4. Why Is CO Gas Monitoring Important?

Health and Safety

CO can interfere with the body’s ability to transport and use oxygen. Exposure can cause symptoms including headache, dizziness, weakness, nausea, confusion, and loss of consciousness. High exposure can be fatal.

NIOSH lists a recommended exposure limit of 35 ppm as an 8-hour time-weighted average, a ceiling value of 200 ppm, and an IDLH value of 1,200 ppm for CO. OSHA lists a permissible exposure limit of 50 ppm as an 8-hour TWA. These are occupational exposure criteria and should not be treated as universal alarm thresholds for every application.

Why Electronic Monitoring Is Useful

A CO transmitter can continuously convert the gas concentration into a signal that a control system can use.

For example:

CO sensor → PLC → Controller logic → Ventilation fan / Alarm / BMS

This allows a system to monitor CO continuously and respond according to its programmed safety strategy.

A sensor itself should not automatically be considered a complete life-safety system. The complete installation should be designed according to the applicable safety requirements and the sensor manufacturer’s instructions.

5- How Carbon Monoxide Gas Sensors Work?

Gas Detection Principle a gas sensor detects CO molecules in the surrounding atmosphere and converts their presence into an electrical response.

There are several sensing principles, but two are particularly important for electronics projects:

Semiconductor sensing

• A semiconductor sensor uses a gas-sensitive material whose electrical conductivity changes when CO interacts with its surface.
• The sensor therefore behaves approximately as a variable resistance.
• A resistor can be connected with the sensing element to create a voltage that a microcontroller can measure.

Semiconductor sensor→ resistance change → voltage measurement

Electrochemical sensing

• An electrochemical CO sensor generates a small electrical current through a chemical reaction involving CO and oxygen.
• Many industrial CO instruments use electrochemical sensing cells, in which a chemical reaction involving the target gas produces an electrical signal related to gas concentration.
• From Gas Concentration to Electrical Signal the sensor’s internal electronics process the sensing element’s response and generate an output corresponding to CO concentration, and Depending on the model that output signal

Electrochemical sensor → small current → transimpedance amplifier → voltage measurement

6. Types of Carbon Monoxide Gas Sensors

CO sensors can generally be divided into several categories:

Semiconductor CO Sensors

Semiconductor sensors use materials such as metal oxides.

Electrochemical CO Sensors

Electrochemical sensors are designed specifically to measure CO through an electrochemical reaction.

Integrated CO Sensor Modules

A module combines the sensing element with electronics for signal conditioning and communication.

For a beginner, an integrated module can significantly simplify the electronics.

7. Comparation Between Semiconductor vs. Electrochemical CO Sensors

The choice of sensor technology strongly affects the design of the project.

The important point is that sensor technology alone does not make a complete CO detector safe or certified.

A certified detector requires appropriate sensor characterization, calibration, electronics, alarm thresholds, environmental testing, fault detection, enclosure design, and compliance with applicable standards.

8. Main specifications of Gas Sensors

Before connecting any gas sensor to a microcontroller, several specifications should be examined.

• Detection Range
• Sensitivity
• Heater Voltage
• Heater Power
• Warm-Up Time

9. Understanding CO Sensor Measurement in ppm

What Does ppm Mean?

CO concentration is normally expressed in parts per million (ppm).

For example:

100 ppm CO means approximately 100 parts of CO per million parts of the measured air mixture.

The four models discussed here use a nominal 0–2000 ppm CO measurement range in their product names. The three analog models explicitly list 0–2000 ppm on their product pages.

Why a 0–2000 ppm Range Does Not Mean Normal CO Levels Should Be 2000 ppm

A sensor’s measurement range describes what the instrument can measure; it is not a recommended operating or exposure level.

For example, NIOSH identifies 1,200 ppm as the IDLH value for CO, illustrating why a 0–2000 ppm sensor range extends into concentrations that can represent a severe hazard.

The appropriate alarm thresholds must therefore be selected according to the application, applicable standards, exposure requirements, and safety design—not simply according to the sensor’s maximum range.

10. Analog and Digital Sensor Outputs

0–5V and 0–10V

Voltage-output sensors provide an analog voltage proportional to the measured CO concentration.

For a nominal 0–2000 ppm range:

0–5V version:

• 0 ppm → approximately 0 V
• 1000 ppm → approximately 2.5 V
• 2000 ppm → approximately 5 V

0–10V version:

• 0 ppm → approximately 0 V
• 1000 ppm → approximately 5 V
• 2000 ppm → approximately 10 V

These examples assume the output is linearly scaled across the full nominal range.

