About: Founded in 2005, Atlas Scientific specializes in building laboratory grade sensing equipment for robots, appliances, and industrial control systems. Thousands of Atlas Scientific sensors can be found in a wi...

In this tutorial, we will be calibrating Atlas Scientific's EZO ORP (oxidation-reduction potential) sensor using Arduino Uno.


The most important part of calibration is watching the readings during the calibration process. It is easiest to calibrate the device in its default state (UART mode, with continuous readings enabled). Switching the device to I2C mode after calibration will not affect the stored calibration. If the device must be calibrated in I2C mode, be sure to continuously request readings so you can see the output from the probe. In this tutorial, the calibration will be done in UART mode.

The Atlas EZO ORP circuit has a flexible calibration protocol, allowing single point calibration to any off the shelf calibration solution. However, if this is the first time calibrating the circuit, Atlas Scientific recommends using the 225mV calibration solution.


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The kit includes 1 EZO ORP circuit, 1 ORP probe, 1 female BNC connector, 1 4oz 225mV calibration solution, 1 4oz ORP storage solution, 1 optional inline voltage isolator.

Ensure that the ORP circuit is in UART mode. For instructions on switching between protocols, refer to the following LINK.

Use the breadboard to mount the circuit and BNC connector. Wire the ORP circuit to the Arduino Uno as shown in the schematic above and connect the probe to the BNC connector.


a) Download the sample code from this LINK. It will be in a folder titled "arduino_UNO_ORP_sample_code".

b) Connect the Arduino to your computer.

c) Open the code downloaded from step a, in your Arduino IDE. If you do not have the IDE you can download it from HERE.

d) Compile and upload the code to Arduino.

c) Open the serial monitor. For access go to Tools -> Serial Monitor or press Ctrl+Shift+M on your keyboard. Set the baud rate to 9600 and select "Carriage return". You should now be able to communicate with the ORP circuit. As a test, enter the command i which will return the device information.


a) Remove the soaker bottle and rinse off the pH probe.

b) Insert the ORP probe directly into the 225mV calibration solution bottle. Let the probe sit in the solution until the readings stabilize (small movement from one reading to the next is normal).

c) Once the readings have stabilized issue the calibration command cal,n where n is the value of the calibration solution. In this case, it is cal,225

Note: Calibration should be done at least once per year. If the ORP that's being read is continuously on the extremes of the scale ( ~ -900mV or ~ +900mV) calibration may have to be done more often. The exact frequency of calibration will have to be determined by your engineering team.

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