Principles of gas detection

1. Principles of gas detection Catalytic, Electrochemical cell, Infrared

2. Core Sensing Technologies • Core sensing technologies • Manufactured by ZA • Lead through innovation • Flammable gases • Catalytic bead • Infrared • Toxic gases • Chemcassette • Electrochemical • Infrared • Used world-wide Electrochemical Catalytic bead Infrared Chemcassette

3. Gas Hazards There are three main types of gas hazard • Flammable • Risk of fire and or explosion,e.g. Methane, Butane, Propane • Toxic • Risk of poisoning,e.g. Carbon Monoxide, Hydrogen Sulfide, Chlorine • Asphyxiant • Risk of suffocation,e.g. Oxygen deficiency, Nitrogen, Carbon Dioxide

4. Flammable Risk oxygen Fuel FIRE Heat • Fire Triangle Three factors are always needed to cause combustion: • A source of ignition (heat) • Oxygen (air) • Fuel in the form of a gasor vapour

5. Each gas / air mixture is ignitable over it’s flammable range European limits EN 61779 In force from July 2003 Some differences to current national standards North American limits NFPA & CSA standards Some differences to European standards Flammable Risk 100% v/v gas 0% v/v air too rich U.E.L. (upper explosive limit) flammable range L.E.L. (lower explosive limit) too lean 0% v/v gas 100% v/v air

6. Flash Point • Flash Point (F.P. oC) • The flash point of a flammable liquid is the lowest temperature at which the surface of the liquid emits sufficient vapour to be ignited by a small flame. • Don’t confuse with Ignition Temperature as the two can be very different:

7. Ignition Temperature • Flammable gases also have a temperature where ignition will take place, even without an external ignition source such as a spark or flame • This temperature is called the Ignition Temperature • Apparatus for use in a hazardous area must does not have a surface temperature that exceeds the ignition temperature • Apparatus is therefore marked with a maximum surface temperature or T rating

8. Toxic Risk • Some gases are poisonous and can be dangerous to life at very low concentrations. • Some toxic gases have strong smells like the distinctive ‘rotten eggs’ smell of H2S • Others are completely odourless like Carbon Monoxide

9. Toxic gas limits & terminology • Time Weighted Average (TWA) • Toxic gas limits related to concentration & time • Short Term Exposure Limit (STEL) • The maximum allowable concentration over 10 minutes. • Long Term Exposure Limit (LTEL) • The maximum allowable concentration over an 8 hour period. • Units of measure • Parts per million (ppm) • Milligrams per cubic metre (mg/m3) • Levels • COSHH • OSHA, NIOSH

10. Toxic Risk 1 million balls 1 red ball • The measurement most often used for the concentration of toxic gases is parts per million (ppm). • For example 1ppm would be equivalent to a room filled with a total of 1 million balls and 1 of those balls being red. The red ball would represent 1ppm.

11. Air is made up of several different gases including oxygen. Normal ambient air contains an oxygen concentration of 20.9% v/v. When the oxygen level dips below 19.5% v/v, the air is considered oxygen-deficient. Oxygen concentrations below 16% v/v are considered unsafe for humans. Asphyxiant (oxygen deficiency) Risk Air Composition

12. Asphyxiant gas limits 100% v/v O2 • Oxygen, Nitrogen • Not flammable or toxic • < 6% v/v O2 FATAL • > ambient changes flammable limits • O2 depletion caused by: • Displacement • Combustion • Oxidation • Chemical reaction 23.0% v/v O2 High alarm 20.9% v/v O2 Ambient 19.0% v/v O2 Low alarm 0% v/v O2

13. Oxygen Enrichment • It is often forgotten that Oxygen enrichment can also cause a risk. • At increased O2 levels the flammability of materials and gases increases. • At levels of 24% items such as clothing can spontaneously combust. • Oxyacetylene welding equipment combines oxygen and acetylene gas to produce an extremely high temperature. • Leaks from the O2 cylinders is the main hazard. • Sensors have to be specially certified for use in O2 enriched atmospheres.

14. Vapour Density • Helps determine sensor placement • The density of a gas / vapour is compared with airwhen air = 1.0 • Vapour density < 1.0 will rise • Vapour density > 1.0 will fall

15. Catalytic gas detection Sensitive Bead ConcentrationMetre ControlCard Non Sensitive Bead • Pellistor (Pellet resistor) • Catalytic beads • Requires Oxygen to operate • 450 - 500OC operating temp • Gas combustion on sensitive bead • Sensitive bead • Platinum wire coil • Rhodium catalyst • Non sensitive bead • Gas coated or restricted inlet • Stability from pressure & temp changes • Poison resistant • Long life porous structure + Gas sinter _

16. Catalytic gas detection pros & cons • Speed of response • 10-20 seconds (T90) • Sensitivity • 0-20 & 0-100% LEL options • Not fail safe • Poisoned by: sulphurs, silicones, phosphors & Leads • Inhibited by: Chlorinated & Fluorinated hydrocarbons • Low powered • Typically 200mA allows reduced battery back up • Costs • Low initial cost • High routine maintenance costs

