O2-CO2 Gas Analyser

A precision gas analysis system built on the SAMD21 microcontroller platform, featuring simultaneous O2 and CO2 measurement with temperature-controlled sampling chamber and Modbus RTU communication.

Features

• O2 Measurement: SGX 4OX electrochemical oxygen sensor with temperature compensation
• CO2 Measurement: SGX INIR2-CD100 infrared CO2 sensor (0-10% range)
• Temperature Control: PID-controlled heater maintaining stable measurement conditions
• Humidity Sensing: Sensirion SHT4x for gas temperature and relative humidity
• Communication: Modbus RTU over RS-485 with configurable baud rate and addressing
• Status Monitoring: Comprehensive status register for system health monitoring
• Calibration: In-field zero and span calibration for both O2 and CO2 sensors

Hardware

• MCU: SAMD21-based custom board (gas_analyser_m0)
• O2 Sensor: SGX 4OX (galvanic cell, 70-130 uA in air)
• CO2 Sensor: SGX INIR2-CD100 (NDIR, UART interface)
• Temperature/Humidity: Sensirion SHT40
• Heater: PWM-controlled with NTC thermistor feedback
• Interface: RS-485 with optional 120 Ohm termination

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Modbus RTU Interface

Default Communication Settings

Parameter	Default Value
Slave ID	100
Baud Rate	9600
Data Bits	8
Parity	None
Stop Bits	1

Supported Function Codes

Function Code	Description
03	Read Holding Registers
04	Read Input Registers
06	Write Single Holding Register
16	Write Multiple Holding Registers

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Register Definitions

Input Registers (Function Code 04) - Read Only

Input registers contain sensor measurements. All floating-point values are stored as IEEE 754 single-precision (32-bit) spanning two consecutive 16-bit registers in big-endian order.

Address	Name	Type	Unit	Description
0-1	O2percent	float	%	Oxygen concentration
2-3	CO2percent	float	%	CO2 concentration (as percentage)
4-5	CO2ppm	float	ppm	CO2 concentration (as ppm)
6-7	gasTempC	float	C	Gas stream temperature (SHT40)
8-9	gasRH	float	%RH	Gas stream relative humidity
10-11	gasAH	float	g/m3	Absolute humidity
12-13	gasVP	float	Pa	Vapour pressure
14-15	ambTempC	float	C	Ambient temperature (NTC)
16-17	heatTempC	float	C	Heater block temperature (NTC)

Holding Registers (Function Code 03/06/16) - Read/Write

Holding registers contain configuration and calibration parameters. Changes are automatically saved to non-volatile memory.

Address	Name	Type	Unit	Description
0-1	status	uint32	-	System status register (see below)
2	modbusSlaveID	uint16	-	Modbus slave address (2-246)
3	modbusBaudrate	uint16	x10	Baud rate / 10 (e.g., 960 = 9600)
4	modbusStopBits	uint16	-	Stop bits (1-3)
5	modbusParity	uint16	-	Parity: 1=Even, 2=Odd, 3=None
6	modbus120Rterm	uint16	-	RS-485 termination: 0=Off, 1=On
7	reserved1	uint16	-	Reserved for future use
8	O2SetZeroPoint	uint16	-	Write 1 to start O2 zero cal
9	CO2SetZeroPoint	uint16	ppm	CO2 zero cal reference (1-4999)
10-11	O2SetXPoint	float	%	O2 span cal point (10.0-25.0)
12-13	CO2SetXPoint	float	ppm	CO2 span cal point (300-50000)
14-15	heaterKp	float	-	PID proportional gain
16-17	heaterKi	float	-	PID integral gain
18-19	heaterKd	float	-	PID derivative gain
20-21	heaterMaxI	float	-	PID integral limit (0-255)
22-23	heaterSetpointC	float	C	Heater setpoint (20.0-50.0)
24-25	heaterMinAmbientDeltaC	float	C	Min delta above ambient (0-20)

Valid Baud Rate Values

Register Value	Actual Baud Rate
960	9600
1920	19200
3840	38400
5760	57600
11520	115200

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Status Register

The status register (holding registers 0-1) is a 32-bit value containing system health information. Each 4-bit nibble represents a different subsystem.