4–20mA

The 4–20 mA model uses a current signal rather than a voltage signal.

For a nominal 0–2000 ppm scaling:

• 0 ppm → approximately 4 mA
• 500 ppm → approximately 8 mA
• 1000 ppm → approximately 12 mA
• 1500 ppm → approximately 16 mA
• 2000 ppm → approximately 20 mA

The important advantage of 4–20 mA in industrial systems is that the signal is represented by current in the loop, making it well suited to longer cable runs and electrically noisy environments when the complete loop is correctly designed.

RS485 / Modbus-RTU

RS485 is an electrical communication interface rather than a measurement unit.

The RS-COWS model uses RS485 with Modbus-RTU, allowing a controller to digitally read the sensor’s measurements. The product page specifies CO, temperature, and humidity measurements and a 2-second data update interval.

11. Applications of CO Sensor

Parking Garages and Vehicle Areas

• Vehicle exhaust can produce CO, making CO monitoring relevant in enclosed or semi-enclosed parking environments.
• A sensor can provide input to ventilation-control systems, where the control strategy is designed according to the applicable requirements.

Mechanical and Boiler Rooms

• Combustion equipment can be a source of CO.
• Continuous monitoring can therefore be incorporated into building or industrial monitoring systems.

HVAC and Building Automation

• The analog and RS485 versions can be integrated into HVAC or building-management systems.
• For example: CO Sensor → Controller → Ventilation System
• The actual control thresholds should be determined by the system designer using applicable regulations and safety requirements.

Industrial Monitoring

• CO monitoring can also be used around combustion equipment, engines, generators, industrial processes, and other environments where CO may be generated.
• CDC/NIOSH specifically identifies generators, small engines, marine engines, pressure washers, forklifts, and other equipment as potential workplace CO sources.

12. Choosing the Right CO Sensor

Choose RS-CO-V10-2-2000P-2 When

• Choose the 0–10 V version when the receiving PLC, HVAC controller, or automation system provides a compatible 0–10 V analog input.
• It is particularly convenient when 0–10 V is already the standard analog interface used throughout the control system.

Choose RS-CO-V05-2-2000P-2 When

• Choose the 0–5 V version when the controller, ADC, or data-acquisition system is designed for 0–5 V analog signals.
• This can be convenient for electronics projects and controllers using 5 V-compatible analog inputs.

Choose RS-CO-I20-2-2000P-2 When

• Choose the 4–20 mA version when the control system uses industrial current-loop inputs.
• This is a practical interface for industrial installations where signal transmission over longer cable distances and noise immunity are important design considerations.

Choose RS-COWS-N01-2-2000P-5 When

Choose the RS485 / Modbus-RTU version when you want digital communication and need more than just a single analog CO signal.

It is especially useful when the system also needs:

• Temperature
• Relative humidity
• Digital communication
• PLC/BMS/SCADA integration
• Multiple networked devices

The product listing identifies the RS-COWS as a combined CO, temperature, and humidity sensor with RS485/Modbus-RTU communication.

Quick Selection Guide

• Need 0–10 V?  → RS-CO-V10-2-2000P-2
• Need 0–5 V?  → RS-CO-V05-2-2000P-2
• Need 4–20 mA?  → RS-CO-I20-2-2000P-2
• Need RS485 + Modbus + CO + Temperature + Humidity?  → RS-COWS-N01-2-2000P-5

13. RS-CO-V10-2-2000P-2: 0–10V CO Sensor

Main Characteristics

The RS-CO-V10-2-2000P-2 is a fixed CO gas sensor with an analog 0–10V output and a nominal 0–2000 ppm measurement range. The product page describes it as intended for continuous ambient monitoring and integration with controllers, PLCs, and data-acquisition systems.

The listed supply range is approximately 10–30 VDC, while the product page describes several environmental and stability features. Because the published page labels some specifications as approximate, the exact installation should follow the supplied product documentation.

When 0–10V Is Useful

This version is appropriate when the receiving controller has a 0–10V analog input.

Typical examples include:

• HVAC controllers
• Building automation systems
• PLC analog input modules
• Data acquisition equipment
• Industrial controllers

The main consideration is that the controller’s analog input must be compatible with a 0–10 V signal.

14. RS-CO-V05-2-2000P-2: 0–5V CO Sensor

Main Characteristics

The RS-CO-V05-2-2000P-2 provides a 0–5V analog output and a listed 0–2000 ppm CO measurement range. The product is designed for fixed, continuous CO monitoring and can interface with controllers and analog acquisition systems.

