Gas Detection 15
is proportional to the amount of combustible present in the atmosphere being monitored. TCD type sensors are often paired with a pellistor type sensor in the same instrument. The pellistor sensor (or mode) is used for 0–100% LEL range measurement, while the TCD is used for high range 0–100% volume measurement. In fact, a common approach is to put both types of sensor into a single housing that shares the same flame arrestor and certification as a flame proof device.
• Photoionisation Detectors (PID) for VOC measurement
Figure 2: The G460 atmospheric monitor can support a wide variety of sensors including pellistor type LEL, PID and infrared combustible gases
Clearly, from a toxic exposure limit standpoint a different detection technique is required. Another limitation of pellistor type sensors is that they require the presence of oxygen in order to oxidise the gas being measured. Most manufacturers stipulate that the
atmosphere must contain at least 10% O2 in order for the LEL sensor to detect gas accurately. Readings are increasingly affected
as the concentration drops below this level. In zero percent O2 pellistor type combustible sensors cannot detect gas at all. For this reason confined space instruments that contain catalytic pellistor type LEL sensors should also include a sensor for measuring oxygen.
Fortunately, there are alternative detection techniques that are not affected by these constraints. It is important to note that these alternative types of sensors should not be seen as replacements for pellistor type LEL sensors. Pellistor sensors are still the best and most cost effective solution for many applications. It is also true, however, that in many cases the best approach is to include one or more additional types of sensor in the instrument.
What Other Types of Sensors are Available for Combustible Gas and VOC Measurement?
The major alternatives for combustible gas and VOC measurement are thermal conductivity detectors (TCDs), photoionisation detectors (PIDs) and non-dispersive infrared (NDIR) sensors.
• Thermal conductivity (TCD) sensors
Thermal conductivity sensors are a specialised type of sensor most frequently used to detect high range concentrations of combustible gas. Thermal conductivity sensors are capable of measuring combustible gas in concentrations up to 100% by volume. The sensor contains two coils of fine wire that are coated with a ceramic material to form beads.
The reference bead is isolated from the air being monitored in a sealed or semi-sealed chamber. The active bead is exposed to the atmosphere being monitored for gas. If a lighter than air combustible gas is present (such as hydrogen or methane), the active bead will dissipate heat in the attenuated atmosphere more efficiently than the reference bead. If a heavier than air gas is present (such as propane) the bead is insulated by the denser atmosphere. The difference in temperature between the two beads
Solvent, fuel and other VOC vapors are pervasively common in many workplace environments. Most have surprisingly low toxic exposure limits. For most VOCs the toxic exposure limit is exceeded long before you reach a concentration sufficient to trigger an LEL alarm. PID equipped instruments are generally the best choice for measurement of VOCs at exposure limit concentrations. Photoionisation detectors use high-energy ultraviolet light from a lamp housed within the detector as a source of energy used to remove an electron from neutrally charged VOC molecules, producing a flow of electrical current proportional to the concentration of contaminant. The amount of energy needed to remove an electron from the target molecule is called the ionisation energy (IE). Larger and / or more reactive molecules have lower ionisation energies than smaller less reactive molecules. Thus, in general, the larger the molecule, the easier it is to detect!
This is exactly the opposite of the performance characteristics of catalytic pellistor type combustible sensor. Pellistor type combustible sensors and photoionisation detectors represent complementary, rather than competing detection techniques. Pellistor sensors are excellent for the measurement of methane, propane, and other common combustible gases that are not detectable by means of a PID. On the other hand, PIDs can detect large VOC and hydrocarbon molecules that are effectively undetectable by pellistor sensors, even when the catalytic sensor is operable in ppm measurement ranges.
The best approach for VOC measurement in many cases is to use a multi-sensor instrument equipped with both a pellistor LEL sensor and a PID sensor.
• Non-dispersive infrared (NDIR) sensors for combustible gas measurement
Non-dispersive infrared (NDIR) sensors measure gas as a function of the absorbance of infrared light at a specific wave-length or range of wavelengths. Different molecules absorb infrared radiation at different wavelengths. When infrared radiation passes through a sensing chamber containing a specific contaminant, only those wavelengths that match the absorbance spectrum of the molecule are absorbed.
The rest of the light is transmitted through the chamber without hindrance. For some types of molecules (like combustible gases) it is possible to find an absorbance peak that is not shared by other types of molecules likely to be present. The active detector in an NDIR combustible gas sensor measures the amount of infrared light absorbed at this wavelength.
