High-performance CO detection for portable, fixed and miniature gas monitors.

Alphasense CO sensors cover industrial safety, car park monitoring, combustion analysis, air quality monitoring and dual gas CO + H₂S detection. Available in multiple mechanical formats (A,B, C, D and H series) and PPB level 4 electrode variants, they give OEMs flexible options for portable instruments, fixed installations and compact wearable devices

Sensor Portfolio Overview

Portable & Industrial Safety – A-Series CO Sensors

Optimised for handheld and portable monitors where board space and size matter.

• CO-A1 – Standard A-Series CO sensor (no filter)
Unfiltered, electrochemical amperometric CO sensor for process and safety applications where only CO is present. The compact A-series format suits portable analyzers
• CO-AE – Extended-range A-Series (up to 1% CO)
Extended-range CO sensor for applications where very high CO levels must be measured. Rated to 1% CO and can be exposed up to 4% without damage, supporting demanding industrial safety and process control environments in a small portable friendly package.
• CO-AF – Filtered A-Series for industrial safety
Standard filtered solution with reduced cross-sensitivity to HS, NO, NO and SO. Used in handheld and portable instruments, as well as car-park and general industrial safety monitors. Frequently selected as the default A-series CO sensor for both portable and fixed instruments.
• CO-AX – Low H₂ cross-sensitivity A-Series
Filtered CO sensor with low hydrogen cross-sensitivity for combustion and industrial safety applications where CO must be measured in the presence of H. Ideal for portable and handheld instruments when reducing H interference is critical.
• CO-A4F – 4-electrode A-Series PPB CO sensor
A-series 4-pin PPB CO sensor for portable air-quality and low-level monitoring. Standard A(4-series) size with internal filter; when used with Alphasense AFE circuitry, noise is reduced to around 20 ppb, with additional digital smoothing available. Widely used in portable air-quality instruments worldwide

Fixed Gas Monitoring & Harsh Environments – B-Series CO Sensors

Designed for stationary systems and outdoor or high-humidity installations where robustness and stability are essential.

• CO-B1 – Unfiltered B-Series CO sensor
Larger-format B-series CO sensor with no filter, used primarily in process applications where only CO is present. Suited to fixed systems and outdoor environments thanks to its larger electrolyte volume, which handles high humidity and varying environmental conditions well.
• CO-BF – Filtered standard B-Series for safety
Standard B-series filtered CO sensor with reduced cross-sensitivity to HS, NO, NO and SO. Recommended for fixed or portable instruments operating in high-humidity, harsh environments—commonly deployed outdoors for industrial safety and car-park monitoring.
• CO-BX – Low H₂ cross-sensitivity B-Series
Filtered B-series CO sensor with low hydrogen cross-sensitivity. Used in fixed and portable systems for combustion and industrial safety where H may be present, and highly recommended for harsh, high-humidity conditions. The B-series format is ideal for fixed and outdoor applications.
• CO-B4 – 4-electrode B-Series PPB CO sensor
PPB-level, 4-pin B-series CO sensor for environmental air-quality and atmospheric monitoring. Standard B (7-series) size with internal filter; used with Alphasense ISB circuitry, it achieves noise levels around 4 ppb. Widely used in fixed air-quality networks worldwide.

Combustion Analysis & Stack Monitoring – C-Series CO Sensor

EN 50379-compliant CO sensors for combustion analyzers, flue-gas monitoring and emissions testing.

• CO-CE – High-concentration C-Series CO sensor
High-concentration, EN 50379-compliant C-series CO sensor for stack applications, emissions and combustion analysis. Features extended height for an additional carbon filter, reducing the impact of other combustion gases while accommodating very high CO levels.
• CO-CF – Filtered C-Series CO sensor
EN 50379-compliant filtered sensor for combustion-analysis CO measurement. The extended housing provides extra carbon filtering to reduce interference from combustion gases, delivering stable readings in stack and emissions monitoring.
• CO-CX – Low H₂ cross-sensitivity C-Series
EN 50379-compliant CO sensor with low hydrogen cross-sensitivity for combustion analysis in flue and stack environments where H is present. Extended height and carbon filtering reduce both combustion-gas cross-sensitivities and H interference for reliable CO measurements in demanding applications

Miniature & Wearable – D-Series and H-Series CO Sensors

Ultra-compact CO sensors designed for clip-on personal detectors, wearables and space-constrained devices.

