8.3 KiB
SGP30
Arduino library for SGP30 environment sensor
Warning: experimental, library is not functional complete yet.
Description
The SGP30 from Sensirion is an environment sensor that measures H2 and Ethanol in the air. From these numbers an intern algorithm in the sensor derives an CO2 equivalent and a TVOC measurement. The library has an experimental conversion for H2 and Ethanol.
The CO2 units are ppm, the TVOC units are ppb. Units for H2 and Ethanol are ppm. Note that for larger concentrations the resolution of the measurements drops, see datasheet.
The library supports 2 types of interfaces, a synchronous and an asynchronous interface. The sync interface is blocking for up to 40 milliseconds which was enough to trigger the implementation of an async interface.
Note: the sync interface is implemented with the async interface.
Note: versions prior to 0.2.0 are obsolete due to a bug in setBaseline().
Sample frequency
The CO2 and TVOC values can be read up to once per second (1 Hz). Ethanol and H2, the raw data can be sampled up to 40 Hz.
The first 15 seconds the sensor needs to stabilize. Thereafter one gets real data.
I2C performance
The SGP30 works with I2C bus at 100 KHz and 400 KHz. In a short test it worked well up to 500 KHz. A faster I2C clock does not give the sync interface much (relative) gain, however for the async interface the relative gain is much more.
(indicative test run with UNO - IDE 1.8.13, CO2 and TVOC only, times in micros)
| I2C speed | measure() | request() | read() |
|---|---|---|---|
| 100 kHz | 12360 | 336 | 732 |
| 200 kHz | 12212 | 196 | 408 |
| 300 kHz | 12168 | 144 | 300 |
| 400 kHz | 12140 | 132 | 264 |
| 500 kHz | 12128 | 124 | 236 |
Note the blocking of measure() takes 11 to 12 milliseconds extra.
Multiple sensors.
The SGP30 sensor has a fixed I2C address 0x58 so only one sensor per I2C bus can be used. If one needs more, one should use an I2C multiplexer or an MCU with multiple I2C buses or switch the VCC as a sort of ChipSelect signal.
- https://github.com/RobTillaart/TCA9548 (I2C 8 channel multiplexer)
Interface
#include "SGP30.h"
Constructor
- SGP30(TwoWire *wire = &Wire) Constructor with optional the Wire interface as parameter.
- bool begin() starts the I2C bus and returns true if the device address 0x58 is visible on the I2C bus.
- bool begin(uint8_t sda, uint8_t scl) idem, for the ESP32 where one can choose the I2C pins.
- bool isConnected() checks if the address 0x58 is visible on the I2C bus.
- void GenericReset() WARNING resets all I2C devices on the bus that support this call!
Meta
- bool getID() reads the sensor ID into 12 bytes. (needs rework).
- uint16_t getFeatureSet() returns 0x0022, indicates that commands used in this library are supported.
- bool measureTest() verify the chip is working.
Synchronous measurements
- uint32_t lastMeasurement() timestamp in milliseconds of the last sync measurement made. This convenience function is useful to prevent reading the sensor too often.
- bool measure(bool all = false) if all == false, only the TVOC and CO2 are updated (slow due to blocking), if all == true, also the H2 and the Ethanol values are updated (even slower). Note the measurement is slow as there is an active blocking until the sensor is done. If the last measurement is less than a second ago, no measurement is made and the function returns false.
A-synchronous measurements
With the async interface, the user should control that reads are at least one second apart. The user should also take care not to mix up different requests. See examples.
- void request() sends a request to the sensor to read CO2 and TVOC.
- bool read() returns true if the last request is more than 12 milliseconds ago the CO2 and TVOC are read and updated. Otherwise false is returned.
- void requestRaw() sends a request to the sensor to read H2 and Ethanol.
- bool readRaw() returns true if the last request is more than 25 milliseconds ago the H2 and Ethanol are read and updated. Otherwise false is returned.
Get the data
The library caches the last read values, and these are the functions to access them.
- uint16_t getTVOC() gets the TVOC concentration (ppb)
- uint16_t getCO2() gets the CO2 equivalent concentration (ppm)
- uint16_t getH2_raw() gets the raw H2. Units unknown.
- uint16_t getEthanol_raw() gets the raw Ethanol. Units unknown.
Calibration
Check the datasheet for operating range, figure 7.
- float setRelHumidity(float T, float RH) sets the compensation for temperature (5-55°C) and relative humidity (10-95%). These values can be obtained e.g. from an SHT30, DHT22 or similar sensor. The function returns the absolute humidity.
- void setAbsHumidity(float absoluteHumidity) sets the compensation for absolute humidity. Concentration is in gram per cubic meter (g/m3)
Baseline functions
The baseline functions give the sensor a reference value. After running in a known condition e.g. outside in open air, one can get the baseline values as a sort of calibration. Please read the datasheet carefully before using these functions.
Note: if the sensor has no reads done, these values tend to go to zero. This is because the baselines are based upon recent reads.
- bool getBaseline(uint16_t *CO2, uint16_t *TVOC) retrieves the baseline values from the sensor.
- void setBaseline(uint16_t CO2, uint16_t TVOC) sets the baseline values.
For faster accurate results for the TVOC under bad air conditions, read Inceptive Baseline for TVOC measurements (not tested)
- bool getTVOCBaseline(uint16_t *TVOC) retrieves the TVOC start value from the sensor.
- void setTVOCBaseline(uint16_t TVOC) sets the TVOC start value.
Miscellaneous
- int lastError() returns last error. (needs rework)
Experimental H2 Ethanol
use at own risk.
Since 0.1.2 the library has experimental support for H2 and Ethanol concentration in ppm.
One should use these functions more as a relative indication than as an absolute measurement as it is definitely not calibrated. Runs with different temperature and humidity (different days) give very different values including overflow and infinity.
- float getH2() gets the H2 concentration. Units ppm.
- float getEthanol() gets the Ethanol concentration. Units ppm.
The used references are based upon
(1) averaging raw data in outside air at 22°C @ 1 meter and
(2) the assumption that this is 0.4 resp 0.5 ppm.
(Note only 1 significant digit) as mentioned in datasheet P2.
- void setSrefH2(uint16_t s = 13119) // 13119 is my measurement.
- uint16_t getSrefH2() returns value set.
- void setSrefEthanol(uint16_t s = 18472) // 18472 is my measurement.
- uint16_t getSrefEthanol() returns value set.
Operational
See examples
Links
https://www.adafruit.com/product/3709 - the sensor.
Future
Must
- improve documentation
Should
- test
- different boards
- different gasses / afmosphere if possible.
Could
- redo getID()
- make defines for the magic numbers (commands)
- move code from .h to .cpp
- improve/merge the private command() function
- add/extend error handling
- better name for measureTest()
The CRC checking + error handling (since 0.1.4) adds around 330 bytes PROGMEM on an UNO. There might be a need for a minimal class that only reads CO2 and TVOC, no baselines etc. for the smallest platforms.