Monitoring
ASK THE EXPERT:
Dave Wagner Answers Questions about
Gas Detection
Dave Wagner is global director of product knowledge & iNet product
manager for Industrial Scientific Corp. in Pittsburgh. He has hosted a
popular blog on the company’s website, AskDave, since 2011.
By David D. Wagner
I
have received a number of questions from our online
visitors covering a multitude of gas detection issues.
For this month’s Ask the Expert segment, EHS Today
asked me to reprise some of the more interesting and
challenging topics for you. So in the next paragraphs, I
will give you some of the best of AskDave.
Gas Sensor Cross Interferences
Gas sensor cross interferences have been the topic of
many questions over the years. In 2011, firefighters in
Phoenix, Ariz., were called to an incident at a fast food
restaurant after a worker had collapsed walking up the
stairs from the basement. The firefighters, after nearly
being overcome themselves, were surprised to see high
readings on their combustible gas sensors from what
they later learned was carbon dioxide (CO2), which
clearly is not a combustible gas.
12
So, “Do LEL sensors really have a cross interference to CO2?”
Well they do, but not in the way that we are accustomed to seeing cross interference on other gas sensors.
Typical catalytic bead or pellistor LEL sensors are made
up of two coils of very fine wire. The resistance of these
coils will change as they heat and burn gas. The change
in resistance due to the change in temperature produces
the signal that we measure as the gas concentration.
If the thermal conductivity of the atmosphere changes
without the presence of any combustible gas, the resistance of the sensor coils also will change. So, if there is
gas present in a significant enough quantity to change the
thermal conductivity of the atmosphere from what it is
in normal air, the resistance of the sensor elements will
change and a combustible gas reading will be displayed. It
is important to note that the reading from the combustible
sensor in this situation either may be positive or negative.
www. EH STodAY.c om I FE B RUARY 2 0 1 6 I EH S To d ay
The firefighters involved in this incident stated that they observed an oxygen reading of 17.3 percent on their
instrument display. From this reading,
you could calculate that the CO2 concentration in this instance was on the
order of 17.5 percent. This concentration
certainly is high enough to change the
thermal conductivity of the atmosphere
and produce a change on the LEL sensor.
In this situation, you also should note
that because of the high level of CO2,
the oxygen sensor probably was reading
higher than the actual gas concentration
due to interference with that sensor. The
reality of this particular incident was that
the actual oxygen concentration was more
likely in the 15-16 percent range, the CO2
concentration was more in the 23-28 percent range and both the oxygen concentration and carbon dioxide concentration
were presenting very significant hazards.
Understanding the reactions of different sensors in an incident like this is the
key to properly assessing and resolving
the situation. Outside of going into the
basement without a gas detector in the
first place when they suspected a problem, these firefighters did it the right
way. The bottom line is any situation,
when the gas detector alarm sounds or
you sense that something is wrong, GET
OUT first and ask questions later.
Another frequently asked question
is: “Why do I sometimes see negative
readings on my gas monitor?”
All electrochemical or catalytic gas
sensors can be prone to both positive
and negative drift due to environmental
factors such as changes in temperature
and humidity. However, these are not
the most common causes of negative
sensor readings.
Negative sensor readings more commonly occur when your instrument has
been “zeroed” in a contaminated atmosphere, where small levels of the sensors’ target gases are present. When the
instrument is moved to a clean-air environment, the sensors will show a negative reading that corresponds to the concentration on the contaminant that was
present during the zeroing operation.
For example, if there is 5 PPM carbon
monoxide present when the sensor is
zeroed, the reading will be -5 PPM when
the sensor is returned to clean air.
Negative gas readings also may occur
when the sensor is exposed to a gas that
produces a negative cross interference. If
a sulfur dioxide (SO2) sensor, which typically has a -100 percent cross interference
to nitrogen dioxide (NO2) is exposed to 2
PPM NO2, the resultant SO2 reading on
your instrument will be -2 PPM.
Does this mean that you should avoid
using sensors that have negative cross
interferences to each other in the same
instrument? Absolutely not! If you have
NO2 and SO2 present in the same atmosphere, the only way that you can understand the true concentration of each gas
is by having both sensors. In the example
that we used above, if your atmosphere
contained 2 PPM SO 2 along with the
2 PPM NO2, the resultant SO2 reading
due to the negative cross interference
would be zero. The only way that you
could know that you have 2 PPM SO2
present is by recognizing the presence of
the NO2 gas and understanding its effect
on the SO2 sensor. Eliminating one of
the sensors from the instrument does not
eliminate the hazard to which it, and you,
are being exposed.
Customers sometimes will say that
they have never seen a negative reading
on an instrument before but that they recently changed monitors and now seem
to see them all the time. This observation
is because some instruments block negative gas readings from appearing on the
instrument display, showing all negative
readings as zero.
This practice can serve to keep you
from seeing and recognizing the hazards
that exist. If an H2S sensor has an offset
of -10 PPM due to drift or a false zero
operation that has been masked by the
instrument, exposure in a true concentration of +10 PPM still will produce a
zero reading and a concentration of +20
PPM only would be displayed as +10
PPM. This situation would be easier to
recognize if the negative reading was
displayed in the first place.
So, while negative readings are puzzling and uncomfortable to most gas monitor users, they are not always a bad thing.
If you understand the circumstances that
cause the negative readings, you will get
more information from your instrument
and have a better understanding of the
environment you are working in.
