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Fixed or Portable?

Published: 01st Jan 2003

ARTICLE CONTINUES BELOW

Streamlining the Gas Monitor Selection Process

Multi-gas monitor. Toxic gas detector. Personal air monitor. Portable gas instrument. Stationary gas monitor. Leak detector. Fixed point system. Meter. Sniffer. CGI.

With so many terms for gas detection instrumentation cluttering the workplace, it is no wonder you can become confused when looking for information on gas detection equipment.

Gas detection and monitoring systems are used as safety devices to alert workers of the potential danger of poisoning by toxic gas exposure, asphyxiation due to lack of oxygen, and fire or explosion caused by combustible gases. Choosing the right monitor is critical - in fact, lives depend on it.

There is a plethora of brands, model types, sensor configurations and sensor types on the market. To help focus your search, you will need to arm yourself with some basic selection criteria. The more you know about the hazardous conditions in your workplace, the potential for exposure to hazardous gases and the best technology for the detection of these gases, the better equipped you will be to select the right monitor for your situation.

1. Which gas hazards are present?

The most basic question relates to the hazardous gas conditions present in the workplace. Are there several areas of concern in the workplace? Which gas hazards are present in each area of concern? Also consider those unique processes that may create a hazardous state once or twice a year. Depending on your industry, you may have a variety of applications with diverse gas hazards, or you may have one source of concern with one known hazard. The most common atmospheric dangers in general industry include flammable gases such as methane or natural gas, oxygen deficiency or enrichment, carbon monoxide and hydrogen sulfide. In documented fatal confined space asphyxiations, at least 70% of the known atmospheric conditions included one or all of these four gas hazards. Knowing what gas/gases need to be monitored will determine which gas sensors should be installed in the instrument. Also, knowing the specifics about the environment, including temperature, humidity, other gases present, etc., will dictate the most appropriate sensor technology. This, in turn, will help you select a suitable monitor.

2. Portable, fixed or both?

The next fundamental decision depends upon the potential for personnel exposure to the hazardous gases. Is there concern for your workers’ exposure to gas hazards, or is area monitoring applicable? If the danger is contained to areas where personnel are restricted, then area monitoring with permanently installed monitors (fixed systems) is an option. However, when workers are under the threat of continuous exposure or face unpredictable conditions, personal monitoring with handheld or portable gas monitors is the safest measure. Often times, fixed gas detectors are installed to monitor flammable gas levels to gauge the danger of an explosion and trigger alarms. Or, in process operations, an area monitor is used to sense a leak and alert the plant manager of the condition. In either situation, workers in the same area may be equipped with gas monitors to ensure personnel safety.

3. Single-gas vs. multi-gas monitor?

How many different gases are present in your workplace? If only one gas hazard exists, then a single-gas portable monitor or single-gas fixed transmitter is appropriate. On the other hand, typical confined space atmospheres require pre-entry testing for combustible gases, oxygen deficiency or enrichment, and specific toxic gas measurements. Depending on the industry a mix of two or three toxic gas sensors is often required to cover the noxious dangers. For personal protection, portable gas monitors in single or multi-gas configurations are available. Many companies have a mix of both, based on the balance of needs. Stationary monitors typically accommodate one or two sensors per transmitter, so rooms or areas with multiple gas hazards will require multiple sensor transmitters located in appropriate locations.

4. How many gas detectors do we need?

There are many ways to look at total usage for your gas detection needs; ultimately you need to look at the potential for risk in every inch of your workplace. How many areas have the possibility of containing a gas hazard? How many employees will work continuously in those areas or pass through those possible pockets of risk? If you have personnel working in an area that has the potential for a gas leak or where unsuspecting pockets of gas may be encountered, then you will want to arm all workers with a personal detector. If the area has been classified as a confined space then the entrants and attendants should follow the guidelines of the regulatory procedures to ensure protection from harmful atmospheres. Most companies have corporate procedures that mandate safety practices including personal protective equipment requirements. Because each company and each worksite is different, there are no rules that cover all situations. Many companies that purchase gas detection equipment only use them during a 2-week maintenance shutdown time, while others use the equipment 24-hours every day of the year. Fixed gas detectors are typically used to cover an area where the location and number of sensor heads depends on the type of gas being monitored, the number of leak points, and the size and configuration of the area. Manufacturers and distributors of fixed gas transmitters will assist in calculating the appropriate type and number of sensor heads after a detailed evaluation of the application.

