LEL/Toxic Gas Analysis

Explosive and toxic gases are a hazard to any job site. This is why analyzing and monitoring the atmosphere is important.

Monitoring hazard es gases for safety, process control and air quality is a fairly complex topic. Gas measurement is more complicated than measuring other elements such as voltage, humidity and temperature. For, there are literally hundreds of different types of gases and each application has a unique requirement.

For example, some applications may require the detection of a specific gas and have to eliminate the measurement of other surrounding gases. In other words, in a methane and propane environment methane may want to be detected and surrounding propane eliminated (separate detection). It may be desirable in some cases to measure a quantitative value off all gases present.

Most sensors are not specific to one gas but rather to a group or family of the gas. In order to select a proper detector for optimum results one must understand the types of sensors available and there response characteristics to the appropriate gas.

Job site air quality detectors must be durable, weather resistant, dust proof and explosion proof. Also, they should be inexpensive, have long life expectancy and be suitable for multi sensor systems. A minimally skilled person should be able to operate and maintain it. So, it would be a good idea to choose a simple system.

There are 2 main categories of work area sensors:

• Toxic monitoring for human health. This requires the use of a sensor that is sensitive to low concentrations of gases.
  
• Combustibility monitoring for safety. This requires a sensor that can monitor high concentrations of gas.

The Occupational Health and Safety Organization's (OH&S) permissible exposure limit (PEL) in a 8 hour work day for carbon monoxide (CO) is 50 ppm. However, the lower explosive limit (LEL) is 12.5% by volume concentration. So, considering this gas is toxic and combustible, 2 sensors need to be used to monitor concentration because they are widely disparate.

Currently, the 5 most common air quality and safety monitoring devices are:

• Electrochemical sensor is a oxidation reduction sensor (redox). This sensor consists of a sensing electrode and counter electrode. Or in other words, a anode and cathode see picture below. These 2 electrodes are separated by a thin electrolyte. Any gas that enters the sensor reacts with the anode at the surface and involves either a oxidation or reduction reaction. The anode materials catalyzes the reaction and needs to specifically developed for a given gas. A electric current is generated proportional to the gas concentration. This sensor consumes little power.This makes it ideal for PEL applications because it can be a portable battery operated unit. It is gas selective and not suitable for combustible gas detection and has a 2-3 year life expectancy. Some specify a exposure dosage for maximum concentration. For example, if a sensor has a rate of 4000 ppm/hour and is exposed to 40 ppm constantly the sensor would last 100 hours. These sensors can effectively measure around 20 gases in low ppm ranges like CO, H2S, SO2, NO2 and Chlorine.



• Catalytic combustible sensor ignites the surrounding gas molecules when they come into contact with the coil. The coil is a platinum wire coated with catalytic ally treated metal oxide and is used within a Wheatstone bridge as seen in picture below. A combustible gas mixture will only burn when it reaches ignition temperature. In the presents of catalytic materials the ignition temperature is reduced, so the gas mixture will ignite at a lower temperature. When the gas burns on the surface of the coil the temperature of the platinum wire is increased and the wire's electric resistance is decreased, thus creating the input to output relationship. This detector is good for general purpose and suitable for both portable and stationary continues measurement. It can be used for most hydrocarbon gases, but most common is methane. Other gases need a correction factor implemented for proper and accurate measurement. It is important to know that this sensor can be poisoned. Certain chemicals like silicon compounds, sulfur compounds and chlorine will deactivate the catalyst and make the sensor unresponsive. This phenomenon is known as catalytic poisoning.



• Solid state sensor uses the transfer of electrons. The sensor consists of transition metal oxides. These metal oxide are prepared structured to form a bead or thin layer chip sensor. When gas comes into contact with the metal oxide the gas dissociates into charged ions or complexes. A heater within the sensor keeps optimal gas detection temperature see picture below. Two biases electrodes are embedded into the metal oxide to measure the change in conductivity. Probably one of the most versatile, this sensor can be used for low concentration toxic gas or high range combustible gases. The detection response characteristics can be changed by using different types of metal oxides, changing processing techniques or changing the process temperature. It is constructed in a simple manner, thus giving a problem free life from 10-25 years. The sensor is robust, so vibration and shock do not affect the operation of the sensor. However, selectivity is limited. Background gases could trigger false alarms.



• Infrared (IR) sensor measures the temperature of the gas molecules in relation to IR absorption. The absorbed energy from the IR cause the gas to increase in temperature which relates to concentration. The temperature change is sensed by the pyroelectric crystal which creates a current flow proportional to temperature see picture below. Also, the wavelengths of the light can be measured for a qualitative analysis. Dissimilar gases absorb light at different wavelengths which means every gas has a finger print. These finger prints are archived for identification purposes. IR sensors are widely used in analyzers and monitors. A certain group of sensors are simple, rugged and suitable for high hydrocarbon and CO2 concentration measurement. Sensor poisoning, burnout and sensor fatigue due to high concentrations is non existent. For, the sensor does not contact the gas. It is protected by optical devices. If the zero gas calibration is maintained the response and span remain acceptably accurate. Any signal loss will result in a alarm. Good choice for high concentration measurement.
  


• Photoionization detector (PIDs) uses ultraviolet (UV) light to ionize gas molecules. This sensor can detect volatile organic compounds (VOCs). UV radiation is generated by a special lamp to generate energy to ionize the gas molecules. The resulting free electrons collect at the surface of the electrodes and create a current flow. The current flow is proportional to the gas concentration. The lamp's energy is in electron volts (eV). Standard lamp's have energy levels from 8.4, 9.6, 10.6, and 11.7 eV. The energy magnitude can be determined by the lamp used. 11.7 eV lamp, for instance, uses lithium fluoride. The ionization potential of the gas has to below the output of the lamp. If benzene was being detected, with has a potential of 9.24 eV, a 9.6, 10.6, or 11.7 eV would need to be used. This detector can detect many VOCs at low concentrations and any gas with a energy potential lower than the lamp. The lamp comes into contact with the gas, so needs frequent cleaning. This makes it undesirable for continues or online monitoring. This sensor is good for portable use and periodic readings.