1. Sensor Operating Theory
Gas Detection Systems
Sensor Operating Theory

2. Gas/Vapor Hazards- Flammable/Combustible Toxic Oxygen-deficiency
Oxygen-enrichment

3. Gas Detection Objectives
Measure gas before hazardous concentration is present
Provide outputs for Emergency Action
Suitable for extreme environments (arctic, tropic, etc)
Complement to Flame Detection equipment
Cost-effective protection

4. Gas Sensing Technologies
Combustibles: Catalytic and Infrared
Hydrogen Sulfide: Electrochemical & MOS
Oxygen: Electrochemical
Carbon Monoxide: Electrochemical
Chlorine: Electrochemical
Sulfur Dioxide: Electrochemical
Nitrogen Dioxide: Electrochemical

5. Key Gas Detection Definitions
Measurement range: gas concentration at full scale sensor output
Unit of Measurement: %LEL, PPM, or %volume
Flashpoint: minimum temperature of liquid where enough vapor is given off to sustain a fire or explosion
Vapor Density: tendency of a pure gas of vapor to rise or sink in clean air
Calibration: process of matching transmitter signal to sensor measurement

6. Combustible Gases-Lower & Upper Limits*
*No differentiation between the terms “Explosive” and “Flammable” as applied to the lower and upper limits of flammability
Too lean for ignition
Too rich for ignition
Clean Air
Combustibles
Lower
Explosive
Limit (LEL)
Upper
Limit (UEL)
Typical Combustible
Gas Detector Range
of Measurement
50%

7. Flammable Limits & Volumetric Equivalents
Methane Gas: 100% LEL= 5% by volume in air
75% = “
50% = 2.5% “
25% = 1.25% “
10% = .50% “

8. Catalytic Sensor Operation
Contact-based
Destructive measurement
Traditional method
Only option for hydrogen gas detection
Used w/ transmitter module to provide 4-20 ma signal output
Typical life of 3-6 years
2 year mfgr warranty

9. Catalytic Sensor Operation
Active Element - Hot wire resistor coated with catalytic material
Reference Element - Same as active but without catalyst
Catalytic Oxidation of flammable gas generates heat & changes resistance of Active Element
Explosion is prevented by Flame Arrestor
Constant U or I power source required
Must calibrate on start-up & routinely for accuracy
Flammable
gas

10. Catalytic Sensor Operation

11. Catalytic Sensor Limitations
FAIL TO DANGER
Poisoning of Catalyst
Plugging of Flame Arrestor
Read High Gas Levels
Frequent Calibration
Service Life - Variable (Months)

12. Infrared Combustible Detectors
reference
absorption band
active absorption band
Principal of Operation
Gases are Transparent to Visible Light BUT...
Gases are Strongly Opaque to Infrared Light at Some Wavelengths
Methane
Propane

13. Point Infrared Detector
Principle of Operation
Flammable
hydrocarbon
gas

14. Fail-Safe Operation

15. Open Path IR Gas Detector
Instrumental Layout m
Flammable
hydrocarbon
gas
Optical Beamsplitter
Measurement
Signal Detector
IR Source
Optical Filters
Microprocessor
and
Electronics
Reference
Signal Detector
Gas
Concentration
(LEL)

16. Drivers for Optical Technology
Safety Driven Design Requirements
FAIL SAFE
No Unrevealed Sources of Failure
HIGH AVAILABILITY
Long Service Life
Low and Easy Maintenance

17. Electrochemical Toxic Sensor Theory
Specific to toxic gas type (H2S, CO, SO2, NO2, CL2 )
Consists of sensing(working), reference, and counter electrodes
Vapor diffusion into capillary results in oxidation/ion reduction
The signal (current) from the working electrode is converted to a voltage by operational amplifier
Capillary Entrance
O-ring
Membrane
Electrolytic fluid
Sensing Electrode
Reference Electrode
Counter Electrode

18. Electrochemical Toxic Sensor Operation
All provide 4-20 ma signal output
0-100 ppm range typical
Carbon monoxide ranges: 0-100, 0-500, or ppm- must specify
Nitrogen dioxide range: 0-20 ppm
Hydrophobic filter provides humidity protection
All designed to meet X-proof req. except CL2, SO2 sensors (I.S. barrier required)
All designed for calibration by a transmitter, controller or external control system.

19. Traditional Sensor Calibration
Matching transmitter output to raw sensor signal in clean air (zero) & mid-scale (span) conditions
Manual or Automatic depending upon transmitter type
Adjustments necessary if drifting has occurred
Requires compressed, accurate calibration gases
Always start-up & after sensor replacement
Every days thereafter typical

20. Gas Detector Calibration Requirements
Catalytic Sensors: Field Calibration required for accuracy
DEC IR Detectors: Routine calibration not required; Factory calibrated for methane only
Electrochemical Sensors: Field Calibration required for accuracy
DEC Open Path IR System: Routine calibration not required; Factory calibrated for methane only