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Gas Detection Equipment , How to Select
Dated: 19 May 2008
Gas detection equipment comes in many forms. Choosing the right one for a specific application can mean the difference between life and refrigerant recovery machine.


The definition of a gas in physics is a substance possessing perfect molecular mobility and the property of indefinite expansion, as opposed to a solid or liquid. Gas can be colorless, odorless, opaque. It can be toxic or harmless. While the viscosity is usually low, some can be relatively dense and settle into low lying areas. Small changes in temperature or pressure can have large effects on the characteristics of gas.

Since many gases are invisible, we must sometimes be able to measure gases to ensure safety. Technology provides a variety of tools that will accomplish such a task.

Gas can be difficult to confine. The tiniest hole can allow a gas to escape. This can lead to a great deal of damage and result in huge costs if the gas leaves the property area.

Gas detection equipment can be deployed in many fashions to assist identifying gas leaks. This equipment can be used to sound alarms, or shut down processes to avoid damage to human health and the environment. The equipment can be designed to be manually operated, or connected to computer controls to automatically take action as needed according to programmed parameters.


Confined space

There are many gases that are heavier than air and will tend to settle in confined areas. Also, gases can concentrate in confined areas and reach concentrations that replace oxygen needed by humans and animals.

Another common example that all homeowners can relate to is CO gas. A malfunctioning gas furnace in a home can spew insufficiently oxidized natural gas as CO. This gas can build up within a home and eventually cause harm to residents confined inside.

Safety rules today require that workers take a number of steps before entering a confined area. One of those steps includes incorporating gas detection equipment to insure the safety of humans and animals.


Exposure

Human noses are very sensitive but are not reliable as a warning system. For example, most of us are familiar with the distinct odor of H2S gas; the sulfur gives it a smell like rotten eggs. The human nose can easily detect a few parts per billion in the air. However, the nose will quickly become inured to the smell. While various states have differing safety levels for exposure, many think that exposure to levels as low as 10 ppb for an hour can be cause for damage. Exposure over 250 bbp can be lethal. Know exposure levels published by state authorities and understand what exposure times are important.


Tubes


The Eagle from RKI Instruments is a portable gas monitor that can detect six combinations of gases simultaneously.  

There are several methods that form the basis of gas sampling technology. Perhaps the most cost-effective and versatile methods for measuring gas levels would be gas tubes. A measured volume of gas is drawn through a glass tube that contains special chemicals. A color change indicates the level of gas in the sample.

The tubes are specific to the gas being analyzed. There are dozens of compounds that can be detected with tubes. The glass vessels are specially manufactured to contain the correct chemicals and materials, at the exact specified diameter and length. The tubes are hermetically sealed at the factory.

In the field, the operator breaks off the tips of the tube and places it in a rubber boot on the pump. The pump is usually manually operated and specifically designed for the tube's size and length. The pump is operated for a defined number of times to draw a measured volume of gas through the tube. The gas reacts with the chemicals in the tube and a color change results. The tubes can have graduation marks and a number is read from the tube depending on how far the color change traveled within it. That number is placed within a formula that is provided by the manufacturer to calculate the concentration of the specific gas at that time.

There are limitations with this equipment. The manual pump cannot be remotely operated, so the operator must be in the same environment. When conditions are unknown, safety gear such as full face scuba air supplies and personal protection clothing should be worn until the area is tested safe. Extension tubing should be avoided. If it is necessary to add an extension tube, know that the tube must be flushed with gas to be sampled, and that the results might not be as accurate because the flow will not be as reliable.


Tedlar bags


Maintaining safe emission levels from indoor motorized vehicles can save lives. Equipment such as TSI Inc.'s CA-Calc exhaust analyzers can prove invaluable.

Tedlar bags are used to grab samples in situations that require laboratory quality analysis. Tedlar is a brand named product from DuPont. It is a polyvinyl fluoride material that is shaped into a sealed bag of known volume. The bag contains a special valve assembly. The bags are generally inert and will not react with most compounds.

A special pump is used to fill the bag. The pump can be programmable to collect samples at specified intervals or periods (its speed is variable to control sample volume). The sample bag is then sealed and transported to an environmental laboratory for quantified analysis. By knowing the sample volume and how it was collected, an exposure time can be calculated.

The advantage of this type of sampling system is the results are very accurate. If the sample might contain a variety of gases, one sample can provide data on all the species. Exposure levels are easily determined.

The disadvantage of this system is that it takes time. The sample is often collected over two, four or eight hours. The sample then must be carefully handled; chain-of-custody protocols are required. Laboratories can take as long as three weeks to turn around a typical sample result.


Collection containers

Gas collection containers are similar to tedlar bag sampling systems. However, a big difference is the pump. Gas collection containers are usually made of stainless steel. The container is cleaned and a vacuum is placed in the container. The sample is controlled by a special valve that allows sample gas to be drawn into the container. This valve can be programmed to allow defined volumes of gas to enter the container at timed intervals or all at once and then reseal.

There is less chance of carryover with the steel vacuum bottles than with bag samples. It is often suggested that tedlar bags should be discarded and steel containers can be reused. The container is less prone to puncture as the steel is more rugged. There is less chance of seam rupture with the steel tank. The advantage of accurate analytical results and the disadvantage of time are the same.


Single and multi-gas detectors


Confined space entry personal monitors, such as RAE System's EntryRAE, have protected workers for years.

A number of suppliers have single and multiple-gas detectors. These detectors come in a variety of configurations depending upon the task they are employed to accomplish. These machines can provide real-time reading. Multi-gas detectors are used to provide information on up to six gases.

While there are many sensors for a variety of gases, there are five common ones. These are usually, percentage of oxygen, H2S, CO, explosive gases and ammonia.

Meters can be permanently affixed to provide continuous or intermittent readings. These meters can be linked to alarms and computers to take specified actions. Processes can be shut down, exhaust fans can be activated, readings can be collected, reports can be generated or the computer can even call a human by telephone and make a report.

Detectors can be small enough to be carried on a belt or in a pocket. Handheld machines can be programmed to take interval readings, which can later be downloaded to a computer.

The advantage of such equipment is readings are available right away. The disadvantage is that sensors are limited. These are also more costly remedies.


Lasers and infrared cameras

The last piece of gas detection is relatively new on the market. Special laser or infrared cameras can be applied to locate escaping gas. These devices are either handheld or mounted. They can quickly cover a large area and are often used to detect gas leaks from piping systems. They cannot determine the type of, but in application, that is usually not the question.

The advantage of using this equipment is that it covers a large area in a short time. Gas leaks are shown on a screen as phases. Reading can be gathered from a long distance to protect the worker. If a leak is detected, it will be easier to gather the additional information needed to quantify the problem.

The variety of methods that are available can meet any demand for most budgets. The level of safety such equipment can provide is essential in any potentially hazardous situation. No matter how good the maintenance schedule is and how rigorously it is adhered to, problems can always creep up, and having these alternatives as backup can be invaluable. PE


Roy Bigham
roy@pollutionengineering.com
Roy D. Bigham has been the editor of Pollution Engineering since 2002. Bigham attended Eastern Michigan University where he majored and computer science with an associates degree in mathematics. He has worked as a laboratory technician at a research laboratory, managed an electroplating operation and an associated analytical laboratory. He spent three years overseeing environmental operations of five domestic and five overseas operations for a major manufacturer in the Detroit area. He then managed a field services department for an environmental analytical laboratory before moving on to a position as an environmental engineer for a construction aggregates company.
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