The determination that areas can be classified as hazardous locations is based on the following: 

• the possible presence of an explosive atmosphere such as flammable gases, vapors, or liquids (Class I), combustible dusts (Class II) or ignitable fibers & flyings (Class III); 
• the likelihood that the explosive atmosphere is present when equipment is operating; and 
• the ignition-related properties of the explosive atmosphere that is present. 

This approach to classifying hazardous locations is used by the United States (National Electrical Code - NEC) and Canada (Canadian Electrical Code - CEC), 

The hazardous locations information provided on these pages is intended to answer questions associated with U.S. and Canadian classified hazardous locations. 

An area may also be considered "hazardous" for other reasons. These may include the use of electrical equipment in the vicinity of water, the risk of personal injury from moving or falling parts, or even the presence of biological hazards. While hazards are associated with all of these conditions, areas are only considered hazardous (classified) locations under conditions defined by the NEC (US) and CEC (Canada). 

Division I - There is a high probability of an explosive atmosphere in normal operation. This can be for part of the time, up to all the time. 

Division II - There is a low probability of an explosive atmosphere being present during normal operation. 

The North American class designations are: 

Class I - Contains flammable gases or vapors in quantities large enough to produce an explosion. 

Class II - Is hazardous due to the presence of combustible dust in the air. 

Class III - Contains easily ignitable fibers or flyings in the air. However, the quantities of fibers and flyings suspended in the air are not likely to be large enough to cause an explosion. 

Group designations further define the types of gases, and dusts: 

Groups Type of Atmosphere

The term "accuracy" is routinely used to describe a wide array of performance specifications when talking about measuring instruments. The designer, manufacturer and user of a particular instrument may use the same word, but can mean entirely different things, with associated varying expectations. For example,

If someone indicates that they want a carbon monoxide monitor with a range up to 200 parts per million (ppm) with an accuracy of 1.0 ppm, what do they really want?

· Does the 1.0 ppm refer a deviation of ±0.5 ppm (for a total of 1.0PPM) or ± 1.0PPM (for a total of2.0 ppm)?

· Do they require consistency by having repetitions with ±1.0ppm?

· Is the user looking for resolution of 1.0 ppm?

· Is a bi-directional measurement required?

· Is the user looking for stability over a wide range of environmental conditions (temperature, humidity, pressure, etc)?

· What effect will EMI and RFI have on the signal?

More generally, accuracy is often used as a catch phrase for many of the following terms:

Resolution - the smallest distinguishable, discrete unit. If the resolution of a sensing system is greater than separation of the reading or indicator, then "accuracy" has no relevant meaning. If on the other hand, the resolution is too fine, the user may be paying for something they do not need and may pay a price when it comes to response time or stability. Repeatability - the figure describing an instrument's ability to achieve the same result, in repeated tests from the same direction. Under identical conditions, specifications state the tolerance within which, the device will give the same output signal in repetitive cycles.

Without this information, resolution loses its practical meaning. What would be the purpose of excellent resolution if the tolerance for repeating the output signal was, for example, greater than the resolution? Repeatability is generally specified as a percentage for Full Scale, with ± understood.

Linearity - the deviation from straight-line output vs linear input. With most gas sensing devices the output is advertised as "linear" or "linearized." With electrochemical sensors, output vs concentration is very close to being linear. With solid state and catalytic sensors, outputs are non-linear, but may be linearized (output is modified to compensate for the response curve of the sensor). linearity is generally specified as a percentage of Full Scale.

Temperature Drift - the variation in output readings as a function of temperature changes. Temperature drift is one of the more simple "accuracy" parameters with the exception of the fact that there is no uniformity in the way it is specified by manufacturers of sensors and transducers. Typically, it can be specified as ± XX % full scale (or ppm) per degree F or degree C. This figure can have a great effect on final readings and should therefore be carefully taken into consideration.

Noise - the variation superimposed on the output signal resulting from either outside influences such as RFI, ground loop feedback, power source variations EMI, etc., or inherent eccentricities of the device itself. Because of the nature of noise, it cannot be specified and the general rule is to try to figure out the source of the noise and minimize it. In general terms, noise becomes more of an issue as resolution becomes tighter.

The information shown above shows that accuracy is a term that can mean many things to different people. As such, it should be used sparingly when discussing sensing devices.

Diesel exhaust contains thousands of gaseous and particulate substances, but the presence of nitrogen dioxide (NO₂), nitric oxide (NO), carbon monoxide (CO), carbon dioxide (CO₂),  sulfur dioxide (SO₂), hydrocarbons and aldehydes are of concern for the purposes of air quality monitoring.

As a diesel engine runs, the complex make-up of diesel exhaust is constantly changing because of various load conditions. These conditions are met by changes in injected fuel quantities, which are then ignited with air in the combustion chamber. The composition of exhaust gas from a diesel engine changes constantly as the fuel/air ratio is altered to meet variable demands for power.

Particulate matter is also found in the exhaust of diesel engines. This is generally what is considered to be the "smoke" of diesel emissions, and is not to be confused with the other exhaust components. Diesel particulate is a complex mixture of compounds composed of non-volatile carbon, large numbers of different adsorbed or condensed hydrocarbons sulfates and trace quantities of metallic compounds. Emissions are influenced by factors as engine type, duty cycle, fuel quality, engine maintenance, intake ambient conditions, operator work practices and emission controls.

