MTP40F

Arduino library for MTP40F CO2 sensor.

Description

The library for the MTP40F CO2 sensor is experimental as not all functionality is tested. It is based upon my https://github.com/RobTillaart/MTP40C library.

The MTP40F sensor is an NDIR (Non Dispersive InfraRed) CO2 sensor. It returns the CO2 level in PPM = parts per million, and in normal outside air it should be between 400 - 420.

The sensor communicates over a 9600 baud serial (TTL) interface with a microprocessor or PC. This implies that calls which can take up to 25 bytes can take as much as about 20 milliseconds.

On the other hand this low baud rate implies it will work over relative long distances. This signal quality over longer distances is not investigated.

The MTP40F has more interface options, I2C, PWM and ALARM. This library does not support these other interfaces for now. However minimal examples are added to have a starter but these need to be tested if and how well these work.

Warnings

During tests with an UNO the communication over Software Serial did fail a few times. Therefore it is important to always check return values to make your project more robust.

During tests it became clear that the sensor needs time to process commands e.g. setSelfCalibration().

CRC errors

Since 0.2.0 the CRC of the sensor responses are verified by the library. This will make the communication more robust by identifying the CRC errors.

The low level request() checks the CRC of the communication with the device. A new error-code MTP40F_INVALID_CRC is defined, however this error code need to be integrated better in the library.

Hardware interface MTP40-F

Has TTL level RS232, I2C and PWM IO.

                       TOPVIEW MTP40-F
                       +-------------+
                       |             | 
    VCC (3v3 out)  5 --|             |-- 1 Vin
    TX / SDA       6 --|             |-- 2 GND
    RX / SCL       7 --|             |-- 3 ALARM
    R/T            8 --|             |-- 4 PWM / I2C
    GND            9 --|             |
                       |             |
                       +-------------+

Links

• 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/MHZCO2
• https://github.com/RobTillaart/Cozir
• https://github.com/RobTillaart/Pressure - converters

Multi device

Sometimes you need to control more devices than possible. This is possible with a multiplexer e.g. HC4052 which can select between four devices.

Drawback of using a multiplexer is that it takes more administration in your code e.g. which device is on which channel. Furthermore using a multiplexer will slow down the access.

• https://github.com/RobTillaart/HC4052 (4 x 2)
• https://github.com/RobTillaart/ADG725 (16 x 2)

Interface

#include "MTP40F.h"

Constructor

• MTP40F(Stream * str) constructor. Should get a Serial port as parameter e.g. \&Serial, \&Serial1 or a software Serial port. That Serial port must connect to the sensor, check datasheet for pins.
• bool begin() initialize the internal settings for the device.
• uint8_t getType() returns type, see below. Return 255 for the MTP40 base class.

CO2 Measurement

• uint16_t getGasConcentration() returns the CO2 concentration in PPM. The function returns MTP40_INVALID_GAS_LEVEL if the request fails.
• void suppressError(bool se) sets or clears a flag that replaces the error value with the last read value if the request fails. This is useful when plotting the values and one do not want a sudden spike. One can still check lastError() to see if the value was OK.
• bool getSuppressError() gets the value of the suppress flag.
• int lastError() returns last error set by getGasConcentration() or by getAirPressureReference() Reading resets internal error to MTP40F_OK;

Timeout communication

The library can set a maximum timeout in the communication with the sensor. Normally this is not needed to set as the default of 100 milliseconds is long enough for even the longest command. This timeout is needed if the sensor did not process the command correctly, preventing the host to wait indefinitely.

• void setTimeout(uint32_t timeOut = 100) sets the timeout. If no parameter is given a default timeout of 100 milliseconds is set.
• uint32_t getTimeout() get the value set above or the default. Value returned is time in milliseconds.

Calibration

Please read datasheet before using these functions to understand the process of calibration.

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/

Air pressure calibration

• float getAirPressureReference() returns the air pressure reference from the device. Returns MTP40F_INVALID_AIR_PRESSURE in case request fails. Default is 1013 (note no decimals) == 1 ATM.
• bool setAirPressureReference(float apr = 1013) to calibrate the air pressure. Default value = 1013 hPa (1 Atm). One can calibrate the sensor with an external device. Value for air pressure should normally between 700 and 1100. The function returns false if the parameter is out of range or if the request fails.

For pressure conversion - https://github.com/RobTillaart/pressure

SinglePointCorrection calibration

It takes a relative short time (few minutes) to calibrate the sensor in a known gas concentration.

• bool setSinglePointCorrection(uint32_t spc) takes several minutes. See datasheet. The parameter spc should be between 400 and 2000 (assumption). The function returns false if the parameter is out of range or if the request fails.
• bool getSinglePointCorrectionReady() To see if setting the SPC has finished or not. The call also fails if the request fails.

As far as known the SPC value can not be retrieved from the sensor.

Self calibration

Self calibration is a process in which the sensor takes the minimum values over a longer period between 24 - 720 hours as the reference for minimum outdoor values. Note that 720 hours is 30 days / 1 month. Default seems to be 168 hours == 1 week.

• bool openSelfCalibration() start the self calibration cycle.
• bool closeSelfCalibration() stop the self calibration cycle.
• uint8_t getSelfCalibrationStatus() Returns if the selfCalibration is open or closed. The function returned 0x00 for CLOSED and 0xFF for OPEN.
• bool setSelfCalibrationHours(uint16_t hours = 168) Sets the number of hours between self calibration moments. Valid values for hours are 24 - 720 (1 day up to 1 month). Default value = 168 hours = 1 week.
• uint16_t getSelfCalibrationHours() returns the value set above.

Note: read datasheet!

Error codes

Future

Must

• update documentation
  • reorder

Should

• setSinglePointCorrection(spc) investigate parameter. datasheet states 0x2000 but that is 8192 which is rather strange. Assumption 2000 decimal is meant.
• improve CRC error code
• ERROR handling.
  • some functions returning bool should return int to handle errors better MTP40F_OK or ERROR flag.
  • would break the interface

Could

• getAirPressureReference() could be smarter
  • always same value?
  • from cache
  • dirty flag (-1);
• reuse command buffer as return buffer?
  • saves a bit.
• move all code from .h to .cpp file

Wont

• store SPC point in the class?

Support

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Thank you,