Free chlorine is produced by simple electrolysis of low-salt pool water (3g/l) using an electrolysis cell with platinum electrodes.

We set a Redox rate to reach (in my case 700mV) and we produce free chlorine (2 H 2 O liq ⟶ 2 H 2 gas + O2 gas ) by operating the electrolyser.

But it's not all that simple: the Redox probe is fragile, and to work properly it is almost essential to install it in a dedicated measuring chamber. The water flow rate through this measuring cell must be around 40 to 60 l/h.

In the past, I created a device to control this flow rate and incidentally trigger an alert when the flow rate is too low (clogged filter that needs cleaning): https://www.elektormagazine.fr/labs/esp32-swimming-pool-water-flow-control

And every time I have had a problem related to this free chlorine regulation, I have a doubt about the origin of the problem: is it the probe that measures badly and that must be regenerated or changed, is it the measuring device (in my case it is a Bayrol Salt Pro) that has a measurement fault and that must be recalibrated or restarted?

To remove this doubt, I created this Redox meter with the following specifications :

-          The measurement accuracy should be approximately 0.1mV and the display is between -2000 and +2000mV.

-          The display should be bright enough with large characters to see the measurement.

-          There must be a reliable calibration procedure.

-          The input impedance of the probe must be very high in order to be able to measure the Redox value directly on the probe and in parallel with the Bayrol without disturbing the measurement.

-          The whole thing should be able to be integrated into a small box powered by a USB C port. I didn't try to have a standalone device.

-          And finally, the measurements must be stored in the Cloud and easily accessible from a mobile phone or computer over a long period (3 months in my case with a measurement every 5 minutes).

-          The device described is designed to measure the Redox couple, but can be very easily transformed into a pH meter. There is only one switch to toggle on the pH/ORP card and you just have to adapt the display accordingly.

 

-                     I used a Heltec WiFi kit 32 v3 CPU which uses the ESP32 S3 processor widely described in the Elektor magazine and which integrates a good quality Oled screen. You can also connect a Lithium battery, the charger being integrated into the card .

-                     In order to preserve the life of the screen (because the device remains in operation throughout the summer season), I added a button to turn on the screen for 5 minutes .

-                     I used a pH/ORP 1130 measuring adapter : https://www.gotronic.fr/art-adapter-ph-orp-1130-12112.htm

-                     This module works very well and the residual background noise is very low, even though this adapter integrates a small 8-bit microcontroller to process its measurement.

-                     However, the power supply is necessarily 5V and I therefore took care with the decoupling of the power supply for this part .

-          It is recommended to galvanically isolate the measuring chain, this is done for the entire device via the power adapter which is itself galvanically isolated. This is the principle of any autonomous measuring device!

-          For A/D conversion I chose the ADS1114 converter which perfectly meets my needs and which has also been extensively described in Elektor magazine.

-          The case is the Hammond 1591CTCL model in transparent Polycarbonate which contains all the electronics.

-          Power is supplied via a USB Type-C socket located on one side of the device.

-          There is also a calibration button. You just need to enter the reference value (for my part I have a 470mV solution) in the PC or mobile interface. By pressing the cal button, the system measures the actual measured value compared to the standard value. The process lasts about 30s because the Redox measurement must be stable and within a range of +/-0.5 mV. A coefficient stored in EEPROM then allows the value to be corrected. On my prototype the correction value is 1.00 which means that the ORP measurement + adapter + Ana is almost perfect and does not require correction!

-          Calibration can also be launched from your mobile phone, and a status line indicates the steps in progress.

-          For the iPhone and PC interface, I used the Arduino IoT environment which is really well done.

-          The complete environment integrates Wi-Fi connection management, an asynchronous WebSocket server, cloud management via an MQTT server, and all graphical management on a mobile phone or PC.

-          No more Javascript or HTML, you can really focus on your project!

-          I have already described the use of this environment in the article dedicated to water leak detection (Water Leak): https://www.elektormagazine.fr/labs/esp32-water-leak-detector-connected-to-iot-arduino

I used Arduino IDE version 2.3.5 for compilation (see screenshots)

You must select Board : ESP32S3 dev Module with the following parameters:

-          Flash size: 16M

-          Partition Scheme: 16M Flash: (3MBAPP / 9MB FATFS)

-          A/ D Library: ADS1115_WE by Wolfgang Ewald

-          Board package: Arduino ESP32 Boards v2.0.18

Arduino_secrets.h tab you must enter, in addition to the SECRET_DEVICE_KEY, the network parameters (or hdcp in the SECRET_LOCAL_IP field), and the WiFi network credentials .

Just compile, upload the new firmware via USB port and try!

You will of course need to enter the parameters of the Dashboard you want to use.

It's really very simple!

Hardware :

See the attached schematic for more details.
The DS1114 ANA converter board and the ORP adapter are powered by 5V after filtering with a pair of 100nF and 47uF capacitors.
The ADS114's ADRR pin is connected to ground and defines the I2C bus address 0x48.
The ANA input A0 is decoupled by a 220nF capacitor. It is directly connected to the output of the ORP adapter, which provides an analog voltage that is converted to an ORP value using the formula: ORP(V°) = (2.5 - voltage)/1.037. The correction coefficient is applied to the converted ORP value (should be close to 1).
If you want to use this device as a pH meter, you need to flip the small switch on the adapter board to Ph. The conversion formula can be found on the attached datasheet of the 1130 module.
The two inputs (Screen and Calibration) are connected by a 10K ohm pull-up resistor.
Power is supplied by a small (good quality!) USB power supply via a USB C connector.

I have been using this device for many weeks, and little by little all the bugs seem to be fixed.

For the moment the measurements made by my device and the Bayrol are strictly identical and now I am impatiently waiting for summer to check the quality of the water in my swimming pool!