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Cerex Air Sentry FTIR

Optical, remote sensing, sub parts-per-billion (ppb) to percent level gas analyzer

The Cerex Air Sentry was the first open path FTIR to be commercialized and has continued to lead the industry with reliable performance and innovative features. It performs chemical detection and quantification of compounds using open-path, Fourier Transform InfraRed (FTIR) Differential Optical Absorption Spectroscopy (DOAS).

It is the recognized choice in open-path FTIR and the most reliable for monitoring industrial facilities, accidental releases, and hazardous waste site emissions.

  • Monitors from 1 to 1,000 meters
  • Eliminates need for many single point gas-specific sensors
  • No lengthy calibrations or time-consuming laboratory analysis
  • Deep sensitivity – detects concentration of gases that traditional sensor arrays can’t

Cerex Air Sentry FTIR

of optical path length
0 gases
to monitor simultaneously
0 parts
that require regular maintenance

Air Sentry OP-FTIR (Standard)

Weight 150 lbs (68kg)
Measurements with Telescope 42” x 23″ x 20″ (109cm x 59cm x 51cm)
Enclosure NEMA 4X – IP65, thermoelectrically cooled

Power 10A/6A @ 120/220 VAC
Operating Temperature -40°F – 140°F (-40°C – 60°C)
Optical Path Length 5m – 1000m

A lighter weight, air cooled, version is available for some applications.


  • Suitable for corrosive environment system deployment
  • Tripod mountable for semi-permanent installations

We recommend sheltering against wind and solar loading for permanent installations


  • Recommend semi-annual inspections of dirt buildup on the exterior heat-sink fins.
  • Rinse air conditioner fins as needed
  • Fans are rated to 40,000 hours at 40°C
  • IR source life time is approximately 2 years
  • Stirling cooler (detector cryocooler) guaranteed for 2 years before refurbishment is required
  • A maintenance free, thermoelectrically (TE) cooled, detector is available for some applications

How it Works

Open Path FTIR Technology

The Cerex AirSentry FTIR operates by sending a beam of Infrared light through the open air. In a mono-static installation, the IR beam is reflected back to the analyzer by a retro-reflector array, where the absorption due to target gases is measured and recorded. The analyzer uses a classical least squares regression analysis to compare the measured absorption spectrum to calibrated reference absorption spectra files per USEPA TO-16 Methodology. Beer’s Law is used to report accurate gas concentrations.

Fourier transform infrared spectroscopy (FTIR) is the preferred technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid, and gas.

The Cerex AirSentry FTIR is the only commercial open-path FTIR evaluated in 2 US EPA site studies and proven capable of detecting, identifying, and quantifying air toxins.


(Above) Process of open-path monitoring (depicted with a Cerex UV Sentry). Similarly, the AirSentry FTIR mounts to a precision pan and tilt head which may be used with a tripod for rapid deployment, or with a fixed mount for permanent installations. The retro-reflector array is housed in a NEMA enclosure and may be tripod or fixed mounted to a mast, pole or superstructure. Monitoring path lengths are typically configured to meet minimum detection limits required for the project. Paths to 1 km are possible. The IR beam is eye-safe and the analyzer may be located directly in the breathing zone. Gases present in the beam path are identified and quantified in near real time. Credit: STI

The most cost effective perimeter & fenceline monitoring solution

Offering real time quantification of multiple compounds, the Cerex AirSentry FTIR provides a cost effective multi-component perimeter fugitive emissions or LDAR monitoring solution.

Low maintenance and low detection limits

Monitoring as few as two gases with the AirSentry FTIR often results in lower initial and lower ongoing operational costs than utilization of competing gas detection technologies, while simultaneously delivering reliable U.S.E.P.A. TO-16 regulatory compliant air quality monitoring.

Individual species of prolific hazardous air pollutants

The AirSentry FTIR is offered fully configured to meet the requirements of your air quality monitoring application. Target gases are selected from Cerex’s library of 385 compounds.

Common Detectable Gases

Extensive Library with Options for Customization

Cerex has over 385 compounds available for detection with the AirSentry. If your compound is not one of the compounds already available within our database, the Cerex AirSentry FTIR may import gas spectra from the numerous FTIR databases on the web (NIST, EPA, PNNL, etc.)

If you have a compound that is not available in a public database, we may be able to work with you to develop a custom spectrum. Detection limits are a function of the IR detector, installed path length, and site conditions. The detection limits shown have been empirically determined from bump tests of fielded instruments at full path-length.

