July 12, 2010
Today, portable gas detection instruments go well beyond the “canary in a coal mine.” With many instruments available for numerous applications, workers can find the choices confusing. No single instrument is going to be suitable for all situations, so selections must be made carefully. A thorough knowledge of the intended application and an understanding of the features available will help you choose the proper instrument for the job.
Portable atmosphere testingPortable instruments for atmosphere testing are usually designed for use as level detectors for safety requirements or as leak detectors. This can often be a source of confusion because instruments intended for one application may not work well for the other.
Level detectors are used for testing an atmosphere for unsafe conditions â€” where it is necessary to measure the amount of toxic or hazardous gases (or the amount of oxygen) present. Leak detectors are intended to pinpoint the source of gas leaks, which usually involves physically moving a sensor on the instrument toward areas of increasing concentration until the leak source is located. A leak detector that incorporates a calibrated output indication can often be used as a level detector, but a level detector usually does not have the characteristics to perform well as a leak detector.
Level detectorsWhile level detectors for gas monitoring are often in fixed installations, there are many applications for portable level detection, especially when workers are temporarily working in an area that could contain hazardous levels of a toxic or combustible gas. At a minimum, these instruments are calibrated to alarm at a specific level of a monitored gas or indicate the specific quantity of gas detected. While frequently electronic based, some level detectors use chemicals to cause a color change to display the amount of a gas present, as with colorimetric indicator tubes.
Leak detectorsLeak detectors must be able to operate over a large range while producing a proportional output indication for small changes in concentration. The range must be large because gas concentrations can vary from parts-per-million or less far from a leak to concentrations nearing 100 percent at the source. While the output indication of a leak detector may include an analog or digital display, an audio indication using a tone or Geiger-counter-like “tick rate” is a popular feature. To accommodate a large range of gas levels, the sensitivity level of the instrument output can be changed as the sensor is brought closer to the point of the leak and greater concentrations of gas are encountered. This prevents off-scale indications and keeps “tick rate” type outputs in a range of maximum sensitivity to changes.
The leak detector response time is more critical than that of the level detector. The method of locating a leak by moving the sensor toward the area of increasing concentrations works best with immediate feedback. Those sensors with response times beyond a few seconds can make the task of finding a leak slow or impractical.
Choosing a detectorWhether choosing a portable gas detector for level measurement or leak detection, several criteria common to both should be evaluated, including the type of gas to be measured, the environment in which the instrument will be used, features of the instruments, and cost of the instruments.
Type of gas. Toxic and combustible gasses are not the same. Toxic substances are harmful to human health, often in small (parts-per-million) concentrations. Combustible gases pose a risk to both life and property, and they usually pose a risk at much higher concentrations of several percent (by volume) and upward. Specifying portable gas detectors for combustible substances takes on the added requirement of determining whether or not the instrument should be intrinsically safe. Intrinsically safe instruments are by design not capable of igniting combustible atmospheres. Electronic instruments are required to be intrinsically safe when used in hazardous (classified) locations, which is often the case when using combustible gas detection instruments.
Type of sensor. There are too many sensor types to list here, but the most common include catalytic, colorimetric, electrochemical, infrared, photoionization and semiconductor. The selectivity, accuracy and range of these sensors are all important considerations. Many level detection instruments are capable of measuring multiple gases simultaneously and will incorporate a number of different sensors.
Instrument features. The obvious physical features of size and weight become important when workers must carry instruments for long periods. Ruggedness also can be an important consideration, as well as the operating environment. Temperature, humidity and airflow can all affect the operation of the various gas sensing methods; make sure to check the operating limits of any instruments you may choose. Heated sensors (catalytic and semiconductor types) can be affected by airflow, and various methods may be required to alleviate the problem. In particular, finding gas leaks where there is a lot of air movement can be problematic.
For portable electronic instruments, a battery power source is usual. Consider battery life, the ability to use standard batteries and specialized charging requirements.
Important features on electronic instruments include the indication system, which can be both visual and audio. Level detectors often have digital displays, and leak detectors often use audio feedback as an indication method. Displays should be visible under a variety of lighting conditions, and include audio accessories such as earphones that allow monitoring in noisy environments.
Cost. Cost almost always has a major impact on the instrument choice. Beyond the basic cost of the unit, several other factors will affect the cost of ownership over time. Operator training, calibration requirements and instrument maintenance costs all come into play. All instruments require some level of instruction to be operated properly, and those instruments that incorporate lots of features may require additional equipment and training. Some instruments need calibration and check procedures that require calibration gas cylinders, gas regulators and gas flow control accessories. Other sensors, such as the colorimetric type, don’t require calibration but are disposable devices and must be replaced with each use. Electrochemical cells used for toxic gases are typically limited to a few years of usable life. Replacement costs for these can add up, especially on multi-gas instruments, while catalytic and semiconductor types for combustible gases usually have a much longer life.