• CO-D4 – Miniature D-Series CO sensor
Smallest industrial-safety CO sensor in the portfolio, designed for handheld and wearable instruments where size is at a premium. Incorporates a filter to reduce cross-sensitivity to common interfering gases while maintaining robust performance for personal safety devices.
• CO-D4X – Miniature D-Series with low H₂ cross-sensitivity
As with CO-D4, but with additional design measures to reduce hydrogen cross-sensitivity. Ideal for clip-on and wearable safety devices used in environments where H is present, providing more accurate CO readings in very compact form factors.
• CO-H4 – H-Series CO sensor
H-series CO sensor in a compact package tailored for clip devices and handheld instruments where space is limited. Designed specifically for Honeywell-forma t instruments, supporting high-volume portable devices.
• CO-H4X – H-Series low H₂ cross-sensitivity CO sensor
CO-H4X provides CO sensing with low cross-sensitivity to hydrogen in the same H-series package. Miniature format, surface-mount–ready, and optimised for clip devices and handheld instruments requiring reduced H interference.

Dual-Gas CO + H₂S Sensors – Space-Saving Safety Solutions

Dual-gas CO + H₂S sensors reduce instrument footprint and cost by combining two toxic-gas channels in a single package.

• COH-A2 – A-Series dual CO/H₂S sensor
Standard A (4-series) dual-gas sensor combining CO (0–1000 ppm) and HS (0–100 ppm).Uses a patented low-hydrogen-sensitivity CO electrode and high-capacity filtering over the CO working electrode to minimise HS cross-sensitivity. Enables significant size and cost reduction for portable and handheld safety instruments.
• COH-D2 – D-Series miniature dual CO/H₂S sensor
Miniature dual-gas sensor (CO 0–1000 ppm / HS 0–100 ppm) in a D-series package. High-capacity filter over the CO working electrode eliminates HS cross-sensitivity to CO, proven in long-term field use under challenging conditions. Ideal for clip-on personal detectors and compact industrial safety devices where space is limited.
• COH-H2 – H-Series dual CO/H₂S sensor
COH-H2 places the COH-D2 dual-gas sensing technology into an H-series package designed specifically for Honeywell-format instruments. Suitable for clip devices and handheld monitors where both footprint and profile must be minimised.

Integrated Smart Technology (IST) CO Sensor Versions

Many CO and CO/H₂S sensors are available with IST – Integrated Smart Technology, adding on-sensor electronics and digital features to simplify OEM integration.

IST versions include, for example:

• CO-A1-IST, CO-AE-IST, CO-AF-IST, CO-AX-IST – A+-series digital variants related to the corresponding A-series CO sensors.
• CO-B1-IST, CO-BF-IST, CO-BX-IST – B+-series digital variants for fixed and harsh-environment installations.
• CO-A4F-IST, CO-B4-IST – PPB CO sensors with IST for air-quality and low-level monitoring.
• COH-A2-IST – Dual CO/H₂S A+-series IST sensor for compact multi-gas instruments.

IST provides:

• On-sensor electronics with digital output
• Storage of factory calibration data
• Simplified OEM integration and board-level design
• Plug and play operation

Choosing the Right CO Sensor by Application

General industrial safety (portable / personal):

• CO-AF, CO-AX – A-series filtered options for handheld and portable safety monitors, including car-park and general industrial safety applications. CO-AX is preferred where H₂ interference must be minimised.
• CO-D4, CO-D4X – Miniature D-series sensors for clip devices, wearables and space-constrained portables; choose CO-D4X when low H₂ cross-sensitivity is required.
• CO-H4, CO-H4X – H-series options designed specifically for Honeywell-format portable instruments, with CO-H4X offering reduced H₂ cross-sensitivity.

Fixed installations & harsh / high-humidity environments:

• CO-BF, CO-BX – Filtered B-series sensors recommended for fixed systems and outdoor or high-humidity environments; CO-BX is the preferred option where H₂ may be present.
• CO-B1 – Unfiltered B-series option for process applications where only CO is present, with a larger format that handles varying environmental conditions well.

High-range CO / process & combustion:

• CO-AE – A-series extended-range sensor (up to 1% CO, tolerant up to 4%) for high-level CO in industrial safety and process-control applications.
• CO-CE, CO-CF, CO-CX – C-series EN 50379-compliant sensors for stack gases and combustion analyzers; select CO-CX when CO must be measured in the presence of H₂.

Air-quality & PPB-level monitoring:

• CO-A4F – A-series 4-electrode PPB sensor for portable air-quality and low-level CO monitoring using Alphasense AFE boards.
• CO-B4 – B-series 4-electrode PPB sensor for fixed air-quality and atmospheric monitoring with extremely low noise when used with ISB circuitry.

Dual gas / space-constrained multi-gas detectors:

• COH-A2, COH-D2, COH-H2 – Dual CO/H₂S sensors in A-, D- and H-series formats for portable, clip-on and Honeywell-format instruments where board space and cost must be minimised without sacrificing performance.

