Accurate Measurement of Flammable Gases

Combination instruments provide the flexibility to do general-purpose safety monitoring, as well as detection of very high levels of flammable gases and vapors.

• By Bruce Holcom
• May 01, 2004

TODAY there are a variety of sensors used to detect flammable gases. There are also many different flammable conditions, each requiring a specific type of sensor to be accurately measured. When selecting a gas monitor for detecting flammable gas, it is critical you understand the differences and choose the appropriate sensor for the environment to be measured.

Catalytic Bead
Traditionally, catalytic bead type sensors have been used in instruments designed for detection of flammable gases and vapors.

The catalytic bead is a very fine wire that is coated with a catalyst material, typically platinum or palladium. This bead is then heated to a temperature sufficient to promote combustion of a flammable gas or vapor on the surface of the catalyst. This combustion generates heat, which in turn changes the resistance of the wire inside the bead. The resistance change is monitored by an electrical circuit (typically a Wheatstone bridge) and used to generate an electrical signal proportional to the concentration of the gas or vapor. The electrical signal is received by a microprocessor, which then generates a digital reading onto a display.

For more than 50 years the catalytic sensor has proven to be a very reliable way of determining whether the atmosphere in enclosed or confined spaces contains an unsafe level of gas that could, with the addition of an ignition source, result in a fire or explosion. However, a potential disadvantage of the catalytic bead sensor is that it requires oxygen to operate. Because the catalytic bead sensor must combust the gas to produce a reading, oxygen is required when this sensor is used.

LEL and UEL Ranges
The familiar "fire triangle" advises us that three things are needed to support a fire or explosion:

• A source of fuel (e.g., flammable gas or vapor)
• Air (oxygen)
• A source of ignition (e.g., spark, open flame, or high-temperature surface)

Many commonly encountered gases and vapors (natural gas, methane, propane, hydrogen, alcohols, etc.) are flammable within a range of concentration known as the explosive or flammable range. This range is defined by the lower explosive limit (LEL) and upper explosive limit (UEL) and is different for each flammable gas. For methane, the LEL and UEL in air are 5.0 percent and 15.0 percent by volume, respectively [NFPA reference]. At concentrations below the LEL, the mixture is too lean (insufficient fuel with respect to oxygen) to sustain combustion, and at concentrations above the UEL, the mixture is too rich (too much fuel with respect to oxygen) to sustain combustion.

Safety instruments using catalytic bead type sensors are intended for use below the LEL and typically are scaled from 0-100 percent LEL. This means the full-scale indication on the monitor is the minimum concentration that could sustain combustion.

NDIR (Non-Dispersive Infrared)
For higher detection ranges, NDIR (non-dispersive infrared) detectors are sometimes employed. This detector operates on the principle that most gases will absorb infrared energy at certain wavelengths.

This method offers some distinct advantages. Most flammable gas sensors are directly exposed to the gas in order to react. The catalytic bead sensor, for example, needs to be exposed to the flammable gas and oxygen in order to combust and obtain a reading. NDIR sensors are not exposed directly to the gas and do not require oxygen to operate.

The infrared sensor typically is isolated from the sample gas stream by a window or optical filter. This helps the NDIR sensor to last longer and require less maintenance. The NDIR method also has some serious disadvantages, the primary one being the detector will not detect all flammable gases and vapors generally tuned to a fairly narrow band wavelength. For instance, no infrared detector will detect hydrogen, and an NDIR detector optimized for alkane hydrocarbons such as methane or propane will not adequately detect flammable aromatics (e.g., benzene or toluene), or unsaturated hydrocarbons (e.g., acetylene).

Monitoring Above the LEL Range
While most safety-related instruments focus on the sub-LEL range, there are many applications or industries where it is necessary to measure concentrations of flammable gases and vapors above the LEL. For instance, a utility company trying to pinpoint a gas leak may subject its leak detector to concentrations far above the LEL. Another common application is in landfills, where inspection wells are monitored to track the condition of the landfill or to verify methane produced in the fill is not migrating offsite. In monitoring of these inspection wells, concentrations of methane can be well above the LEL or even above the UEL and are often deficient in oxygen.