It is efficient as an alarm and easily integrated into your existing alarm system.

*NOTE: If desired target gases are available within the UV spectrum, we recommend the Cerex UV SENTRY for better pricing options. The UV Sentry is an open-path analyzer that uses UV-DOAS (Ultra Violet Differential Optical Absorption Spectroscopy). Find more information about UV-DOAS and FTIR techniques here.

Air Sentry FTIR Common Target Gases

Acetaldehyde (C2H4O)
Acetic Acid (C2H4O2)
Acetone (CH3COCH3)
Acetylene (C2H2)
Acrolein (C3H4O)
Benzaldehyde (C7H6O)
Butane (C4H10)
n-Butyl (C6H12O2)
Carbon Dioxide (CO2)
Carbon Monoxide (CO)
Chlorine Dioxide (ClO2)
Ethane (C2H6)
Ethanol (C2H6O)
Formaldehyde (CH2O)
Formic Acid (CH2O2)
n-Hexane (C6H14)
Hydrogen Bromide (HBr)
Hydrogen Chloride (HCl)
Hydrogen Fluoride (HF)
Methane (CH4)
Nitric Oxide (NO)
Nitrogen Dioxide (N2O)
Phenol (C6H5OH)
Phosgene (CCl2O)
Water Vapor (H2O)


Results You Can Rely On

Cerex recognizes that detailed gas monitoring results need to be reliable and available quickly to various stakeholders. For this reason, Cerex engineers developed software to accurately detect and display gases within the air in real time from any location.

On an Air Sentry FTIR, a user can see results remotely on a phone, tablet, or computer anywhere in the world via a virtual connection. GPS and cellular configurations make this possible in isolated environments.

All data is automatically saved in an easy to access .csv file; this feature helps our clients in various ways - from daily use to crucial events such as evidence in court. Early detection translates into quicker response times, reduced environmental impact, decreased plant downtime, and reduced exposure to site personnel and the community.


Real time spectral subtraction and synthetic background implementation allow the Air Sentry to clearly display accurate concentrations of target gases over 1000 meters without interference from other gas species


Hundreds of successful installations

In 2019 alone, 190 Air Sentry units were provided to various clients

Cerex has been manufacturing the AirSentry FTIR since 1994 and continues to provide support for these fielded instruments. With hundreds of detectable compounds, the AirSentry FTIR is the monitoring solution for petrochemical industries, semiconductor plants, agriculture, landfill, and site surveys of historically polluted areas.

The AirSentry FTIR, with its hundreds of detectable compounds, is suitable choice for compliance with Rule 1180 and refinery fenceline air monitoring. A flexible analyzer platform, the AirSentry may be rapidly converted from open path mode to point analyzer mode with the inclusion of an optional sample cell. In open path mode the AirSentry FTIR offers full USEPA TO-16 compliant monitoring. In point analyzer mode the AirSentry FTIR meets a variety of method compliance standards.

All Applications

Acid Plant Monitoring

Brownfield Remediation

Chemical Depot Monitoring

Chemical Manufacturing

Concrete Manufacturing

Electrowinning Emissions Monitoring

Fence-line Monitoring System (FLMS)

Manufactured Gas Plant Remediation (MGP)

Ore Processing

Perimeter Ambient Monitoring System (PAMS)

Refinery Operations and Monitoring

Superfund Site Remediation

Tank Farm Monitoring

Regulation Compliance

USEPA Method 318
Extractive FTIR Method for Measurement of Emissions from Mineral Wool and Wool Fiberglass Industries

USEPA Performance Specification 15
Extractive FTIR CEMS in Stationary Sources

USEPA Method 320
Vapor Phase Organic and Inorganic Emissions by FTIR

USEPA Method 321
Determination of HCl for Portland Cement Industries

NIOSH Method 3800
Organic and Inorganic Gases by Extractive FTIR Spectrometry

ASTM Method D6348-03
Determination of Gaseous Compounds by Extractive Direct Interface Fourier Transform Infrared Spectroscopy

View current installations of the Air Sentry on our portfolio page

Air Sentry Specifications

General Parameters
Part Number Shepherd FTIR
Analyzer Type Multi-gas portable point analyzer
Technology Fourier Transform InfraRed (FTIR) Differential Optical Absorption Spectroscopy (DOAS)
Performance Available with standard HAP (385 compound & 62 Compound), Criteria, Hospital and custom gas detection libraries.
Data Output Rate User selectable from 5 seconds and up
Typical output rates are 30s, 60s, 120s, or 300s
Time weighted average available for user defined periods (8 hour, 12 hour, 24 hour)
Measurement Principle Beer-Lambert Law
Measurement Technique Classical Least Squares (CLS) regression analysis; optional Partial Least Squares (PLS)
Operating Temperature 0°C to 35°C (Optional Air Cooled Model)