EH S To d ay I F E BRUA RY 2 0 1 6 I w w w. E H S To day. c o m
13
Monitoring
OSHA defines the breathing
zone as the area “within a
10-inch radius of the worker’s
nose and mouth.”
Over the years, I have been asked
many questions about the “breathing
zone” and how it relates to using portable
gas monitors intended for personal protection: “What is all this ‘breathing zone’
stuff about anyhow?”
OSHA defines the breathing zone as
the area “within a 10-inch radius of the
worker’s nose and mouth.” That would
indicate that an instrument used primarily for personal protection from toxic
hazards such as H2S should be worn on
the collar, the lapel, on a breast pocket
or even on the brim of a hard hat; somewhere within a 10-inch radius of your
nose and mouth.
Some would suggest that because
gases like H2S are heavier than air that
the instrument used to protect against
them should be worn lower on the body,
around the knees or attached to the top
of the boot. While there may be some
validity to this argument, I believe that
this puts the instrument itself in danger
of being damaged in the working environment or even lost without notice and
may make it more difficult to recognize
that the instrument is alarming in high
noise areas.
You must not ignore the fact that in
most cases, a gas monitor for personal protection is intended to provide direct protec14
tion from a respiratory hazard. So with
that in mind, keep breathing, keep safe and
keep it within the “breathing zone.”
Next question: “Portable gas monitoring instruments typically are operated in a passive (diffusion) mode or
in an aspirated (pumped) mode. How
do I know which one I should use and
whether or not one mode is better than
the other?”
This is a common question that really
has a simple answer, but it does require
some explanation.
To pump or not to pump? That is the
question. Most sensors intended for use
in portable gas monitoring instruments
are designed to operate in the passive
mode. These designs are such that gas in
air diffuses through normal air currents
into openings on the face of the sensor
and accumulates on and reacts with the
sensor’s working electrode. The sensors
will function and perform normally in
a properly designed diffusion based instrument with no help from a sampling
pump at all.
However, there are many who believe
that a pump is necessary to draw air into
the instrument and sensors and that an
instrument with a pump can detect gas
in a wider area than a simple diffusion
monitor. The truth is that the flow rates
w ww. EH STodAY. c o m I FE B RUARY 2 0 1 6 I EH STo d ay
of pumps used with portable instruments
are relatively low, so that the only gas that
is sensed or air that is drawn in is from
the immediate end of the sampling hose
or inlet of the sampling pump. Whether
the instrument is operated in a diffusion
mode or in a pumped mode, it only is a
point detector and only can detect gas that
is at the immediate face of the sensors or
inlet to the sampling pump.
So when should an instrument be used
with a sampling pump? As I said earlier,
the answer is quite simple. The only time
that it is necessary to use a sampling
pump with a portable gas monitoring instrument is when it is necessary to sample
the conditions of the air in an area located
remotely from the location of the instrument. Confined space regulations require
that atmospheres of confined spaces
are tested prior to entry, and to do that,
a pump is required to draw the air from
within the space out to the instrument.
The rule of thumb for whether or not
you need to use a pump with your instrument is that if you are standing at point A
and need to know the gas concentration at
point B, you need a pump.
Finally, I would be remiss if I did not
address the issue of bump testing and calibration. I have answered countless questions for gas detection users and one in
The only way you can be sure that your
gas detector will respond to gas is to
check it with gas.
particular: “Should I be doing a bump
test or a calibration check on my instrument and what is the difference?”
It’s definitely an interesting question
and among the most relevant every day.
By most definitions, a bump test is a brief
exposure of the monitor to gas in order
to verify that the sensors respond and the
instrument alarms function accordingly.
The bump test, by this definition, does
not check the accuracy of the instrument.
This is where the calibration check
comes in. A calibration check is performed by exposing the monitor to a
certified concentration of gas for a particular time to verify that it provides an
accurate reading.
What was confusing to this user was
that the manufacturer of his monitor was
telling him to bump test and verify the ac-
curacy of the monitor before use but was
not specifying how long the gas should
be applied and what the reading tolerance
should be.
In most applications, knowing that the
instrument will respond and produce an
alarm that might save your life if a threatening gas hazard is encountered is all you
need. In other applications, the accuracy
of the reading is more important.
With the instruments available today,
if you are concerned about the accuracy
of your readings before you use your instrument, you are better off to calibrate it
rather than do a calibration check. It will
take the same amount of time, use the
same amount of gas and will guarantee
the accuracy of the instrument readings
when it is completed. If you are doing a
calibration check, and the readings fall
outside of the desired or specified accuracy, you will have to do the calibration
anyway, so you might as well do it the first
time and get the guaranteed result.
In the end, it really doesn’t matter
whether you choose to do the bump test, a
calibration check or a full calibration. Pick
the one that is right for you. The important thing is that before you take your gas
monitor out and use it on a job where an
employee’s life might be in danger, check
it with gas in some manner and verify that
it works properly.
If you follow me, you have heard me
say this many times: The only way you
can be sure that your gas detector will respond to gas is to check it with gas. Do it
every time!
EHS
David D. Wagner is global director of
product knowledge & iNet product manager for Industrial Scientific Corp., Pittsburgh, Pa. You can find his “Ask Dave”
blog at www.askdaveblog.com.
For related content, go to:
ehstoday.com/real-detection
CIRCLE 107 ON READER CARD OR LINK TO THE VENDOR ONLINE AT www.ehsrs.biz/61801-107
E H S To d ay I F ebruary 2 0 1 6 I w w w. E H S To day. c o m
15