© Chevron 2009
Gas and Vapor
Detection
Awareness Plus
Rev. 1
© Chevron 2009
Outline
Critical Components for Gas Testing include:
 During planning stage determining what types of
gas test is required
 When applicable refer to the MSDS Sheets for the
product the equipment to be isolated
 Verifying the gas testing instruments to be used
are in good operating condition
 Ensure that all safety precautions required to
perform gas testing are in place and followed
 Conduct required gas tests
 Evaluate, interpret and record the test results
We will also look at the operation principles of
some portable gas detection instruments to
understand some of their shortcomings.
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Introduction
 In the experience of a leading expert in the
field of gas and vapor testing the majority of
people he has encountered over the years of
training and discussion, mainly in the
petroleum industry, have led him to the
conclusion that most could have been or were
using even the simplest of gas detectors in
misleading, and possibly dangerous ways.
Such misuse of the gas detector can not only
produce erroneous readings, but also
endanger the lives of everyone in the vicinity.
Note: The safety of everyone involved depends upon
the recognition of potential contaminant gases or vapors
and the correct selection and use of the gas detection
instruments to measure these.
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Why do we need to perform gas
testing?
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Determining Gas or Vapour
Concentrations
 Gas testing is undertaken to determine
the presence of flammable and or toxic
vapour or gas, and at what
concentration are present.
 Gas Testing is done to ensure that no
hazardous work conditions exist from
the presence of such gases or vapors.
 The principal areas of concern for
human safety are:
 Determining oxygen levels (deficiency or enrichment)
 Determining explosive risks (flammability)
 Determining health risks (toxicity)
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Planning for Gas Tests
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Prior to commencing work in an area which may
contain hazardous gases or vapors the person in
charge should identify hazardous substances or
conditions that may require gas testing, such as:
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Oxygen deficiency or enrichment
Flammable gas or vapor
Toxic vapors such as:
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benzene or organic lead
Hydrogen sulfide
Carbon monoxide
Carbon dioxide
 During the planning stage for proposed work the
following should be determined;
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The types of gases that need to be tested for;
What the acceptable limits are;
The frequency of gas testing required
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Calibration and Response Testing
 All gas detection equipment shall be calibrated
in accordance with the manufacturers
guidelines and/or local standard requirements.
 Field calibration testing should be done and the
results recorded on a monthly bases.
 The gas test instrument should be response
checked (bump tested) before each work shift,
or before use if not used on every work shift.
 Response check (bump test) requires the
manufacturer’s calibration gas to be used.
 Response checks/bump tests should be carried
out using the manufacturer’s recommended
test procedures and equipment
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Review Manufacturers Precautions
 Prior to using gas detection equipment the user must be
aware of any manufacturers precautions. Review the
Owner’s Manual and Operating Manual for your specific
gas detection equipment (if you don’t have these, get
them!).
 These may vary depending on the manufacturer and type
of sensor being used.
 Typical warnings that may apply include:
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Oxygen deficient or oxygen enriched environments may cause
erroneous LEL readings.
Ensure sufficient battery charge before use.
Only recharge or change batteries in a non-hazardous area.
Do not draw liquid into the sampling line.
Only zero the instrument in clean air.
Sampling environments with more than 100% LEL may provide
erroneous LEL readings.
Do not remove instrument from it’s case when in a hazardous
environment.
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Conduct Gas Tests
 Only Qualified Gas Testers are authorized to
conduct gas tests
 Many client Company will not lend their gas
testing equipment to contractors, they require
them to have their own gas testing equipment
 Test the work area atmosphere for the
identified hazards.
 Testing should start in a known safe area and
move into the work area to be tested.
 Tests should be done in the following order:
 Oxygen
 Flammable gases
 Toxic vapors
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Conduct Gas Tests cont.
 Test in area where flammable or toxic
gases/vapors are likely to accumulate.
 Any tests which indicate readings outside the
acceptable range should result in the area
being cleared of all personnel and the area
ventilated until a safe environment is
obtained.
 Appropriate PPE must be worn when testing
(e.g. respiratory protect required when initially testing
inside a confined space such as a tank).
 Any alarm activation on the test instrument
should result in an immediate evacuation of all
personnel out of the area until the reason for
the alarm is determined and corrective
measures taken.
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Interpreting Gas Test Results
 All work must be stopped whenever a test indicates
a hazardous condition exists
 Gas detection instruments may read out in a number
of scales:
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ppm - parts per million (or ppb, parts per billion)
mg/m³ - milligrams per cubic metre
% v/v – percentage volume per volume
% LEL – percentage of lower explosive limit
 It is important to understand what measurement
scale is being used on the instrument you have and
ensure this matches the acceptable limits scale.
Note: some multi-gas instruments may read in several different
ways (e.g. read flammable atmosphere in %LEL, oxygen level in
% v/v, and H2S level in ppm)
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Typical Scenarios for Gas Testing
 The following are some typical
examples where gas testing would be
required at a facility:
 Before performing Hot Work or entering a
Confined Space.
 When responding to a spill of hazardous
materials.
