Gas and Vapour Monitoring
Of all the physical states of matter that may be
encountered at Hazmat incidents, gases and
vapours are the hardest to control. They will
spread further than liquids and solids,
extending safety cordons and potentially lead to
public safety concerns beyond the area of
operations.
Determining the extent of any hazard zone is
key to ensuring the safety of responders and
the public. However, an overly cautious
approach can lead to unnecessary disruption to
the public and critical infrastructure. This places
an unreasonable burden on responders and
other agencies involved in dealing with the
incident.
Several systems provide estimates of hazard
zones for initial responders. These will be the
worst case estimates and should be refined
wherever possible.
Various fixed and handheld monitors can supply
crucial information to help inform a tactical
decision when assigning hazard zones or
providing controls to deployment.
Understanding a monitor’s capabilities and
limitations is vital to ensuring that the data
gained, is interpreted and applied in an
accurate way to ensure safety.
Combined portable gas monitors
These are the most common gas/vapour
monitoring equipment types available to
responders. Often referred to as 4 or 5 channel
monitors, their capabilities vary between
suppliers. Typically, they will have an oxygen
sensor, a flammable sensor and a number of
toxic gas sensors.
Oxygen sensor
Oxygen exists in air at about 20.9 % or
approximately 1/5th with the remaining 4/5ths of
air made up of Nitrogen.
Monitoring the oxygen level will ensure that the
atmosphere is breathable. Most monitors have
a warning alarm at 19.5 % oxygen in air. This is
the level below which the atmosphere is
considered to be oxygen deficient. At 17 % the
symptoms from lack of oxygen may impair your
ability to escape.
Reasons for oxygen deficiency:
1. Oxygen is consumed during combustion,
increasing levels of carbon dioxide and
other combustion gases such as carbon
monoxide.
2. Oxygen is used up by micro-organisms is
some confined spaces such as sewers and
fermentation vessels.
3. Oxygen is consumed when metals rust.
4. Air is displaced by another gas / vapour
either by design such as inert gases or by an
accident release.
If oxygen levels drop, this should trigger control
measures such as breathing apparatus but also
lead to an investigation as to the cause of the
drop. If the reduction cannot be explained by
the first 3 points above, this should trigger the
question, what else is in the air?
Most meters’ sensitivity level will be 0.1 %
before a change can be observed. If 0.1 %
oxygen has been displaced from the
atmosphere, the gas displacing the oxygen must
also have displaced the nitrogen in the air as
well. Given that there is 4 time more nitrogen
than oxygen in the air, a drop in oxygen of 0.1 %
will also mean a drop of 0.4 % nitrogen. This
gives a total of 0.5 % of an unknown gas or
vapour that must be present.
will cause ignition of any flammable vapour
which enters. The process of combustion
increases the heat of the wire and as electrical
resistance is proportional to heat, the meter
then measures this change in resistance.
1 % is equal to 10,000 ppm (parts per million) so
a 0.5 % drop in air means that 5,000 ppm of
something else is present. This is potentially
very significant given that many gases or
vapours are harmful if not toxic when inhaled at
concentrations significantly less than 5,000
ppm, an oxygen meter is a poor substitute for a
toxic gas sensor.
Explosive Limits
Unknown gas
displacing air
Air
20.80%
20.90%
79.10
%
Oxygen
The meter displays this information as a
percentage of the lower explosive limit for the
gas it has been calibrated against. LEL meters
do not display the % of flammable gas in the
atmosphere
Combustible materials in air will only burn if the
fuel concentration lies within well-defined
lower and upper bounds referred to as
flammability limits or explosive limits.
0.50%
78.70
%
Nitrogen
Oxygen
Unknown gas
Nitrogen
Oxygen meters will also identify oxygen
enrichment in air. Most alarm at 23.5 %. Oxygen
enrichment may be produced by certain
chemical reactions, but is most commonly
encountered by leaking oxygen cylinders under
pressure. Oxygen enriched atmospheres
present a significant fire and explosion risk.