The listed supply is approximately 10–30VDC.

When 0–5V Is Useful

The 0–5V interface is particularly convenient when the receiving electronics already operate with a 0–5 V analog input.

For example, a system designer may use:

CO Sensor → 0–5V ADC Input → Microcontroller → Display / Alarm

The main limitation is that the controller or ADC must be able to accept the sensor’s output range.

When interfacing with a microcontroller, always verify the maximum allowed analog-input voltage. Do not connect a sensor output directly to an input that cannot tolerate its full output range.

15. RS-CO-I20-2-2000P-2: 4–20mA CO Sensor

Main Characteristics

The RS-CO-I20-2-2000P-2 is the 4–20 mA version of the CO sensor family.

It provides:

• CO measurement
• 0–2000 ppm nominal range
• 4–20 mA analog output
• Approximately 10–30 V DC supply according to the product listing
• Fixed installation for continuous monitoring

Understanding the 4–20mA Signal

For a linear 0–2000 ppm configuration:

CO concentration = (I − 4mA) / 16mA × 2000 ppm

For example, if the controller measures 12mA:

CO = (12 − 4) / 16 × 2000

CO = 1000 ppm

The important point is that 4mA represents the bottom of the measurement scale, rather than 0 mA.

This also provides a useful distinction between a normal zero-scale reading and certain types of wiring or loop faults, depending on the transmitter and receiver configuration.

Why 4–20mA Is Common in Industry

4–20 mA is widely used in industrial instrumentation because current-loop signaling is practical for long cable runs and is relatively resistant to voltage-drop and electrical-noise problems compared with simple voltage signaling.

The actual loop design still depends on the transmitter type, supply voltage, receiver input resistance, grounding, cable installation, and manufacturer’s wiring instructions.

16. RS-COWS-N01-2-2000P-5: RS485 CO Sensor

Main Characteristics

The RS-COWS-N01-2-2000P-5 is different from the three analog models because it communicates digitally using RS485 / Modbus-RTU.

It combines:

• CO measurement
• Temperature measurement
• Relative humidity measurement
• RS485 communication
• Modbus-RTU protocol

The product page lists a 10–30 VDC supply and a data update interval of approximately 2 seconds.

Why Use RS485?

With an analog sensor, the controller reads a voltage or current.

With the RS485 model, the controller communicates with the sensor and requests digital measurement data.

A simplified architecture is:

CO Sensor → RS485 Bus → PLC / PC / BMS / SCADA

Multiple RS485 devices can also be integrated into a properly designed bus network, with device addressing handled by the communication protocol.

Why Temperature and Humidity Are Useful

Environmental conditions can be important when interpreting gas measurements and evaluating the surrounding environment.

The integrated temperature and humidity measurements also mean that one device can provide multiple environmental parameters rather than requiring separate sensors.

17. Comparison of the Four CO Sensor Models

Technical Comparison

The analog model information is taken from the corresponding product pages, while the RS-COWS information comes from its product listing.

The Key Difference

• The three RS-CO models perform essentially the same basic job—measuring CO and providing an analog output—but the output interface changes.
• The RS-COWS model adds digital communication and additional environmental measurements.
• Therefore, choosing between these sensors is not simply a matter of asking which sensor detects CO better.
• The system designer should first determine how the measurement will be received and processed.

18. Installation and Operating Considerations

Sensor Location

• Sensor placement is an important part of gas monitoring.
• The correct position depends on the source of CO, room geometry, airflow, ventilation system, expected accumulation points, and applicable installation standards.
• A sensor should not simply be placed wherever it is convenient to mount.

Avoid Obstructing the Sensing Area

• The gas must be able to reach the sensing element. Avoid covering or blocking the sensor’s gas-entry area.
• Do not install the device in a location where dust, water, chemicals, or mechanical damage could interfere with its operation unless the sensor is specifically rated for those conditions.

Warm-Up and Stabilization

• Gas sensors may require a warm-up period after power is applied.
• The RS-COWS product page, for example, specifies approximately 2 minutes of warm-up, with additional time recommended for full precision.
• Always allow the specified stabilization time before relying on the measurement.

Calibration and Maintenance

• Gas sensors are measurement instruments, not permanent zero-error devices.
• Calibration requirements depend on the sensor technology, operating environment, manufacturer, and application.
• For safety-critical applications, calibration should be performed according to the manufacturer’s procedure and the requirements applicable to the installation.