5. Which sensor technology is appropriate?

In any gas monitor, the sensors are the heart of the instrument and the foundation of gas detection. Many models of multi-gas monitors use a combination of sensor technologies in the same package to accommodate the best total solution for applications involving multiple gas hazards. Sensor technologies are not suitable across all gas types or applications. The advantages and disadvantages of the technologies help clarify the appropriate usage.

The catalytic sensor, also referred to as the catalytic bead sensor, is the most common technology used to detect and measure combustible gases and vapors, such as methane, from 0-100% LEL (lower explosive limit). The sensor’s response to a combustible gas depends on the chemical composition, the molecular weight and vapor pressure of the gas. Also, a minimum oxygen concentration of 5-10% by volume in the mix of diffused gas is generally required for the sensor to operate properly. The catalytic sensor is less sensitive to temperature and humidity effects, offers repeatable performance and is relatively stable.

However, it is susceptible to poisoning or inhibition from some gases, which may decrease its sensitivity or damage the sensor beyond recovery. The more common poisons and inhibitors are lead, silicon, phosphorus and sulfur. Also, catalytic sensors have a high energy consumption which dictates the power requirements in portable monitors. The dependable catalytic sensor is used in both portable and fixed gas detection systems for combustible gas measurement and offers a long life of 3 - 5 years.

Electrochemical sensors are widely used for the detection of specific toxic gases at the PPM level and for oxygen in levels of percent of volume (% vol). Toxic gas sensors are available for a wide range of gases, including carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen dioxide, chlorine and many others. Electrochemical sensors are usually small (typically = 1 inch diameter) and require little power usage which is beneficial for portable gas monitors.

The sensors can be used over a wide temperature range (-20° to +50°C is common), though for improved accuracy temperature compensation is often built into the instrument electronics. Routine calibration against a known concentration of the target gas, commonly available in disposable cylinders, is necessary for stability and accuracy of the sensor response. Overall, electrochemical sensors offer very good performance for the routine monitoring of toxic gases and percent of volume oxygen presence and are available for both portable and fixed gas monitors.

The non-dispersive infrared (NDIR) sensor, commonly referred to simply as the infrared sensor, is based on the principle that gases absorb light energy at a specific wavelength, typically in the infrared range. The limitation of NDIR technology for gas detection is dependent on the uniqueness of the absorption spectrum of a particular gas. Infrared sensors can detect gases in inert atmospheres (little or no oxygen present), are not susceptible to poisons, and can be made very specific to a particular target gas. NDIR sensors are also extremely stable, quick to respond to gas, can tolerate long calibration intervals and have a lifespan of five years or more. Infrared sensors are commonly used to detect methane, carbon dioxide and nitric oxides in both portable and fixed gas detection instrumentation.

For many years, the thermal conductivity sensor has been used in instruments for measuring combustible gases above the % LEL range and for leak detection. The sensor consists of two elements, both comprised of a wire coil. The elements are heated to an operating temperature of approximately 250°C. Heat is transferred from the element to the surrounding gas. The amount of heat transferred depends on the thermal conductivity of the gas.

When the heat transfer occurs, the element’s resistance changes and that change is measured using a bridge circuit. Some advantages of the thermal conductivity sensor are that it does not require oxygen to operate, and it is not susceptible to poisons. One drawback is that it cannot measure gases with thermal conductivities similar to the reference gas (i.e. Nitrogen). Thermal conductivity sensors are used primarily in portable gas leak detectors.

A variety of metal oxide sensors (MOS) are available for the detection of combustible gases, chlorinated solvents and some toxic gases, such as carbon monoxide and hydrogen sulfide. MOS sensors, also referred to as solid-state, are inherently nonspecific, and as a result are useful in applications where the atmospheric hazards are unknown. The most common type of MOS sensor consists of a sensing element that is enclosed in a metal housing with a stainless steel mesh cover. The cover acts as both a flame suppressor and a gas entry port. During operation, the sensing element is heated to 250°– 350°C. When gas enters the sensor it reacts with the oxide coating which causes a decrease in resistance between the two electrodes.

