7.4 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 0.2.1
Minor breaking change as Measure() will only update lastMeasurement in case the measurement was successful. See #11.
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.
Related
- 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. begin() also resets all internal variables.
- uint32_t uptime() returns milliseconds since 'instantiation'.
Range
To set the max range of the actual sensor.
- void setPPM(uint16_t PPM) PPM = 2000, 5000, 10000. The value set is persistent over reboots (see issue #9).
- uint16_t getPPM() returns the cached PPM value. Note: initially this function returns zero / 0 as the cache is not filled by setPPM() yet.
Measure
- int measure() workhorse, send command to read the sensor and waits until an answer is received. Return values see below. Will only update lastMeasurement if the measurement is successful (0.2.1)
- uint32_t lastMeasurement() timestamp in milliseconds of last measurement.
- int getCO2() returns CO2 PPM last measurement.
- int getTemperature() returns temperature last measurement. Note that the temperature can be negative.
- int getAccuracy() returns accuracy last measurement. Note: check datasheet.
- 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 | took too long to receive an answer. |
| -11 | MHZCO2_ERROR_CRC | Checksum error, handle answer with care. |
CRC error
In case of an checksum error the values received may be corrupted. Best strategy is to ignore the measurement. However this is not always possible so a strategy might be to compare the values of the measurement with the previous ones. Often one can determine the failing ones but definitely not always.
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/
Timeout
These functions are used to set the default timeout in the receive function. The faster the baud rate used, the smaller this value can be. As the receive() call has to read 9 bytes at (default) 9600 baud. This is max 1 character per millisecond. To receive 9 bytes one needs therefore at least 10 to 15 milliseconds under ideal circumstances. So setting the timeout lower makes little sense.
If the timeout is set to zero / 0 there will be no time out checking.
Use with care!
- void setTimeOut(uint16_t timeout = 1000) set the stream reading timeout, defaults to 1000 milliseconds.
- uint16_t getTimeOut() returns the set stream reading timeout.
Future
Must
- improve documentation
- buy hardware MHZ19B / MHZ19C
- test with hardware
- verify timeout
Should
- check 5000 and 10000 PPM and possible others?
- is there an effect on the PWM output.
- should the PPM be a parameter of the constructor?
- default to 2000? or model based?
Could
- add type info for derived classes?
- A .. E ?
- save RAM? possible?
- all arrays start with 0xFF, 0x01
- reduce data types?
- uint16_t send() could return bytes send?
- would allow higher level functions to check.
- add int maxCO2() by keeping track myself?
- extend unit tests if needed.
- fix SoftwareSerial - https://github.com/Arduino-CI/arduino_ci/issues/346
Won't
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