 Entering an oily water separator to clean or
carry out repairs.
 Entering inside a bulk storage tank, or any
other confined space.
 Downwind of a tank being degassed, including
boundary measurements if near a site
boundary.
 Carrying out welding (or other hot work)
inside a tank compound.
 Opening of live electrical boxes inside a
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Gas Detection Devices
 Instruments may use a number of different
principles of operation to measure gas and vapor
concentrations.
These
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may include:
Colorimetric
Catalytic combustion
Electrochemical
 Instruments in common day-to-day used typically
have catalytic combustion sensor for LEL and
electrochemical sensors for Oxygen, HES, CO, etc.
 All Sensors have a limited life expectancy, refer to the
manufacturer’s recommendations.
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Catalytic Combustion
 Flammable Gas
Detection Principal
 Catalytic combustion
on heated filament
 Filaments are part of
Wheatstone bridge
 Increase in
temperature causes
an imbalance in the
bridge and an
increased current
flow
 Current flow is
measured and is
proportional to
concentration of
combustible gas
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Catalytic Combustion
Instruments
 Require oxygen to be present to enable
combustion process.
 There must be a minimum of 16% Oxygen present
for LEL sensor to read properly.
 Only reads the sample that passing the sensing
wires where the catalytic reaction occurs.
 Require the sample to be ‘drawn’ past the
sensing wire (at a defined rate of flow).
 Refer to the Gas Test Instrument’s manufacturer’s
specifications
 Give an indicative reading only, not a specific
concentration reading.
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Filament ‘Poisons’
 Poisoning of filaments results in loss of catalytic
ability which leads to reduced sensitivity.
Some typical ‘poisons’ for catalytic elements
include :
 Tetraethyl lead.
 Silicon containing products (e.g. furniture polishes,
silicone rubber compounds, etc.).
 Halogenated hydrocarbons.
 Some hydraulic fluids and lubricants (if very hot).
 Volatile sodium and potassium compounds.
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Electrochemical Sensors
 Essentially the same principle as a battery.
 They use two ‘electrodes’ in a cell, separated
by an ‘electrolyte’.
 Sample is normally by diffusion onto the cell.
 Works by ions being collected which creats an
electron flow in the presence of the target gas
e.g. Oxygen + Lead = Lead Oxide + Electricity
 Flow of electrons created is directly
proportional to the amount of gas present.
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Oxygen Meters
 Detection Principle
 Galvanic cell filled with electrolyte
 Oxygen diffuses into cell and goes into solution
 Factors that may Influence Accuracy
 Temperature
 Altitude
 Humidity
 Limitations/Advantages
 Oxygen specific – no significant interferences
 Calibrate the instrument at the temperature and
altitude at which it will be used.
 Once opened, the oxygen cell has a limited life span.
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Typically 1 - 2 years (depending on manufacturer)
Replace the cell when it reaches the end of its life
expectancy or;
Replace the cell whenever it can no longer be
calibrated to 20.9%
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Use of Colormetric Pump
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A colormetric pump and relevant tubes are often
used to test for the presence and concentrations
of various toxic vapors that may be present.
For example there are colormetric tubes in various
ranges for detection of Benzene, Carbon Monoxide,
Carbon Dioxide, Mercury, Hydrogen Sulfide, etc
 Colormetric pumps and tubes may also be used
to determine the total hydrocarbons present at a
job site to determine if respiratory protection is
required.
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Detection Requirements

In order for a gas detection instrument to work
correctly:
 The product being detected must be in a vapour or
gaseous state.
 The instrument must be drawing a sample from where
the gas/vapour will have accumulated. (i.e. Is the
gas/vapour potential to be present lighter or heavier than
air).
 Must ensure the gas test instrument’s sensor filament
has not been ‘poisoned’ by conducting required bump
testing.
 The equipment must be given time to allow temperature
of the instrument to stabilize if it is stored in a location
where the temperature is different that the location tests
to be performed in.
 Be aware of CHANGING CONDITIONS.
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Other Issues to Remember
 Avoid sucking liquid into the instrument
 Ensure any calibration gas is the correct gas and
concentration for the instrument to be calibrated.
 Response check (bump test) kits are generally not
interchangeable between various manufacture
models.
 Most gas detection instruments are indicators only
so treat any reading as a potential danger until
you can validate the reason for the reading.
 Adopt a ‘Read and Run” principle to gas testing –
always be prepared to retreat to a safe area when
testing.
 Gas testing for toxic vapors such as Benzene, etc
is normally be performed using a colormetric
pump and tubes of the proper type and range.
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Other Issues to Remember (cont)

Flammable hydrocarbons with a high flash point
will not under normal conditions give off vapors
unless their flashpoint temperatures are reached.
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For example diesel will not give off any vapors at
ambient temperatures less that its flash point.
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Gas Testing – Additional
Information
For further instructions and additional information for Gas
Testing please refer to:
 Gas Detection Safety Bulletin
 Your companies operating procedures and gas
testing equipment manufacture’s Owner’s Manual
and Operating Instructions;
 Your Company’s Safety Officer
 The clients Safety Department’s Representative
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