I have received a number of questions from visitors to the Industrial Scientific Corp. online blog covering a multitude of gas detection issues. 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 our blog, 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.
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.
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 SO2 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.
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 protection from a respiratory hazard. So with that in mind, keep breathing, keep safe and keep it within the “breathing zone.”