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Photoionization detectors (PID) have
featured in Dräger Safety's product range
for quite some time now. A while ago, for
example, the Dräger Multi-PID was presented
(see Dräger Review 86 and 90), and
has now been superseded by the ATEXcertified
Multi-PID 2. As the basic functional
principle of a PID was already described in
detail in Dräger Review 86, this article will
discuss the new features of the Multi-PID 2
and its variety of possible applications.
Dräger Multi-PID 2: new device,
new possibilities
The Multi-PID 2 is the direct successor to
the Multi-PID 1, and remains unchanged as
far as its detector technology is concerned.
The Multi-PID 2 still offers high measurement
sensitivity, accuracy, reproducibility
of the measurement signal and low crosssensitivity
to water vapour (humidity). The
Multi-PID 1's familiar substance library and
response factors have also been retained,
meaning that no correction factors need
to be taken into account when personnel
switch to using the Multi-PID 2. As regards
handling and the instrument's display, however,
the previous model has undergone
some important changes:
Substance name displayed on screen.
The name of the substance currently
calibrated is displayed in plain text on the
screen, above the substance concentration.
If other substances occur at the site
of measurement in addition to the one
currently being detected, the total sum of
organic substances is displayed, based on
the calibrated substance.
Additional concentration information displayed
on screen.
As well as the current
concentration, the 15-minute average
value (STEL), the 8-hour average value
(TWL), the maximum recorded concentration
(PEAK) and the elapsed measurement
time are displayed on the screen in
a smaller font size.
Data recording mode displayed on screen.
In the top right-hand section of the screen
a symbol indicates whether the interval
mode (i.e. continuous data recording) or
the TAG mode (i.e. manual data recording)
is activated, or whether no data are
currently being recorded. Just like its
predecessor, the Multi-PID 2 can store
15,000 measured values. The Dräger
GasVision then allows convenient processing
and archiving of these data.
GasVision outputs the maximum,
minimum and average readings for each
measurement interval.
One-key calibration.
Because PIDs
generally require daily calibration, this
process has been simplified in the Multi-
PID 2 through the use of just one key.
This means it is no longer necessary to
work through several steps in the menu,
so calibration could not be easier to
perform.
Five languages in one device.
While users of the Multi-PID had to choose a
language at the time of purchase, the
Multi-PID 2 allows the operating language
to be changed at will: German, English,
French, Spanish and Italian are available,
so personnel with different mother
tongues can easily use the device too.
Applications
PIDs offer multifunctional use when it comes
to measuring organic and some inorganic
substances. The main difference as compared
with other sensors (e.g. catalytic
Ex-sensor or infrared Ex sensor) is the fact
that the PID is also able to conduct
measurements right down to the sub-ppm
range (with detection limits of approx. 0.5
ppm). A number of typical applications are
described below.
Leak detection and environmental
analysis
Thanks to its rapid response time and high
level of sensitivity, a PID is well-suited to
detecting leaks in many different settings,
e.g. in pipelines in refineries, in tanks in
chemicals factories, refineries and petrol
stations, and on ships and other means of
transport. If a leak is identified or an accident
involving hazardous goods occurs, it is
of course important to determine whether
the soil in the vicinity of the leak has been
contaminated. Thanks to its high sensitivity,
the Multi-PID 2 is also suited to such tasks,
as well as to conducting a preliminary classification
of soil samples (by way of a screening
test to decide whether a laboratory
analysis is necessary).
Arson investigation
Arson investigations are often conducted to
determine whether fire accelerants such as
petrol, hydrocarbons or alcohol were used.
