5.6 KiB
MHZCO2
Arduino Library for MHZ series CO2 sensors.
Description
The MHZCO2 is an experimental library for the MHZ19B CO2 sensor with a Serial (RS232-TTL) interface and compatibles.
The library offers a base class and derived classes to prepare for specific functionality. The base class is based upon the MHZ19B specification. This might change in the future as compatibles might differ on detail.
Reference: user manual MHZ129B 2019-04-25 version 1.4
Version 0.2.0
Version 0.2.0 fixes a bug in setPPM() which makes older versions obsolete.
Compatibles
This list is not verified although these devices should be compatible based upon datasheet.
There exists different models of 400-2000 PPM and 400-5000 PPM and 400-10000 PPM. As far as known these have the same interface as there is very little information to be found.
| type | precision | notes |
|---|---|---|
| MHZ1311A | 50ppm + 5% | energy saving version |
| MHZ19 | 50ppm + 5% | |
| MHZ19B | 50ppm + 3% | test device |
| MHZ19C | 50ppm + 5% | (1) |
| MHZ19D | 50ppm + 5% | |
| MHZ19E | 50ppm + 5% |
Note (1): There exists different models of the MHZ19C and probably others. The range can go from 400-2000 PPM, 400-5000 PPM and 400-10000 PPM. As far as known these have the same interface as there is very little information to be found. See #9.
Note: The calibration of the MHZ1311A is different than MHZ19x series
If there are compatible devices missing in this list, please let me know.
In previous versions the MTP40F was incorrectly mentioned as compatible.
Links
- https://emariete.com/en/sensor-co2-mh-z19b/
- https://emariete.com/en/sensor-co2-low-consumption-mh-z1311a-winsen/
- https://revspace.nl/MHZ19
- https://www.co2.earth/ - current outdoor CO2 level can be used for calibrating.
- https://keelingcurve.ucsd.edu/ - historical outdoor CO2 level.
- https://github.com/RobTillaart/MTP40C
- https://github.com/RobTillaart/MTP40F
- https://github.com/RobTillaart/Cozir
Connection
- check datasheet of your sensor.
Interface
#include "MHZCO2.h"
Constructor
- MHZCO2() base class constructor.
- MHZ19() constructor. Also 19B, C, D, E
- void begin(Stream * str) set the Serial port to use, e.g Serial1, or a softwareSerial port.
- uint32_t uptime() returns milliseconds since 'instantiation'.
Range
- void setPPM(uint16_t PPM) PPM = 2000, 5000, 10000.
- uint16_t getPPM() returns (cached) PPM value.
Measure
- int measure() workhorse, send command to read the sensor and waits until an answer is received. Return values see below.
- uint32_t lastMeasurement() timestamp in milliseconds of last measurement.
- int getCO2() returns CO2 PPM last measurement.
- int getTemperature() returns temperature last measurement.
- int getAccuracy() returns accuracy last measurement.
- int getMinCO2() returns minCO2 last measurement.
The latter two might not be supported by all MH sensors.
Return values of measure()
| value | Name | Description |
|---|---|---|
| 0 | MHZCO2_OK | measurement succeeded. |
| -10 | MHZCO2_TIMEOUT | to too long to receive an answer |
| -11 | MHZCO2_ERROR_CRC | Checksum error, handle answer with care. |
Calibration
WARNING: use with care, read the datasheet as these commands may disrupt your sensor.
- void calibrateZero() Only use when sensor is at least 30 minutes in a calibrated 400 PPM environment.
- void calibrateSpan(uint16_t span) Only use when sensor is at least 30 minutes in a calibrated 2000 PPM environment.
- void calibrateAuto(bool mode = true)
Note the outdoor calibration CO2 level differs per day and one should check a local airport or weather station for a good reference.
The University of San Diego keeps track of CO2 for a long time now. See - https://keelingcurve.ucsd.edu/
Future
Must
- improve documentation
- buy hardware MHZ19B / MHZ19C
- test with hardware
- verify timeout
Should
- check 3000 PPM
- fix SoftwareSerial - https://github.com/Arduino-CI/arduino_ci/issues/346
Could
- investigate configurable timeout. now hardcoded 1 second.
- 2 bytes + 2 functions.
- extend unit tests
- add type info for derived classes?
- A .. E ?
- save RAM? possible?
Won't
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