Gas Sensor Calibration

This guide will walk you through the process of calibrating your gas sensors connected to a Modbus base unit.


Over time, electrochemical sensors can drift due to environmental changes and ageing, which can lead to inaccuracies in measuring gas concentration. This drift offsets the readings from the actual gas concentration. Calibration is the process of minimizing these measurement errors by adjusting the linear equation used to calculate the gas sensor concentration, ensuring accurate readings. Regular calibration, typically performed annually or biannually, is essential to maintain the accuracy and reliability of the sensor.

Calibration Methods

There are two methods used to calibrate gas sensors: peak calibration and average calibration.

Peak Calibration

Peak calibration is a simpler and quicker method of calibrating a gas sensor, requiring no controlled environment. It works by having the base unit measure the minimum and maximum gas concentrations over a specific period. The user must provide a reference maximum value. The minimum value is assumed to be zero, eliminating the need for a reference minimum value.
Here are the steps for conducting a peak calibration:
  1. Start Peak Calibration: To begin the calibration, write '1' (for peak calibration) to the corresponding Modbus register for your sensor. For example, if calibrating a NO2 sensor, you'd write to register 0x0200. This command initiates the period during which the base unit records the minimum and maximum gas concentrations.
  1. Measurement Period: The measurement period lasts from the time you initiate the start command until you issue the stop command. The duration of this period is decided by you, the user. During this period, ensure the sensor is exposed to both zero and higher-than-zero gas concentrations. This variety in exposure will ensure the sensor records a true minimum (zero) and a meaningful maximum value.
  1. Determine Reference Maximum Value: Like in average calibration, you need to determine the reference maximum value. This is the known concentration of the gas that corresponds to the maximum value recorded by the sensor during the measurement period.
  1. Stop Peak Calibration: To conclude the calibration, write the reference maximum value (in ppb or ppm, as appropriate) to the 'stop calibration' register corresponding to your sensor. If calibrating the NO2 sensor, for instance, you'd write the reference max value to register 0x0201.
Even though peak calibration is less comprehensive than average calibration, it is more convenient and quicker. It is suitable for scenarios where creating a controlled environment is not feasible.

Average Calibration

Average calibration is a comprehensive method typically used in controlled environments, such as during sensor production, to ensure the sensor accurately measures gas concentration over time. The calibration process determines an average minimum and maximum value, both corresponding to known concentrations of the gas.
Here are the detailed steps for conducting an average calibration:
  1. Preparation: Create a controlled environment where you can regulate the gas concentration. You will need to simulate two specific conditions – one with 0 ppb/ppm of the gas (minimum) and the other with a known higher concentration (maximum).
  1. Average Minimum Calibration: Set up your controlled environment to have 0 ppb/ppm of the gas. Begin the calibration on the base unit by writing '2' (for average calibration) to the appropriate Modbus register for your sensor. For example, if you're calibrating a NO2 sensor, you'd write to register 0x0200. Allow the base unit to measure the gas concentration over a defined period. The base unit will calculate and store the average minimum value, which should be close to zero in a well-controlled environment.
  1. Average Maximum Calibration: Adjust your environment to maintain a known concentration of the gas, higher than 0 ppb/ppm (e.g., 20 ppb/ppm). To initiate the maximum average calibration, send the 'start calibration' command (write '2') to the same Modbus register again. Allow the base unit to measure this gas concentration over the same period of time used for the minimum average calibration. Consistency in the duration of the minimum and maximum calibration periods is crucial. The base unit will calculate and store this average maximum value. Concurrently, monitor the sensor's readings. The highest reading recorded during this period is considered the 'maximum reference value,' providing essential information about the sensor's behavior at this specific concentration.
  1. Stop Calibration: Finally, to stop the calibration, write the reference maximum value (in ppb or ppm, as appropriate) to the 'stop calibration' register for your sensor. For instance, for a NO2 sensor, you'd write the reference max value to register 0x0201.
Through the average calibration method, the sensors are well-tuned to provide accurate readings both at minimum (zero) and at higher known concentrations. Additionally, by incorporating the sensor's response at the peak concentration, this method offers a more comprehensive calibration than the peak method.

Modbus Registers for Calibration

Here are the Modbus registers you'll use to start and stop calibration for each sensor:
Start Calibration Register (1: Peak, 2: Avg)
Stop Calibration Register (Expects max value in ppb or ppm)


By following these steps, you should be able to successfully calibrate your gas sensors and ensure they continue to provide accurate readings. It is crucial to use the correct Modbus registers for your specific sensor and method of calibration. Remember to perform the calibration process periodically to maintain the sensor's accuracy and reliability. Should you encounter any challenges or have any questions, do not hesitate to reach out for support. Your success in achieving accurate sensor readings is our top priority.