Status Register Bit Layout

Bits 31-24       23-20      19-16               15-12       11-8          7-4    3-0
     [WDT Count] [Amb Temp] [Gas Temp/Humidity] [Heater]    [CO2 Sensor]  [O2]   [System]

Subsystem Status Definitions

System Status (Bits 3-0)

Value	Meaning
0x01	System OK
0x02	WDT Reset Occurred
0x03	System OK + WDT Reset History

O2 Sensor Status (Bits 7-4)

Value	Meaning
0x01	Sensor OK
0x02	Reading too high (>25%)
0x04	Sensor not connected (<1%)

CO2 Sensor Status (Bits 11-8)

Value	Meaning
0x01	Sensor OK
0x02	Sensor not connected (no data)
0x04	Sensor communication error

Heater Status (Bits 15-12)

Value	Meaning
0x01	Temperature stable
0x02	Temperature unstable (>0.2C from setpoint)
0x04	Temperature sensor error

Gas Temperature/Humidity Status (Bits 19-16)

Value	Meaning
0x01	Sensor OK
0x02	Sensor error

Ambient Temperature Status (Bits 23-20)

Value	Meaning
0x01	Sensor OK
0x02	Sensor error

WDT Reset Count (Bits 27-24)

Contains the number of watchdog timer resets since last EEPROM clear (0-255, saturates at 255).

---

Usage Examples

Reading Sensor Values (Python with pymodbus)

from pymodbus.client import ModbusSerialClient
import struct

# Connect to the analyser
client = ModbusSerialClient(
    port='COM3',
    baudrate=9600,
    parity='N',
    stopbits=1,
    bytesize=8,
    timeout=1
)
client.connect()

SLAVE_ID = 100

def read_float(client, address, slave_id):
    """Read a 32-bit float from two consecutive input registers."""
    result = client.read_input_registers(address, 2, slave=slave_id)
    if result.isError():
        return None
    # Combine two 16-bit registers into a 32-bit float (big-endian)
    raw = (result.registers[0] << 16) | result.registers[1]
    return struct.unpack('>f', struct.pack('>I', raw))[0]

# Read all sensor values
o2_percent = read_float(client, 0, SLAVE_ID)
co2_percent = read_float(client, 2, SLAVE_ID)
co2_ppm = read_float(client, 4, SLAVE_ID)
gas_temp = read_float(client, 6, SLAVE_ID)
gas_rh = read_float(client, 8, SLAVE_ID)
heater_temp = read_float(client, 16, SLAVE_ID)

print(f"O2: {o2_percent:.2f}%")
print(f"CO2: {co2_ppm:.0f} ppm ({co2_percent:.3f}%)")
print(f"Gas Temperature: {gas_temp:.1f}C")
print(f"Gas RH: {gas_rh:.1f}%")
print(f"Heater Temperature: {heater_temp:.1f}C")

client.close()

Parsing the Status Register

def parse_status(client, slave_id):
    """Read and parse the status register."""
    result = client.read_holding_registers(0, 2, slave=slave_id)
    if result.isError():
        return None
    
    status = (result.registers[0] << 16) | result.registers[1]
    
    # Extract each nibble
    system_status = status & 0x0F
    o2_status = (status >> 4) & 0x0F
    co2_status = (status >> 8) & 0x0F
    heater_status = (status >> 12) & 0x0F
    gas_th_status = (status >> 16) & 0x0F
    amb_temp_status = (status >> 20) & 0x0F
    wdt_count = (status >> 24) & 0xFF
    
    # Interpret statuses
    status_names = {
        'system': {0x01: 'OK', 0x02: 'WDT Reset', 0x03: 'OK (WDT history)'},
        'o2': {0x01: 'OK', 0x02: 'Too High', 0x04: 'Not Connected'},
        'co2': {0x01: 'OK', 0x02: 'Not Connected', 0x04: 'Error'},
        'heater': {0x01: 'Stable', 0x02: 'Unstable', 0x04: 'Sensor Error'},
        'gas_th': {0x01: 'OK', 0x02: 'Error'},
        'amb_temp': {0x01: 'OK', 0x02: 'Error'}
    }
    
    print(f"System: {status_names['system'].get(system_status, 'Unknown')}")
    print(f"O2 Sensor: {status_names['o2'].get(o2_status, 'Unknown')}")
    print(f"CO2 Sensor: {status_names['co2'].get(co2_status, 'Unknown')}")
    print(f"Heater: {status_names['heater'].get(heater_status, 'Unknown')}")
    print(f"Gas Temp/RH: {status_names['gas_th'].get(gas_th_status, 'Unknown')}")
    print(f"Ambient Temp: {status_names['amb_temp'].get(amb_temp_status, 'Unknown')}")
    print(f"WDT Reset Count: {wdt_count}")
    
    return status

parse_status(client, SLAVE_ID)

