"pentane equivalent" calibration gas mixtures

"pentane equivalent" calibration gas mixtures
July 10, 2003
Applications Note: Use of "pentane equivalent"
calibration gas mixtures
Introduction
The gas that is used to verify accuracy is every bit as important as the detector itself when it
comes to worker safety. Choosing (and using) the right mixture is critical to the success of your
atmospheric monitoring program.
BW understands the importance of calibration, and we are always on the lookout for ways to
improve the process. To that end, BW is pleased to announce the introduction of a new series of
"pentane equivalent" calibration gas mixtures. These mixtures are designed to provide an even
more dependable (and easy) means for the verification of accuracy of instruments that include a
sensor for the detection of combustible gas.
What are "pentane equivalent" mixtures and why are they better?
Using BW’’s new "equivalent" mixtures is exactly like using our older mixtures. In the case of the
GasAlertMicro and GasAlertMax, all you have to do is press the "cal" button, attach the adaptor,
and flow gas to the sensors. All the adjustments are made automatically.
The difference comes from what's in the cylinder of gas. In the past, BW has usually
recommended using mixtures that include 50% LEL methane as the combustible gas used for
general purpose calibration. BW's new equivalent mixtures are still based on methane, but in
concentrations that are designed to produce a level of sensitivity "equivalent" to that provided by
a mixture that contains a 50% LEL concentration of pentane.
The reasoning behind the development of these new formulations has to do with how combustible
sensors detect gas, and what happens to sensitivity in the event that a combustible sensor ever
becomes "poisoned". If sensitivity is lost due to poisoning, it tends to be lost first with regards to
methane. A partially poisoned sensor might still respond accurately to pentane, while showing a
dangerously reduced response to methane. BW’’s equivalent mixtures eliminate this potentially
dangerous source of calibration error. Because BW’’s equivalent mixtures are based on methane,
any loss of sensitivity to methane is detected (and can be corrected) immediately.
Environmental conditions can have an effect on sensor accuracy
There are three types of sensors which are commonly used in confined space monitors; oxygen,
combustible gas (LEL), and toxic gas sensors. Each type of sensor uses a slightly different
detection principle. The kinds of conditions that can affect accuracy vary from one type of sensor
to the next. The type of sensor that is most prone to being affected by the atmosphere in which it
is being used tends to be the combustible sensor. Age and usage can have a serious effect on
sensitivity. Chronic exposure to silicone containing substances (found in many lubricants), the
tetra-ethyl-lead found in "leaded" gasoline, halogenated hydrocarbons (Freons®, or solvents such
as trichloroethylene and methylene chloride), high concentrations of hydrogen sulfide or even
very high concentrations of combustible gas may all lead to degraded combustible sensor
performance. In most cases all this means is that the sensitivity is adjusted upwards at the time
the instrument is calibrated. In the worst case, the sensor may need to be replaced. Verifying the
accuracy of the sensors on a regular basis is essential to assuring worker safety.
July 10, 2003
Applications Note: Use of "pentane equivalent"
calibration gas mixtures
Page 2
How combustible sensors work
The minimum amount of a combustible gas or vapor in air that will explosively burn if a source of
ignition is present is the Lower Explosive Limit (LEL) concentration. Combustible gas readings
are given in percent LEL, with a range of zero to 100 percent explosive.
Combustible sensors contain two coils of fine wire coated with a ceramic material to form beads.
The "active" bead is coated with a palladium-based material that allows catalyzed combustion to
occur on the surface of the bead. The "reference" bead lacks the catalyst coating, but in other
respects exactly resembles the active bead. Any combustible vapors that are present will be
subject to catalytic combustion on the surface of the active bead, heating this bead to a higher
temperature. The temperature of the untreated reference bead is unaffected by the presence of
gas. The difference between the temperatures of the two beads is proportional to the amount of
combustible gas present. Since the beads are strung on the opposite arms of a Wheatstone
Bridge electrical circuit, the instrument perceives this as a change in the electrical resistance in
the circuit. It is this change in resistance due to differential heating that is used by the instrument
to provide a reading.
Relative response
BW combustible gas sensors are non-specific and respond to all combustible gases and vapors.
It is not necessary for the combustible vapor to be present in LEL concentrations. Even trace
amounts of combustible gas can be detected by this method.
Catalytic hot bead sensors respond to a wide range of ignitable gases and vapors. The amount of
heat produced by the combustion of a particular gas/vapor on the active bead will reflect the heat
of combustion for that substance. Heat of combustion varies from one substance to another. For
this reason readings vary between equivalent concentrations of different combustible gases. The
amount of heat provided by oxidation of the molecule on the active bead surface actually is
inversely proportional to the heat of combustion for that gas. This occurs because of differences
in molecular interaction with the catalytic surface. In general, the larger the size of the molecule,
the greater the heat of combustion. On the other hand, the smaller the molecule, the more readily
it is able to penetrate the sintered surface of the bead, and interact with the catalyst in the
oxidation reaction.