19. Connecting CO Sensors to PLCs and Controllers

0–10 V Connection

A typical system concept is:

• Sensor 0–10 V OUT → PLC Analog Input
• Sensor GND → PLC Analog GND
• Sensor V+ → DC Power Supply +

The exact terminal names and wiring arrangement must always be taken from the product’s wiring diagram.

0–5 V Connection

A typical system concept is:

• Sensor 0–5 V OUT → Controller Analog Input
• Sensor GND → Controller GND

The controller must have an analog input capable of measuring the full 0–5 V range.

4–20 mA Connection

• The 4–20 mA sensor is connected to a suitable analog current input or current-loop receiver.
• The exact connection depends on whether the transmitter output is configured as a two-wire loop-powered device or another arrangement.
• Therefore, engineers should not assume that every 4–20 mA transmitter uses identical wiring.

RS485 Connection

• For the RS-COWS model, the communication interface is:  RS485 / Modbus-RTU
• A typical system contains:  Sensor → RS485 A/B → PLC or Modbus Master
• The master communicates with the sensor according to the device’s Modbus configuration.

For a real installation, the engineer should verify:

• Device address
• Baud rate
• Parity
• Stop bits
• Register addresses
• Data format
• A/B polarity
• Cable type
• Bus termination requirements

These parameters should come from the sensor’s communication documentation rather than being assumed.

20. Project Ideas and Interfacing CO Sensors with Microcontrollers

• Carbon monoxide sensors can be used in a wide range of electronics and automation projects.
• The four sensor models discussed in this article provide different interfaces, so the best connection method depends on the microcontroller or controller being used.
• Makers Electronics currently lists the four models as 0–10 V, 0–5 V, 4–20 mA, and RS485 versions.

Project Ideas Using CO Sensors

Some practical project ideas include:

• CO Monitoring System: Display the measured CO concentration on an LCD or OLED display.
• IoT CO Monitor: Connect an ESP32 to the sensor and send CO measurements to a web dashboard or mobile application.
• CO Alarm System: Activate a buzzer, warning LED, or relay when the measured concentration exceeds a programmed threshold.
• Smart Ventilation Controller: Use the CO measurement to control an exhaust fan or ventilation system.
• Data Logger: Record CO concentration over time on an SD card or send measurements to a computer.
• Industrial Monitoring Node: Connect the sensor to a PLC or industrial controller for continuous monitoring.
• Multi-Sensor Environmental Monitor: Use the RS-COWS model to monitor CO together with temperature and humidity.
• RS485 Sensor Network: Connect multiple Modbus sensors to an ESP32 or PLC and collect their measurements through a single communication bus.

For safety-critical applications, however, a hobbyist microcontroller project should not automatically be treated as a certified life-safety system. Alarm thresholds, redundancy, calibration, and applicable safety standards must be considered separately.

Connecting the 0–5 V Sensor

The RS-CO-V05-2-2000P-2 provides a 0–5 V analog output and can be connected to a suitable Arduino analog input.

• Sensor V+ → External DC Supply
• Sensor GND → Arduino GND
• Sensor 0–5 V OUT → Arduino Analog Input

For a 0–2000 ppm range:  CO (ppm) = VOUT × 400

For example:  2.5 V corresponds to approximately 1000 ppm, assuming linear scaling.

With an ESP32, verify the ADC input range first. A voltage divider or suitable signal-conditioning circuit may be required because ESP32 inputs are not designed to accept arbitrary 5 V signals.

Connecting the 0–10 V Sensor

The RS-CO-V10-2-2000P-2 provides a 0–10 V output. It must not be connected directly to a 5 V Arduino input or an ESP32 ADC.

Use a voltage divider or appropriate signal-conditioning circuit to reduce the voltage to a safe ADC range.

For a 0–2000 ppm range: CO (ppm) = VOUT × 200

For example:  5 V represents approximately 1000 ppm.

Connecting the 4–20 mA Sensor

The RS-CO-I20-2-2000P-2 uses an industrial 4–20 mA output. A microcontroller ADC normally requires a voltage, so the current can be converted using a precision resistor or dedicated current-input circuit.

For example:  a 250 Ω resistor produces:

• 4 mA → 1 V
• 20 mA → 5 V

The CO concentration can then be calculated from:  CO (ppm) = (I − 4) / 16 × 2000

where I is the measured current in mA.

Connecting the RS485 Sensor

The RS-COWS-N01-2-2000P-5 uses RS485/Modbus-RTU rather than an analog output. Connect it to an Arduino or ESP32 through an appropriate RS485 transceiver.