A reference detector measures the amount of light at another wavelength where there is no absorbance. The greater the concentration of combustible gas, the greater the reduction in the amount of light that reaches the active detector when compared to the reference signal.
Sensors for measurement of combustible gas and VOCs
Able to dectect LEL range C1-C5 hydro- carbon gases (methane, ethane, propane, butane, pentane and natural gas)
Standard Pellistor type LEL sensor NDIR combustible gas sensor PID (with standard 10.6 eV lamp) Electrochemical H2 sensor
Thermal Conductivity Sensor Table 1: Sensor technology selection chart
* Because of their logarithmic output curve, NDIR sensors show the most sensitivity at the lowest concentration of measured gas. An NDIR combustible gas sensor with 0.1% LEL resolution over 0 – 5% LEL provides 50 ppm step-change resolution for methane. Because the LEL concentration is so much lower, the same sensor would provide 11 ppm step change resolution for n-hexane.
** Although PIDs are able to detect a wide variety of VOC vapours, the ability of the PID to measure LEL range concentrations is limited by the full range of the PID. The 10% LEL concentration for most VOC gases ranges between 1,000 and 3,000 ppm. A PID with a full range of 2,000 ppm would only be able to detect maximum concentrations of 6% to 20% LEL, depending on the VOC being measured.
*** Only if the exact composition of the oxygen deficient atmosphere is known and the instrument is properly calibrated for use in this mixture.
**** TCD sensors that include a catalytic bead or operation mode are vulnerable to sensor poisons as long as the catalytic bead is under power.
www.envirotech-online.com AET Annual Buyers’ Guide 2012
Yes Yes No No Yes
Able to detect LEL range C6-C9 hydro- carbon gases (hexane, hepane, octane, nonane)
Yes Yes
Yes** No Yes
Able to accurately detect LEL range heavy fuel vapours (e.g. diesel, jet fuel, kerosene, etc.)
No Yes
Yes** No No
Able to detect heavy fuel vapors in low ppm range (e.g. diesel, jet fuel, kerosene, etc.)
No
Yes* Yes No No
Able to use in low oxygen
atmospheres
Vulnerable to sensor poisons (e.g. silicones, phosphine, tetraethy lead, H2S, etc.)
No Yes Yes Yes
Yes***
Yes No No No
No****
Able to use for high range combustible gas measurement (100% LEL and higher)
No Yes No No Yes
Able to measure hydrogen (H2)
Figure 3: Response of PID, pellistor type LEL and NDIR combustible gas sensors to warm 130˚F (54˚C) diesel vapor. Readings for all three sensors displayed on LEL scale. PID and IR LEL sensor show strong response. Pellistor LEL sensor shows almost no response.
It is the chemical bonds in the molecules being measured that actually absorb the infrared light. While pellistor type LEL sensors are more sensitive to small molecules like methane than to larger molecules like pentane or nonane; the sensitivity of NDIR sensors depends on how well and how many chemical bonds in the molecule absorb IR light at the measurement wavelength.
Since larger molecules have more chemical bonds holding the atoms in the molecule together, they provide more opportunities for infrared radiation to be absorbed. Thus, an NDIR sensor is very sensitive to molecules such as octane, nonane and the larger molecules in diesel vapor (Figure 3). NDIR combustible gas sensors have a number of other advantages when compared to pellistor type sensors.
NDIR sensors do not have a flame arrestor that limit the ability of large molecules to diffuse into the optical sensing chamber. NDIR sensors do not require oxygen. They are also not subject to damage due to exposure to sensor poisons.
Finally, unlike pellistor type sensors, they can be used for measurement of high concentration combustible gas above the 100% LEL concentration.
One of the most important limitations of NDIR combustible gas sensors is that they cannot be used for measurement of hydrogen
(H2). In applications where H2 may be potentially present, the instrument should be equipped with a type of sensor that does
respond well to H2, such as a pellistor LEL sensor or an electrochemical sensor capable of measuring H2 in the desired range.
Depending on the design, NDIR sensors may or may not be capable of measuring acetylene and certain VOC molecules that do not absorb infrared well at the measurement wavelength. Consult the manufacturer for specific details.
Summation
No single type of sensor is perfect for all applications. The four basic sensors (LEL / O2 / CO / H2S) used in most multi-sensor instruments are a good start, but may not be capable of properly monitoring
for the presence of all of the potential hazards. Table 1 summarises the advantages and limitations of each type of sensor discussed.
The technologies and sensors are readily available, as long as your instrument is capable of supporting their use.
Yes No No Yes Yes
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