Because of this variability it is difficult to define a typical diesel exhaust and in an ideal world, it would be prudent to measure every possible contaminant. However, as a practical, real world solution, because nitrogen dioxide is produced in measurable quantities in most diesel exhaust scenarios, regardless of temperature, fuel mixture, engine type, etc., it is the best component to use as an indicator of overall air quality.

By configuring a ventilation system to begin increased ventilation at 1.0 ppm NO₂, with further increased ventilation and/or an alarm at 3.0 ppm, a reasonable, workable compromise can be made, to best ensure air quality

For diesel and mixed vehicle applications AMC recommends monitoring for both CO and NO₂, with sensors in the breathing zone (4-5' from the floor) to provide the most traceable measurement of occupant exposure. In cases where all vehicles in a space have top-exiting exhausts it may also be advisable to have NO₂ sensors mounted at a higher level to potentially provide for more rapid detection. This should only be done in conjunction with sensors mounted at breathing height.

Traditional HVAC design requires that buildings operate in a state of near constant ventilation at maximum level for the particular type of space. While this certainly ensures plenty of fresh air, it is far from energy efficient.

Most gas detection systems operate on a principal known as "demand control ventilation", where fresh air is brought in as needed, based on predetermined concentrations of specific gases. Two of the more common examples of this in commercial buildings are in underground parking garages, and in the occupied spaces.

Most building codes require constant ventilation of underground parking garages, unless they are equipped with a gas detection system that automatically initiates ventilation at specified gas concentrations. The obvious benefit of the gas detection system is that it ensures that any occupants of the parking garage are not exposed to

dangerous levels of toxic exhaust gases. The other, less widely discussed benefit is that aside from during peak times, it is rare to see gas concentrations above the prescribed levels, which means that the ventilation system is operating unnecessarily. It is our experience that installing a gas detection system in an underground parking garage can reduce fan operation by anywhere from 70-90% which significantly reduces the associated ventilation costs through minimized energy consumption, and subsequently provides a reduction in the building's overall carbon footprint. These savings increase dramatically in parking garages that are heated and/or cooled.

The same theory can be applied to the occupied spaces of the building with similar results. Consider a large conference room with sporadic occupancy (1-2 hours per day). Traditional design would mean that the conference room is ventilated based on the maximum occupancy of the space, regardless of how many people are actually in it. By measuring the carbon dioxide (CO2) level in the space we can get a reasonable indicator of occupancy, and ventilate based on real-time need, rather than on the potential maximum, and effectively reduce the direct ventilation costs, and the related energy required to heat and/or cool the space.

The following is a general guide for the installation, mounting and locating of gas detection sensors. It is not a detailed study of all situations and should not therefore be looked upon as the definitive book on sensor placement. Sound engineering principals and common sense must still apply.

Important
Another factor in reliable leak detection is to be sure the sensor is located between the potential leak source and the ignition site or location of people.

Sensors do not detect in a dispersive manner. They rely on single point monitoring. The leak must reach this single monitoring point, in order to be detected. This is precisely the reason why the sensor location and number of sensor installed is of utmost importance.

Table A - Vapor density

* Air currents often result in inconsistent air circulation. Always be aware of areas that could facilitate irregular air currents and abnormal build-ups of gas (gas pockets).

Gas and vapour dispersion
Sensor installation should be near the potential leak source. Sensor location changes when low volatility liquids are being monitored. In this instance, sensors need to be installed in the immediate area of the leak source. Keep in mind that sensor readings take longer to register with slow dispersion liquids if the sensor is installed too far from the leak source.

Temperature limitations
Ambient temperature can greatly affect the sensor's performance. Whether too hot or too cold, make sure all sensors and electronics are operating within their ambient temperature limitations.

Vibration
Be sure to anchor any sensor installation to a firm base. Securing the sensor to a vibration source compromises the life of the sensor and may void the sensor's warranty.

Moisture
Unless installed with moisture protection accessories, sensors should be mounted away from moisture sources. When exposed to excessive moisture or direct water spray, sensors may fail, or experience shortened life span.

Accessibility
Consider "accessibility" when installing sensors that require periodic calibration. There should always be sufficient maneuverability to ensure a secure connection is achieved with calibration equipment.

Sensor orientation
Ensure the sensor to be installed is not sensitive to or dependent upon its mounting orientation in order to operate effectively.

Dust and dirt
Mount sensors away from areas prone to dust and dirt. If not feasible, make sure dust protection accessories are installed with the sensor.

In addition to being located with consideration of the factors noted in the previous section, sensor instrumentation needs to be calibrated and function tested regularly. The accuracy of the estimation will degrade through time in response to environmental conditions. While it is generally recommended that equipment be calibrated every six months, the extent of degradation and the optimal calibration frequency depends on the nature of the site. For example, a site with excessive vibration, EMI, pressure changes, presence of the target or other gasses, or other factors may require more frequent calibration. Our team can help advise you on what the right calibration schedule is for your site. 

Calibration of gas detection equipment should be undertaken by a trained professional. We can help in a number of ways: 

• We have a large team of field service technicians that travel to customer sites around the world to calibrate equipment. 
• We provide training to contractors, property managers and owners to enable calibration themselves. 
• We offer factory calibration services. With our newest generation of equipment, the sensor module can be easily swapped out and replaced by a factory calibrated sensor module, by most building personnel. The sensor module removed can be sent to us for a credit, thereby helping to reduce waste, save costs and protect the environment.