CO Sensor → RS485 A/B → RS485 Transceiver → Arduino/ESP32

The microcontroller communicates with the sensor using Modbus and can obtain CO, temperature, and humidity measurements.

The sensor’s Modbus address, baud rate, and register information should always be taken from its documentation.

Practical Project Example

A simple IoT CO monitoring system can be built as:

CO Sensor → Signal Interface → ESP32 → CO Calculation → Display / Wi-Fi → Dashboard or Alarm

The ESP32 can display the measured CO concentration, store data, send it over Wi-Fi, or control a ventilation system according to the application’s control logic.

Important:

• Always verify the sensor’s supply voltage, output range, grounding, and microcontroller input limits before making the connection.
• For safety-critical CO monitoring, the project should also follow the applicable safety standards, calibration requirements, and manufacturer instructions.

21. Frequently Asked Questions

What is a CO gas sensor?

A CO gas sensor is an electronic device used to measure the concentration of carbon monoxide in air and convert the measurement into an electrical signal or digital data.

What does 2000 ppm mean on these sensors?

It indicates a nominal measurement range extending to 2000 parts per million of CO. It does not mean that 2000 ppm is a safe concentration. NIOSH lists 1,200 ppm as the IDLH value for CO.

What is the difference between CO and CO₂ sensors?

CO sensors measure carbon monoxide, while CO₂ sensors measure carbon dioxide. They are different gases and require different sensing systems.

What is the difference between 0–5 V and 0–10 V?

Both are analog voltage outputs, but their full-scale voltages are different. A nominal 0–2000 ppm sensor using 0–5 V maps 2000 ppm to 5 V, while the 0–10 V version maps 2000 ppm to 10 V.

Why does a 4–20 mA sensor start at 4 mA instead of 0 mA?

The 4 mA lower limit provides a live-zero signal and is useful in industrial instrumentation. The receiver can distinguish a normal low-scale signal from certain zero-current conditions, depending on the transmitter and loop design.

Is RS485 the same as Modbus?

No. RS485 is an electrical communication interface, while Modbus-RTU is a communication protocol commonly transmitted over RS485.

The RS-COWS model uses both RS485 and Modbus-RTU.

Can I connect the 0–10 V sensor directly to any microcontroller?

Not necessarily. The microcontroller’s ADC input must be able to safely accept the maximum sensor output. If the ADC is limited to 3.3 V, for example, a 0–10 V signal requires an appropriate signal-conditioning circuit.

Which model is suitable for a PLC?

All four can potentially be integrated with suitable PLC hardware, but the choice depends on the PLC input interface:

• 0–10 V analog input → RS-CO-V10
• 0–5 V analog input → RS-CO-V05
• 4–20 mA analog input → RS-CO-I20
• RS485/Modbus interface → RS-COWS

Can these sensors be used as a complete safety system?

A sensor is only one component of a safety system. A complete CO safety installation may require appropriate alarms, ventilation controls, emergency procedures, calibration, maintenance, redundancy, and compliance with applicable standards.

Where can CO come from?

CO can be produced by combustion sources such as generators, vehicles, furnaces, gas appliances, fireplaces, charcoal, and small engines.

22. Conclusion

Carbon monoxide sensors provide an important interface between gas measurement and electronic control systems. The four RS-series models covered in this guide measure CO over a nominal 0–2000 ppm range, but they differ significantly in how the measurement is delivered to the rest of the system.

The RS-CO-V10-2-2000P-2 provides a 0–10 V analog output, while the RS-CO-V05-2-2000P-2 provides 0–5 V. The RS-CO-I20-2-2000P-2 provides a 4–20 mA industrial current-loop output, making it suitable for control systems equipped with current inputs. The RS-COWS-N01-2-2000P-5 uses RS485/Modbus-RTU and additionally measures temperature and humidity.

For electronics and automation engineers, the most important selection criterion is therefore not simply the CO measurement range. It is the interface required by the receiving system:

• 0–10 V → RS-CO-V10
• 0–5 V → RS-CO-V05
• 4–20 mA → RS-CO-I20
• RS485/Modbus + Temperature + Humidity → RS-COWS

Finally, CO is a serious health and workplace hazard. CDC/NIOSH identifies CO as a colorless, odorless, toxic gas that can accumulate rapidly in enclosed or semi-enclosed environments. Proper sensor selection, installation, calibration, maintenance, alarm strategy, and compliance with applicable safety requirements are therefore essential when the sensor is used for real-world protection.