polycarbonate capacitor in series between conductivity cell sense output and J5P3
    A 1 MΩ resistor to ground on J5P2s
    A 1 MΩ resistor to ground on J5P3

    For more details, you can refer from this link in EZ which issue is almost similar from this. 

    Let me know if that helps.

    Cheers,

    Erbe 

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