Writing Configuration (Changing Heater Setpoint)

def write_float(client, address, value, slave_id):
    """Write a 32-bit float to two consecutive holding registers."""
    # Pack float as big-endian 32-bit
    raw = struct.unpack('>I', struct.pack('>f', value))[0]
    high_word = (raw >> 16) & 0xFFFF
    low_word = raw & 0xFFFF
    
    # Write both registers
    result = client.write_registers(address, [high_word, low_word], slave=slave_id)
    return not result.isError()

# Set heater setpoint to 40C
if write_float(client, 21, 40.0, SLAVE_ID):
    print("Heater setpoint updated to 40C")
else:
    print("Failed to update heater setpoint")

Changing Modbus Settings

# Change slave ID to 50
client.write_register(2, 50, slave=SLAVE_ID)

# Change baud rate to 19200 (write 1920 = 19200/10)
client.write_register(3, 1920, slave=SLAVE_ID)

# Enable 120 Ohm termination
client.write_register(6, 1, slave=SLAVE_ID)

# Note: After changing communication settings, 
# you will need to reconnect with the new parameters

---

Calibration Procedures

O2 Sensor Calibration

The O2 sensor supports two-point calibration: zero (0% O2) and span (typically 20.9% in air).

Zero Calibration (0% O2)

1. Flow nitrogen or other zero-O2 gas through the sensor
2. Wait for readings to stabilise
3. Write 1 to register 7 (O2SetZeroPoint)

# Start O2 zero calibration
client.write_register(7, 1, slave=SLAVE_ID)
# Calibration takes up to 60 seconds
# Monitor status - system enters STATE_CALIBRATING_O2

Span Calibration (e.g., 20.9% O2)

1. Flow calibration gas (e.g., ambient air at 20.9% O2)
2. Wait for readings to stabilise
3. Write the known O2 percentage as a float to registers 8-9

# Calibrate with 20.9% O2 (ambient air)
write_float(client, 8, 20.9, SLAVE_ID)
# Calibration takes up to 60 seconds

Calibration Requirements:

• Sensor current must be within 70-130 uA for air
• Reading must stabilise (+/-0.1 uA for 5 consecutive readings)
• Maximum calibration time: 60 seconds

CO2 Sensor Calibration

Zero Calibration

1. Flow nitrogen or zero-CO2 gas through the sensor
2. Wait for readings to stabilise
3. Write the expected zero reading (typically 0-400 ppm) to register 10

# Set CO2 zero point (expects 0 ppm, sensor reads 50 ppm)
# Write 50 to offset the reading
client.write_register(10, 50, slave=SLAVE_ID)

Span Calibration

Span calibration for CO2 is reserved for future implementation (registers 11-12).

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LED Status Indicator

The analyser has an RGB LED that indicates system state:

Colour	Pattern	State
Green	1 Hz blink	Normal operation
Magenta	1 Hz blink	Startup
Amber	0.5 Hz blink	Temperature unstable
Blue	0.5 Hz blink	O2 calibration in progress
Red	0.5 Hz blink	Error condition

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Building and Flashing

This project uses PlatformIO. To build and upload:

# Build
pio run

# Upload via Atmel-ICE
pio run --target upload

Dependencies

All libraries are included in the lib/ folder:

• Adafruit NeoPixel ZeroDMA
• Sensirion I2C SHT4x
• FlashStorage SAMD
• Custom libraries: SGX_4OX, SGX_INIR2_CD100, NTC_Therm, PID_Control, modbus-rtu-slave

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Troubleshooting

No Modbus Response

• Check RS-485 wiring (A/B polarity)
• Verify baud rate and slave ID
• Check termination resistor setting if at end of bus

O2 Reading Shows 0%

• Check O2 sensor connection
• Verify sensor current is within range (status register)
• Sensor may need replacement if current is <70 uA in air

Temperature Unstable (Amber LED)

• Allow 5+ minutes for heater to stabilise after power-on
• Check ambient temperature is not too close to setpoint
• Verify heater NTC is properly installed

CO2 Sensor Not Responding

• Check UART connection (38400 baud)
• Verify sensor is powered and warmed up
• Check status register for sensor errors

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License

This project is provided as-is for reference and educational purposes.