Fail to read pH value by using pH probe (pH electrode probe E-201-C with its module)

Question

I set up the pH sensor using an ESP32. I connected VCC to 3.3V, GND to GND and PO to PIN39 (ADC1_CH3) and extended my pH probe using four meters of alarm cable.

When I do not connect the probe with the module I get a good analog reading, with the voltage measured by using an ammeter (ignoring noise and linearity of the ESP32.)

When I plug the probe in to the module, then the analog reading will jump to either 0 or 4095.

What is the problem? Is the probe broken or is the module broken or are both broken?

This is the module I used:

My program:

    /*!
     * @file DFRobot_PH_Test.h
     * @brief This is the sample code for Gravity: Analog pH Sensor / Meter Kit V2, SKU:SEN0161-V2.
     * @n In order to guarantee precision, a temperature sensor such as DS18B20 is needed, to execute automatic temperature compensation.
     * @n You can send commands in the serial monitor to execute the calibration.
     * @n Serial Commands:
     * @n    enterph -> enter the calibration mode
     * @n    calph   -> calibrate with the standard buffer solution, two buffer solutions(4.0 and 7.0) will be automaticlly recognized
     * @n    exitph  -> save the calibrated parameters and exit from calibration mode
     *
     * @copyright   Copyright (c) 2010 DFRobot Co.Ltd (http://www.dfrobot.com)
     * @license     The MIT License (MIT)
     * @author [Jiawei Zhang](jiawei.zhang@dfrobot.com)
     * @version  V1.0
     * @date  2018-11-06
     * @url https://github.com/DFRobot/DFRobot_PH
     */
    #include "DFRobot_PH.h"
    #include <EEPROM.h>

    #define PH_PIN 39
    float voltage;
    float phValue,temperature = 25;
    DFRobot_PH ph;

    void setup()
    {
        Serial.begin(115200);  
        ph.begin();
    }

    void loop() 
    {
        static unsigned long timepoint = millis();
        if(millis()-timepoint>1000U){                  //time interval: 1s
            timepoint = millis();
            //temperature = readTemperature();         // read your temperature sensor to execute temperature compensation
            voltage = analogRead(PH_PIN)/4096.0*3300;  // read the voltage
            phValue = ph.readPH(voltage,temperature);  // convert voltage to pH with temperature compensation
            Serial.print("Voltage: ");
            Serial.print(voltage);
            Serial.print("   39: ");
            Serial.print(analogRead(39));
            Serial.print("   temperature:");
            Serial.print(temperature,1);
            Serial.print("^C  pH:");
            Serial.println(phValue,2);
        }
        ph.calibration(voltage,temperature);           // calibration process by Serail CMD
    }

    float readTemperature()
    {
      //add your code here to get the temperature from your temperature sensor
    }

Without probe:

Ammeter reading 1.89V without probe.

Ammeter reading 1.45V with probe.

Answer 1

Given that the signal goes to zero volts when you connect the probe on its extended cable, I'd say you probably did something wrong while attaching the extended cable. You most likely shorted the two wires together. It is also possible that you otherwise broke the sensor while working on the cable.

The module is an amplifier. The sensor is either a current source or a voltage source. If the output of the module goes to zero, then either the sensor is dead or you've sorted its cable when you extended it.

As far as my understanding of such sensors goes, they are fairly sensitive to mishandling. I wouldn't try to use an ohmmeter on it for fear of damaging the electrodes. Inspect your work on the extende cable - look for anything that could cause a short circuit. Try removing the extension - restore the sensor cable to its original condition as far as possible.

---

In general, it is best to try things out as received before making modifications.

Try it out, make sure it works as sold. If not, send it back and use another one.

If it works, you can do you modifications. If it then quits working you can be pretty sure that you broke it with your modification.