A combustible gas and vapor reading instrument may be calibrated to any number of different
gases or vapors. If an instrument is only going to be used for a single type of gas over and over
again, it is usually best to calibrate the instrument to that particular hazard. As long as the gas
that is encountered is the same gas that was used during calibration, the readings will be exactly
accurate (to the tolerances of the instrument design). This is what is illustrated in Figure 1.0:
July 10, 2003
Applications Note: Use of "pentane equivalent"
calibration gas mixtures
Page 3
Figure 1.0: Linear response to the gas used in calibration
Note that in a properly calibrated instrument, a concentration of 50 percent LEL produces a meter
response (reading) of 50 percent LEL.
Figure 2.0 illustrates what may be seen when a combustible sensor is used to monitor gases
other than the one to which it was calibrated. The chart shows the "relative response curves" of
the instrument to several different gases.
Figure 2.0: Relative response curves
Note that the response to the gas to which the instrument was calibrated, the "calibration
standard", is still precisely accurate. For the other gases the responses are a little off.
In the case of some gases the readings are a little high. This results in the instrument going into
alarm a little bit early. This type of error is not usually dangerous, since it simply results in workers
exiting the affected area sooner than they otherwise would have.
July 10, 2003
Applications Note: Use of "pentane equivalent"
calibration gas mixtures
Page 4
Gases that produce lower relative readings than the calibration standard can result in a more
potentially dangerous sort of error. One way to reduce the potential for this type of error is to use
a lower alarm setting. It may be seen from the graph that the amount of relative error decreases
the lower the alarm point is set. If the alarm point is set at 10 percent LEL, the differences due to
relative response of the combustible sensor are minimal.
Choosing the right calibration mixture
The other method for reducing the effects of this sort of error is in the choice of the calibration gas
used to calibrate the combustible sensor. The best results are obtained when calibration is done
using the same gas that is expected to be encountered while actually using the instrument. When
it is not possible to calibrate directly to the gas to be measured, or when the combustible gas is
an unknown, a mixture which provides a sensor response that is more typical of the range of
combustible gases and vapors that will be encountered should be selected.
Relative response may be expressed as a ratio and presented in the form of a table. Table 1.0
lists the expected response of a sensor that has been calibrated to pentane, pentane or methane
to a selection of other combustible gases. The closer the relative response comes to 1.0, the
more accurate the reading. For instance, if the sensor is calibrated to pentane, then exposed to
hexane, the response ratio is so close, (0.9 to 1), that for all intents and purposes any error is
trivial.
Combustible Gas /
Vapor Hydrogen Methane Propane n-Butane n-Pentane n-Hexane Relative response
when sensor is
calibrated on
pentane 2.2 2.0 1.3 Relative response
when sensor is
calibrated on
propane 1.7 1.5 Relative response
when sensor is
calibrated on
methane 1.1 1.0 1.0 0.65 1.0 0.75 0.5 0.8 0.6 1.2 0.6 0.7 0.45 2.3 1.75 1.15 Isopropyl Alcohol 1.4 1.05 0.7 Ammonia 2.6 2.0 n-Octane Methanol Ethanol Acetone Toluene Gasoline (Unleaded) 0.9 0.9 1.6
1.4 0.7 1.2 1.2 1.05 0.5 0.9 0.4 0.8 0.7 1.3 0.35 0.6 Table 1.0. Relative response ratios
As may be seen from Table 1.0, when the instrument is calibrated to methane, readings for many
gases on the list are somewhat low. On the other hand, when calibrated to pentane, most of the
July 10, 2003
Applications Note: Use of "pentane equivalent"
calibration gas mixtures
Page 5
gases on the list will produce readings that are quite close to, or a little bit higher than actual. For
many applications pentane, or a mixture which provides a similar level of sensitivity, is the gas
that's "just right" for combustible sensor calibration.
Table 2.0 lists examples of the equivalent methane concentration that provides the same level of
sensitivity as direct calibration to the listed combustible gases and vapors. By varying the
concentration of methane in the calibration gas mixture, it is possible to simulate the
characteristics of any desired combustible gas.
Combustible Gas / Vapor
Hydrogen
Methane
Ethanol
Acetone
Propane
n-Pentane
n-Hexane
n-Octane
Toluene
Relative response when
sensor is calibrated to 2.5%
(50% LEL) methane 1.1 Concentration of methane used
for equivalent 50% LEL
response
2.75% CH4
1.0 2.5% Vol CH4
0.7 1.75% Vol CH4
0.8 0.65 0.5 0.45 0.4 0.35 2.0% Vol CH4
1.62% Vol CH4
1.25% Vol CH4
1.12% Vol CH4
1.0% Vol CH4
0.88% Vol CH4
This technique is particularly useful for target gases and vapors (such as toluene) that are not
available packaged in field portable cylinders in LEL range concentrations.
Summation
The catalytic "hot bead" sensors used in BW instruments are resistant to poisoning, stable, and
have proven to be an exceptionally dependable sensor design. But it's still important to verify
accuracy on a regular basis. Most importantly, if sensitivity is lost due to poisoning, it is
frequently lost first with regards to methane. The safest approach is to verify the function of the
sensor by testing it by exposure to methane, or a methane based calibration gas mixture.
In most cases the loss of sensitivity is incremental, that is, it occurs a little at a time. In some
cases, however, the loss of sensitivity can be almost immediate. This is the reason that gas
detector manufacturers stress the importance of periodically testing their instruments by exposing
them to known concentration calibration gas, and why use of methane based "pentane
equivalent" calibration gas is such a good idea.
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