17. Electro-chemical cells • Speed of response • 10-90 seconds (T90) • Sensitivity • Part Per Million (ppm) • Life • 1 to 2 years • Not fail safe • Except O2 deficiency • Low powered • Ideal for portable devices • Costs • Low initial cost • High routine maintenance costs Gas permeable membrane Measuring electrode V Electrolyte Reference electrode

18. Electro-chemical cells pros & cons • Speed of response • 10-90 seconds (T90) • Sensitivity • Part Per Million (ppm) • Life • 1 to 2 years • Not fail safe • Except O2 deficiency • Low powered • Ideal for portable devices • Costs • Low initial cost • High routine maintenance costs

19. Infrared Gas Detection • Infrared detection is a method of using light to detect the presence of flammable gas hazards. • Light is made up of many different wavelengths known as the Electromagnetic Spectrum. Infrared Light Wavelength Light Source Prism

20. All flammable hydrocarbon gas molecules absorb light in the infrared region of the Spectrum. The specific wavelength used is dependent on a number of factors including gas type to be detected, interference from others gases, strength of signal and effect of water vapour. This absorption characteristic can be used as the basis of a hydrocarbon gas detector. Infrared Gas Detection IR Spectrum of Methane Gas at 3.4 microns

21. Infrared Gas Detection • Infrared gas detectors compare the amount of light at a wavelength where hydrocarbon gas molecules absorb light with an area of the spectrum where no such absorption occurs. • The absorption wavelength is know as the sample wavelength and the wavelength at which no absorption is expected is known as the reference wavelength. • The measurement made is the change in ratio between the sample and reference signals.

22. Infrared Gas Detection Sample & Reference signal strengths IR Source IR Detectors R S Fog, Rain,Snow,Dirt S R Gas S R

23. Infrared Gas Detection Pros & Cons • Fast speed of response • >5 seconds (T90) • Sensitivity • 0-100% LEL or LEL Metres • Works in Oxygen free atmospheres • Fail safe • Self checking diagnostics • Costs • High initial cost • Low routine maintenance costs

24. Semi-Conductor Gas Detection Voltage Source Meter Metal Oxide Silicon Heater • Metal oxide film deposited onto a silicon slice • Similar to computer Silicon ‘Chips’ • Surface heated to 200-400ºC depending on design • Thin film, Thick Film, MOS, MMOS • Absorption of the sample gas on the oxide surface plus catalytic oxidation • Causes change of electrical resistance • Resistance change can be related to sample gas concentration. Electrode Gas Sample

25. Used in cheap domestic detectors in Japan High volume low cost Popular for H2S detection High temp / high humidity applications Low temp / low humidity applications Limited range of gases susceptible to cross interferences from other gases Non Linear output Added circuitry/complexity of reading Susceptible to environmental fluctuations Some designs can ‘fall asleep’ Negative signal drift Frequent gassing often required Some designs very high power Higher cost of battery back up Costs Low initial cost High routine maintenance costs Semi-Conductor Gas Detection

26. Measures the thermal conductivity of the sample gas with another (usually air) Suitable for the measurement of high (%V/V) concentrations of gas The heated sensing element is exposed to the sample The reference is enclosed in a sealed compartment If the thermal conductivity of the sample gas is higher than that of the reference then the temperature of the sensing element decreases Thermal Conductivity Gas Detection Sealed reference gas chamber Sample gas Reference element Sensing element

27. If the thermal conductivity of the sample gas is less than that of the reference then the temperature of the sample element increases. These temperature changes are proportional to the concentration of gas present at the sample element. Mainly used for detecting gases with a thermal conductivity greater than air Eg, Methane and Hydrogen Gases with thermal conductivities close to air cannot be detected. Eg, Ammonia and CO Gases with thermal conductivities less than air are more difficult to detect as water vapour can cause interference E.g Carbon Dioxide and Butane Mixtures of two gases in the absence of air can also be measured using this technique Thermal Conductivity Gas Detection

28. PID Technology: Ionization Cell Gas flow through cell Air molecule VOC molecule UV Lamp cathode anode

29. VOC molecule is Ionized by UV Light Air molecule VOC molecule Ionized VOC Electron + + anode cathode Positive ion moves toward cathode Negative electron towards anode

30. Current generated due to VOC presence Migration of charged species yields a detectable current proportional to the gas concentration + + anode + ve cathode - ve A

31. Paper tape gas detection • Speed of response • 5-15 minutes (T90) • Wide range of standards gases • H2S, Ammonia, Chlorine etc. • Semiconductor doping gases • Arsine, Silane, Diboraine etc. • High Sensitivity • Part Per Billion (ppb) to ppm levels • Low cross sensitivities • Physical evidence of gas • Costs • Higher initial cost to electrochemical cells • High consumable costs

32. Comparison of detection techniques