Gas Detection 13
Figure 2: Changes in humidity as well as exposure to certain contaminants can affect the output of gas detecting sensors in fresh air.
• How can I avoid negative readings?
The best way to reduce or avoid negative readings is simply wait a little longer BEFORE making an Auto Zero adjustment after initially turning the instrument on (Figure 1). Many types of sensors, especially PID and pellistor type LEL sensors, start out with an initially higher output signal, then “count down” as the sensors fi nish warming up in fresh air. To avoid problems make sure the instrument is located in fresh air that does not contain measurable contaminants, and give the sensors time to stabilise completely before using the instrument or making a fresh air adjustment.
Remember that stabilisation can sometimes take quite a bit of time. While pellistor type LEL sensors usually stabilise completely within 6 - 8 minutes, electrochemical sensors that have been exposed to certain interfering contaminants can sometimes take an hour or longer to clear completely. For instance, CO sensors are equipped with an internal fi lter to prevent or limit the sensor’s response to VOC contaminants. However, once the fi lter is saturated, the sensor responds very strongly to solvents and VOCs such as methanol, MTBE (methyl tert-buytyl ether) and toluene. CO sensors are also very responsive to acetylene. Figure 3 shows the clearing time for a CO sensor that was exposed to a 500 ppm (2% LEL) concentration of acetylene (C2
H2 ). Even after
30 minutes in fresh air the reading is still a few ppm above zero. If the instrument is fresh air adjusted before clearing is complete, readings will drop below zero as the sensor continues to stabilise.
• Why do sensor readings go negative?
The potential causes of negative readings are different for each type of sensor.
Standard (pellistor type) LEL sensors detect combustible gas by catalytically oxidising (burning) the gas on an active bead in the sensor. Along with warm-up issues; mechanical damage due to banging or dropping, exposure to sensor poisons and exposure to high concentrations of combustible gas can all affect the zero ofset and lead to negative readings.
Electrochemical sensors used to measure CO, H2 S and many other
substance-specifi c toxic gases detect gas by means of a chemical reaction between the gas and materials in sensor. The chemical reaction causes a change in the electrical output of the sensor that is proportional to the concentration. The most common causes of negative readings are fresh air adjusting the sensor while it is in the presence of measurable contaminants (especially common with CO sensors that are zeroed in areas contaminated with engine exhaust or where people have been smoking), or fresh air adjusting the sensor before it has fi nished stabilising after exposure to an interfering contaminant.
• What should I do to correct negative readings?
Correcting negative readings is extremely easy. Almost all instrument designs include a simple procedure to make an automatic fresh air zero adjustment (Figure 4).
Make sure the instrument is located in fresh air, that the sensors are fully warmed up, and that the readings are stable. Continue to pay attention to the fresh air readings. If readings continue to count downwards it may be necessary to repeat the fresh air zero procedure.
Figure 4: GfG instruments can be automatically fresh air zeroed or calibrated by means of a simple “AutoCal©
” procedure, (press “Air” to
make an automatic fresh air zero adjustment, “Gas” to make a span calibration adjustment, or “Exit” to return to normal operation)
Figure 3: Electrochemical CO sensors can take a long time to clear after exposure to certain interfering contaminants such as acetylene.
Photo-ionization detector (PID) sensors use photons of highly energetic ultraviolet light to ionise VOC molecules present in the atmosphere being sampled. The detection reaction produces a current fl ow of electrons “stripped” from the ionised molecules that is proportional to concentration. Like LEL sensors, PID readings often start high, then count downwards while the sensor is warming up. PID sensors can sometimes take up to 15 or 20 minutes to fully warm up. Once again, it is very important to wait until the readings have stabilised completely before fresh air adjusting and using the PID to measure gas.
The output of PID sensors in fresh air can also be infl uenced by the ambient temperature and humidity. Water vapour molecules can absorb photons of ultraviolet light without being ionised. The more humid the atmosphere the more the signal of the PID in fresh air can be “quenched” by the presence of the water vapour molecules. Fresh air adjusting the PID zeroes out this quenching effect. Changes in humidity from one day to the next, or from one area to another can be the cause of negative readings. The best approach is to fresh air zero PID equipped instruments in the same humidity as the air in which the instrument is used to obtain readings.
Non-dispersive infrared (NDIR) sensors detect gas by measuring the absorbance of infrared light by chemical bonds in the molecules of interest. Just like PID sensors, the output of NDIR sensors can be infl uenced by changes in humidity. While NDIR sensors warm-up and stabilise completely within a few minutes, differences in the humidity between one day and the next can cause negative readings.
Contact Details GFG Europe Ltd• 710 Avenue West, Skyline 120, Great Notley, CM77 7AA • Tel: +44 (0) 1376 342236 Mob: +44 (0) 7771 642512 • Web:
www.gfgeurope.com
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www.envirotech-online.com IET January / February 2016
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