Key Benefits Across the CO Sensor Portfolio

Based on the CO and CO/H₂S product range in the Alphasense portfolio:

• Multiple form factors (A, B, C, D, H) to fit portable, fixed, wearable and OEM-specific instruments
• Filtered and unfiltered options to match application needs and cross-sensitivity constraints
• Low-hydrogen-cross-sensitivity variants (CO-AX, CO-BX, CO-CX, CO-D4X, CO-H4X, COH-A2, COH-D2, COH-H2) for accurate CO measurements in H₂-rich environments
• EN 50379-compliant C-series options specifically tailored for combustion and flue-gas analysis
• PPB-level, 4-electrode CO sensors (CO-A4F, CO-B4) for advanced air-quality and low-level monitoring
• Dual-gas CO + H₂S solutions (COH-A2, COH-D2, COH-H2) to reduce instrument footprint and cost
• IST digital variants for simplified integration and calibration storage

Why OEMs Choose Alphasense

Alphasense has built a global reputation as a leader in gas-sensor technology, with decades of performance data and millions of sensors deployed in the field.

For CO and CO/H₂S sensing, OEMs rely on Alphasense for:

• Proven electrochemical platforms used across multiple toxic-gas families
• Broad portfolio coverage from ppm safety sensors to PPB-level air-quality sensors
• Strong technical support, including AFE/ISB boards designed specifically for 4-electrodeCO sensors
• Long product lifecycles and consistent manufacturing, supporting stable instrument designs over time

How CO Sensors Work

Alphasense CO sensors use electrochemical amperometric technology to convert CO gas concentration into a measurable electrical signal.

• Electrochemical cell
Each sensor contains an electrochemical cell where CO participates in a controlled reaction at the working electrode. The sensor is designed so that the reaction rate is proportional to the CO concentration at the electrode surface.
• Amperometric measurement
Amperometric sensors operate by measuring the current produced by the electrochemical reaction at the working electrode. An external potentiostat circuit is required to measure the current between the working electrode, counter electrode and reference electrode. In a three-electrode sensor, each electrode has a specific use: Depending on the measured gas, the Working electrode either oxidizes or reduces gas. In a four-electrode sensor, an additional auxiliary electrode is used:



 

In a three-electrode sensor, each electrode has a specific use:
• Working electrode – designed to optimise the oxidation or reduction of the toxic gas to be measured.
• Reference electrode – keeps working electrode at required potential – Bias voltage.
• Counter electrode – balances the reaction of the working electrode

Depending on the measured gas, the Working electrode either oxidizes or reduces gas.

In a four-electrode sensor, an additional auxiliary electrode is used:
• Auxiliary electrode -Baseline correction for Zero Current and Temperature shifts that are critical for PPB sensor
  
  
• Signal output to the instrument
The resulting current (or corresponding voltage, depending on the front-end electronics) is proportional to the CO concentration. This signal is then read by the instrument’s analogue front-end or digital electronics which apply calibration data to display CO readings to the user.
• Filtering and cross-sensitivity control
Many CO sensors in the range incorporate internal filters and/or specific electrode designs to reduce cross-sensitivities to other gases such as HS, NO, NO, SO and H, as seen in the AF, BF, BX, CX, D4X, H4X and dual-gas COH models. Together, this architecture delivers a compact, low-power sensing solution for both worker-safety instruments and R&D-grade analyzers that demand stable, repeatable CO measurements.

Why Measure CO Gas?

Carbon monoxide (CO) is one of the most common—and most dangerous—toxic gases encountered in industrial, commercial, and confined-space environments. Because it is colorless, odorless, and non-irritating, workers often have no indication of exposure until symptoms occur, making continuous monitoring essential across safety and R&D applications.

Critical for Worker Safety
CO binds to hemoglobin more readily than oxygen, reducing the body’s ability to transport O₂. Even moderate concentrations can cause headaches, dizziness, or impaired judgement—while higher exposures can rapidly lead to unconsciousness and fatal outcomes. Real-time detection allows safety instruments to provide early alarms and protect personnel working around combustion processes, vehicle emissions, furnaces, or poorly ventilated areas.

Essential in Combustion & Process Control
In industrial and commercial combustion systems, CO is a key indicator of burner efficiency, incomplete combustion, and equipment malfunction. Accurate CO measurement helps operators tune systems, prevent emissions issues, and diagnose developing faults before they become costly or hazardous.

Compliance and Risk Reduction
Many industries—such as petrochemical, steelmaking, HVAC, transportation hubs, and wastewater treatment—require CO monitoring to meet regulatory standards. Reliable CO measurement helps organizations reduce liability, avoid downtime, and ensure compliance with local and international safety requirements.

Protecting Smart Buildings & Public Spaces
In parking garages, warehouses, and enclosed public facilities, CO monitoring is used to control ventilation systems automatically. Continuous sensing prevents unsafe accumulation while minimizing energy consumption.