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Sensirion Gas Index Algorithm Arduino Library

Sensirion's Gas Index Algorithm software provides a VOC and an NOx Index output signal calculated from the SGP40/41 raw signal inputs SRAW_VOC and SRAW_NOX. Note: for SGP40, only SRAW_VOC is available. This algorithm enables robust detection of ambient changes of VOCs and NOx with minimal sensor-to-sensor variation. The algorithm is based on a statistical gain-offset normalization and adapts both parameters constantly applying an exponentially decaying function of the learned parameters to be able to adapt to changing environments.

For the VOC Index output, the software must be instanced as VOC Algorithm while for the NOx Index output, the software must be instanced as NOx Algorithm. It is important to feed the raw signals to the corresponding algorithm (i.e., SRAW_VOC to the VOC Algorithm and SRAW_NOX to the NOx Algorithm) at a constant sampling interval which must coincide with the sampling interval that is used to read out the raw signals from the SGP40/41 sensor. The default sampling interval applied in the algorithm is 1 s. If your sampling interval is different, please use the constructor variant allowing you to pass the sampling interval. The VOC algorithm is tested for a sampling interval of 1s and 10s.

The algorithm calculates the VOC and NOx Index signals recursively from a single raw tick value of SRAW_VOC and SRAW_NOX, respectively, which are both measured by the SGP40/41 sensor at each time step, as well as internal states that are updated at each time step. These internal states are most importantly the recursively estimated mean and variance of the corresponding SRAW signal as well as some additional internal states such as uptime and other counters. After estimating the states, the algorithm converts the raw signals in ticks into either VOC or NOx Index, respectively, and applies an adaptive low-pass filter.

Installation

To install, download the latest release as .zip file and add it to your Arduino IDE via

Sketch => Include Library => Add .ZIP Library...

Dependencies

The dependency to the Sensirion I2C drivers are only needed to run the example, which uses a SGP40 resp SGP41 and a SHT4x sensor.

Quick Start to run the example

The example measures VOC and NOx ticks with a SGP41 sensor using a SHT4x to compensate temperature and humidity. The raw VOC and NOx measurement signals are then processed with the gas index algorithm to get VOC Index and NOx Index values.

For more details about the sensors and breakout boards check out http://sensirion.com/my-sgp-ek/.

Install the driver dependencies listed above the same way as you installed this library (via Add .ZIP Library)
Connect a SGP41 and SHT4x Sensor over I2C to your Arduino

Open the exampleUsage sample project within the Arduino IDE

 File => Examples => Sensirion Gas Index Algorithm => exampleUsage

Click the Upload button in the Arduino IDE or
```
 Sketch => Upload
```
When the upload process has finished, open the Serial Monitor or Serial Plotter via the Tools menu to observe the measurement values and calculated Gas Index value. Note that the Baud Rate in the corresponding window has to be set to 115200 baud.

Low power example (SGP40)

The provided low power example demonstrate how to run the SGP40 sensor in low power mode and apply the VOC index algorithm to the acquired data.

Reduced power consumption is achieved by turning off the heater after each measurement. The heater is then turned back on by calling for a first ignored measurement that preceeds the actual measurement call by 170ms.

The following two low power modes have been tested:

Duty cycle	Sampling interval	Average power consumption at 1.8V
Continuous	1 s	6.3mW
20%	1 s	<2.0mW
2%	10 s	<0.2mW

Contributing

Contributions are welcome!

We develop and test this algorithm using our company internal tools (version control, continuous integration, code review etc.) and automatically synchronize the master branch with GitHub. But this doesn't mean that we don't respond to issues or don't accept pull requests on GitHub. In fact, you're very welcome to open issues or create pull requests :)

This Sensirion library uses clang-format to standardize the formatting of all our .cpp and .h files. Make sure your contributions are formatted accordingly:

The -i flag will apply the format changes to the files listed.

clang-format -i src/*.cpp src/*.h

Note that differences from this formatting will result in a failed build until they are fixed.

License

See LICENSE.