Storage Temperature -40C° to 60C°
Power Supply 100 to 130V or 200 to 240VAC/50-60Hz
Power Consumption 240W MAX
Instrument Cooling Air Cooled
Installation Fixed mount or tripod mount (open path analyzer mode)
Bench-top (point analyzer mode)
Dimensions 42.1" x 21.9" x 23.3" (107cm x 55.7cm x 59.1cm)
Weight 85 lbs (38.6Kg)
Measuring Parameters
Concentration measurement accuracy ±5 % Reading TYP
Concentration measurement precision ±5 % Reading TYP
Interferent Mitigation Real time spectral subtraction and residual subtraction, multi-peak analysis.
Zero point calibration Simultaneous dual mode natural zero and/or synthetic zero on gas by gas basis. Automated compound specific background selection.
Real Time Operating Software Cerex Continuous Monitoring Software (CMS)
Operating System Windows 10 Pro
Post Collection Analysis Software Cerex Data Processor
Operating System Windows 7, 8, 10
Apodized Resolution User selectable: 0.5 cm-1, 1 cm-1, 2 cm-1, 4 cm-1, 8 cm-1, 16 cm-1, 32 cm-1
Scan frequency 6 scans/s @ 32 cm-1
Detector Stirling Cooled MCT (standard)
TE Cooled MCT (application dependent)
Detector temperature 77 K (Stirling cooled MCT)
Source SiC, 1550K
Beamsplitter ZnSe (standard)
Window material ZnSe (standard)
Wavenumber range 600-4200 cm-1 
Temperature stability < 1% per C⁰
Frequency repeatability  < 0.001 cm-1 @ 1918cm-1
Frequency accuracy < 0.06 cm-1 @ 1918cm-1
Short term stability < 0.09%
Maximum signal to noise ratio 50,000: 1 (RMS, 60s, 4 cm-1, at peak response)
Signal sampling 24-bit ADC
Quality Assurance Hardware
Type Internal linear multi-pass cell permanently mounted in beam path. External window removable.
Calibration gas inlet Quick Connect QA Adapter: 1/4" Swagelok
Calibration gas outlet Screw-in Exhaust Adapter: 3/8" Swagelok
Path Length 19.2 m
Volume 3.4 L
Maximum sample pressure 1 PSI
Pressure and temperature measurement Integrated pressure and temperature sensor
HMI Touchscreen (standard model)
Digital Interface RJ45 Ethernet LAN Port for local or internet based remote data and access using MODBUS TCP, VNC, and remote desktop.  Spectral data may be stored locally, on a NAS, or disabled. Industrial external wireless option available. Type A USB for data retrieval and peripheral accessories.
Power Connection Environmentally sealed circular Amphenol bayonet connector.
Integrated Computer Embedded industrial PC
Integrated Wind Mapping Three dimesnional ultrasonic anemometer with temperature measurement. Data integrated into CMS data tables and available via MODBUS.
Integrated Data Acqusition External ports with data integration for particulate or ancillary co-located analyzers (TDLAS laser, electrochemical, etc.).
Data Output MODBUS over RS-232, RS-485.  MODBUS over TCP-IP is standard.
Analog Output 8 channel configurable 0-24mA (4-20mA with extended range)
Type Compound MDL PPB
Criteria Sulfur Dioxide 300
Volatile Organic Compounds Acrolein 15
1,3 Butadiene 4
Styrene 8
BTEX Benzene 30
Toluene 100
Ethylbenzene 25
m-Xylene 100
o-Xylene 300
p-Xylene 300
Alkanes Pentane C5 4
n-Hexane C6 3
TVOC Methane C1 3
NMHC Ethane C2 8
NMNEHC Butane C4 4
Propane C3 4
Formaldehyde 7
Propene (propylene) 6
Ethene (ethylene) 4
Acetylene 4
Acetaldehyde 5
Methanoic Acid 5
Methanol 4
Other Hydrogen Cyanide 10
Hydrogen Fluoride 4
Ammonia 3
Hydrogen Sulfide 450

Discuss your project with our application specialists

Our engineers have decades of experience in creating custom solutions to meet your monitoring challenges