For these situations, a catalytic bead type LEL range detector is not suitable and can indicate dangerously low false readings. Remember, the catalytic bead type sensor must "burn" the gas on the surface of the bead. If the mixture is above the UEL (too rich), the instrument actually may indicate a very low reading or no reading. In addition, subjecting a catalytic bead sensor to very high gas concentrations can result in damage to the sensor, including loss of sensitivity that also can produce erroneously low readings.

For applications of this type, where very high gas concentrations may be encountered, the two most commonly used methods of detection are thermal conductivity (TC) and NDIR (see above).

Thermal Conductivity
A thermal conductivity detector is based on the principle that gases differ in their ability to conduct heat. A TC detector will consist of two elements, typically a pair of wires (filament) or thermistors that are heated above ambient temperature.

One of the elements (active) is exposed to the gas sample to be measured, and the second element (reference) is exposed to a reference gas (typically air for this type of instrument). If the sample gas has a different thermal conductivity as compared to the reference gas, the temperature of the active filament will change as compared to the reference element. As with the catalytic bead type sensor, the resulting temperature change results in a resistance change that is measured with a Wheatstone bridge circuit to produce a reading proportional to the gas concentration.

The thermal conductivity sensor can be used for a variety of gases and does not require oxygen to operate. Because the thermal conductivity of different gases as compared to air can vary widely and be either positive or negative in direction, the thermal conductivity detector must be tuned or calibrated for a specific gas or vapor. The most notable advantage of the TC type of detector is that it can detect concentrations of flammable gases and vapors up to 100 percent by volume, well above LEL and UEL ranges.

Packaging
In the past, you would have to buy a separate instrument in order to obtain each of these sensor types. Fortunately, advances in electronics and software have led to the development of combination instruments that include catalytic bead and thermal conductivity or infrared detectors, as well as electrochemical detectors for oxygen and toxic gases. This provides users with the flexibility to do general-purpose safety monitoring, as well as detection of very high levels of flammable gases and vapors.

Some instruments have become so sophisticated, they can automatically select the appropriate sensor to use based on the gas concentration encountered.

Considerations
Users who require an instrument that monitors flammable gases or vapors should verify that the instrument selected is suitable for the application.

• Equipment ratings for hazardous locations. When dealing with flammable gases and vapors, the equipment should be rated for use in Class I or Zone 0 or 1 hazardous locations, with the rating appropriate for the gases that could be encountered. For instance, an instrument approved for use in methane environments may not be suitable for atmospheres containing hydrogen.
• Choose the right sensor for the job. Make sure the sensor type is suitable for the gas being detected. When using TC or IR detectors, verify with the instrument manufacturer that the instrument is tuned appropriately for the gas to be detected and also calibrated using the recommended gas mixture.
• Multi-sensor/multi-function awareness. If using a unit with multiple flammable sensors, be aware of how the different ranges function. Will the unit automatically range up or down with the gas concentration, or must the range be manually selected? If manual, what is the optimal sequence of operation to determine the appropriate range? What precautions should be taken to ensure the readings are accurate? If an instrument is equipped with two types of flammable sensors, what procedures are necessary to avoid damage to the catalytic bead sensor?
• Training and maintenance. Finally, as with all safety equipment and instrumentation, do not skimp on training! Make sure the operators know how to verify the equipment is working properly (e.g., bump testing, battery and pump check, etc.) and are thoroughly schooled in operation of the equipment and interpretation of readings. For instance, if the display reads "10," is that percent LEL or percent volume?

Also, ensure the personnel responsible for maintenance of the equipment know how to properly calibrate the units, determine whether or not it is functioning properly, and are aware of the manufacturer's recommendation for frequency of calibration and correct calibration gas and concentration. Most manufacturers now offer a variety of training resources, including computer-based training CDs, Web access training material, videos, on-site training, and the traditional operator's manual. Training records, audits, and refresher courses to verify the training is understood and being followed are also of the utmost importance.

In conclusion, most performance and safety standards are too general to be the only source used to specify the proper equipment for applications requiring gas monitoring equipment. Verify with the local jurisdictional authority and with the equipment manufacturer that the instrument is safe and suitable for the intended use and that all operational and maintenance requirements are well understood. In this way, monitoring of hazardous gases can be dealt with in a manner that protects personnel and property and provides an accurate assessment of risk.

This article originally appeared in the May 2004 issue of Occupational Health & Safety.