When police or fire officers inspect the
scene of the fire, the Multi-PID 2 is used to
search for any traces of such substances. In
these situations the Multi-PID 2 is typically
used with the sampling hose as a probe,
allowing sites to be accessed which would
otherwise be difficult to reach. |
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Entering aircraft tanks and
other confined spaces
Aircraft tanks constitute a particularly
extreme example of confined spaces. Since
maintenance and inspection personnel
have to conduct their checks right into the
furthest corners of the tank, the use of respiratory
protective equipment is only possible
to a limited extent. This makes it all
the more important to perform an "all-clear
measurement" to ensure, for example,
that the kerosene concentration is below
10 ppm. Because catalytic Ex and infrared
sensors cannot reliably measure such low
concentrations of organic substances,
this situation is also a typical example of a
PID application. In other tanks used to
store organic substances too, an all-clear
measurement should be conducted using a
PID before personnel enter the tank. There
is a clear trend towards assessing organic
substances not only with respect to the
explosion hazard they pose, but also with
respect to their toxicity. This is something
which can be done for a wide range of
organic substances using the Multi-PID 2.
Fumigation
Methyl bromide is still used, particularly
when timber is transported overseas, but
also during transport of textiles. When unloading
containers, or indeed when they are
inspected by customs officials, it is important
to ensure that concentrations of methyl
bromide and any other fumigants are below
the permissible limit values. The advantage
offered by the Multi-PID 2 here is that the
instrument is able to cover an additional
range of measurement. Concentrations of
between 0.5 and 2000 ppm can be detected
as standard, and the measurement range
can be extended to 20,000 ppm if the dilution
probe is used. This makes the Multi-PID
2 suitable both for monitoring the efficiency
of the fumigation process and for conducting
measurements of relevance to health
protection.
Measuring diesel vapours
Diesel is a mixture of different long-chain
hydrocarbons and is used as fuel in cars,
agricultural vehicles, trains and ships. Up
until now, attention was generally focused
primarily on measuring concentrations of
diesel fuels in the Ex concentration range
(the lower explosive limit range, or LEL).
However, people's health may also be at
risk below this concentration range. Bearing
this in mind, the "American Conference
of Governmental Industrial Hygienists“
(ACGIH) issued a new limit value for the
8-hour mean value in 2002. For the hydrocarbons
in diesel, this limit is 100 mg/m3,
equivalent to approx. 15 ppm. Diesel concentrations
in the toxic concentration range
are relevant when it comes to entering or
cleaning tanks, and also during refilling and
fuelling activities. The Multi-PID 2 enables
diesel concentrations in the ppm range to
be determined. Because diesel is a mixture
of different hydrocarbons, no universally applicable
response factor can be stated. For
lighter diesel types the instrument can be
calibrated to nonane and a response factor
of 1.4 can be used. For heavier diesel types,
such as those used in the shipping industry
for example, a response factor of 1 can be
applied.
Measurements in the chemicals and
petrochemicals industry
This is without doubt the main area of application
for PIDs. Especially when personnel
are to enter tanks, pipelines and other confined
spaces, it is important to ensure that
the concentration of the substances stored
inside them is sufficiently low to prevent any
harm being caused to the health of personnel.
The advantage of the PID, once again,
lies in the fact that it allows measurements
in the ppm range and that it can be calibrated
to different substances. Using the Multi-
PID 2's calibration memory, different substance
calibrations can be stored inside the
instrument. This means that toluene can
be measured first, for example, followed by
an acetone measurement sometime later.
Calibration of the individual memories can
be performed prior to the measurement process,
so that only the calibration memory in
question needs to be selected when it is
time for measurement. A further application
in the chemicals and petrochemicals industry
besides confined space measurements is
of course classic workplace monitoring, for
example when concentrations of hazardous
substances need to be checked during
particular work activities (e.g. filling of substances
from one container to another).
In conclusion, it is clear that the Multi-PID
is able to perform measurements in many
kinds of different applications and work
areas. Through Dräger Safety's technical
applications service, users have access
to far more response factors than those
already stored in the substance library of
the Multi-PID 2. For substances not listed
there it is possible to determine whether
they can be measured by means of PID.
Oliver Schirk
Dräger Safety AG & Co. KGaA
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Dräger Safety AG & Co. KGaA |
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Revalstrasse 1 |
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23560 Luebeck, Germany |
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Tel +49 451 882 0
Fax +49 451 882 2080
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