The output of the MOS sensors varies logarithmically with the gas concentration. This limits the accuracy of the sensor and the overall measuring range of the sensor. Changes in the oxygen concentration, humidity and temperature also affect the sensor performance. Although MOS sensors are relatively low cost, the stability and repeatability of the sensor is sometimes poor. Power consumption is high due to the heating of the element, which restricts the use of this sensor in portable devices. MOS sensors are commonly used in low cost, hard-wired fixed gas detection systems.

Photoionization (PID) detectors are often used in specialised applications where high sensitivity (sub-PPM levels) and limited selectivity (broad-range coverage) is desired. PID’s are commonly used for detecting Volatile Organic Compounds (VOCs) such as benzene/toluene/xylene (BTX), vinyl chloride and hexane, and provide quick response for this growing concern. A PID operates by ionising components of a sample stream and detecting the resulting charged particles collected at an electrode within the detector. A PID responds to compounds having an ionisation potential equal to or less than the energy of the photons emitted by the UV light. The most common lamp energy, 10.6 electron volts (eV), is capable of detecting many typical VOCs such as those listed above.

Many gases, including carbon monoxide, carbon dioxide, sulfur dioxide, oxygen and methane, are not detectable by a PID due to their ionisation characteristics. Advantages of this technology include the fast response time and excellent shelf life (however sensor life is poor). Major disadvantages are that most PIDs suffer from sensor drift and humidity effects, therefore calibration requirements are more demanding than other common gas detectors. PID technology is most commonly found in portable instrumentation.

While some applications require only a gross indication of detection, others may require the precision of specific gas measurement. Marrying the best-suited technology with a solid electronics design gives you the ideal match for the best personal protection against hazardous gases. For the most part, simply knowing which gases you need to monitor will direct you to the best monitoring solution.

Other Considerations

Once you have determined your basic requirements, you will have additional alternatives to consider. In most cases, the options are simply that - optional.

For example, you may have occasional or area-specific applications that require remote sampling of the gases, where an internal or detachable sampling pump would be required. Confined space requirements include pre-testing of the environment prior to entry. This is best accomplished by drawing an air sample from various stages ahead of the entrant through the use of a remote sampling pump.

Or, you may need to record the exposure data for industrial hygiene measurements, so data logging capabilities are critical. Many companies measure Short Term Exposure Limits (STEL) and Time Weighted Average (TWA) readings as part of standard workplace industrial hygiene practices to evaluate employee exposure over the work period.

For high noise areas, loud (greater than 100dB) or vibrating alarms are critical for alarm recognition. Check the manufacturers’ instrument alarm specifications as well as the availability of external alarms that provide audible and visual indications of danger.

For proof of intrinsically-safe equipment, look for third-party certifications or approvals, such as ATEX, CSA, UL, MSHA and CENELEC. Effective June 2003, all units intended for use in hazardous areas must comply with the new ATEX directive when they are brought to the EU market. The directive details the minimum requirements for developing equipment for explosive dust and gas areas. Manufacturers have had the opportunity to have products certified for the past few years, so ask if the gas monitor is ATEX certified.

You will also have power supply selections, display choices, mounting options and an assortment of add-on accessories. In some cases you will need to research the specifications - do not assume your work site’s gas detection requirements are available from all manufacturers. Identify your workplace concerns and match the product to your needs.

Conclusion

What should not be optional, however, is durability. In all cases, look for durable equipment that is easy to use and easy to calibrate. Evaluate the long term cost of ownership, not just the initial purchase price. When evaluating a manufacturer, test its ability to respond to your technical questions or service issues. By all means, talk to your colleagues and ask for recommendations and experience with brands. The selection of a gas monitor should a thorough process - arming yourself with this basic knowledge should allow you to ask informed questions during your search.

Published: 01st Jan 2003 in Health and Safety International

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