Flammable sensor
Also referred to as explosimeters or LEL meters,
these are non-specific meters that will detect
the presence of any flammable gas or vapour.
The majority of meters work using the same
principles. A combustion chamber within the
meter is fitted with a flashback arrestor to
prevent ignition of the main body of the vapour.
Within the combustion chamber a heated wire
Lower explosive limit (LEL): The lowest
concentration (percentage) of a gas or a vapor
in air capable of producing a flash of fire. At a
concentration in air lower than the LEL, gas
mixtures are "too lean" to burn.
Upper explosive limit (UEL): Highest
concentration (percentage) of a gas or a vapor
in air capable of producing a flash of fire.
Concentrations higher than UFL or UEL are "too
rich" to burn.
Calibration
Any combustible gas or vapour will produce
heat and therefore increase resistance when
combusted. However, each gas or vapour will
have different levels of resistance increase
response depending on their molecular size and
the energy released when burnt.
The meter will be calibrated to a specific gas.
Remember every gas has different explosive
limits, the meter will only display a true
percentage of the LEL if it is measuring the
same gas it was calibrated with. Manufacturers
provide conversion tables for common gases,
giving a conversion factor (CF) to correct the
displayed value to the true value.
Some meters will have these conversion factors
programmed to enable selection of the gas or
vapour present and display the true value
without any further calculation needed.
Owing to these factors, when measuring an
unknown gas, it is critical that a safety margin is
built into any interpretation of the risk. With
the first alarm typically set at 10 % of the LEL of
the calibration gas, if the meter is less sensitive
(to the gas being measured), the meter will be
displaying less than the true value. This should
be a clear signal to take additional control
measures. If the responder is in chemical
protective clothing, then a first alarm at 1 % of
LEL should be considered, as chemical
protective clothing generally provides no
thermal protection.
Response time and T90
Another parameter which is safety critical to
understand is response time. In gas detectors
the response time is defined as the time it takes
the output of the sensor to reach 90 % of the
value it will finally reach when exposed to a
change in gas concentration at its sample point;
it is written as T90. The overall response time of
a gas sensing system is governed by three
factors:
 Time taken to transport the sample to
the sensor.
 Time it takes for the gas-sensing
mechanism to respond (this is
dependent on the type of sensor.
 The response time of the signal
processing electronics;
Having an understanding of response time is
key to safe operation of gas monitoring
equipment. For example, moving forward into a
flammable vapour cloud, the lag time on the
monitor could create the following situation.
During the time taken for the monitor to alarm
at its lower value of 10 %, the person deploying
could have continued forward and actually be in
a much higher concentration. This effect,
combined with the potential lower sensitivity in
response for gases other than the calibration
gas, discussed earlier, could mean that the
operator is potentially in an ignitable
atmosphere. It is therefore safety critical that
any operator is fully trained to understand
these effects in order to ensure safe
deployment.
Typical response times can range from a few
seconds to half a minute or more. The response
time will always be higher on passive monitors
compared to those with a pump fitted.
Toxic Sensors
Most gas monitoring manufacturers have a
range of toxic sensors that can be selected at
the point of purchase, however the industry
default for sensors are carbon monoxide and
hydrogen sulphide.
These sensors are electrochemical gas sensors
that measure the concentration of a target gas
by creating a chemical reaction that in turn
generates a small, measurable current which is
proportional to the concentration. There is a
finite amount of chemical present in the cell so
each time the cell is exposed to the target gas, a
small amount of the chemical is consumed. This
means that these cells have a limited life which
will be shortened further, following exposure to
very high concentrations. The electrochemical
sensors can also be affected by other nontarget chemicals creating either a crosssensitive response or “poisoning” the sensor.
These sensors measure in parts per million
(ppm) and are usually pre-programmed with 3
alarms. The first of these is usually similar to the
Workplace Exposure Limit (WEL) for the
substance.
Carbon monoxide – Increasing hazard with
concentration
LEL 12.5%
AEGL3
1,700ppm
UEL 74%
AEGL2
400ppm
LD50 3,760ppm
IDLH 1,200ppm
WEL 30 & 200ppm