May 14, 2008

Glossary

1. What are hazardous locations?
2. What is meant by "Accuracy"?
3. What should I be monitoring for in diesel exhaust applications?
4. Solid state vs. Electrochemical for CO monitoring - which is best?
5. Where should sensors be located?

1. What are hazardous locations?
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:

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2. What is meant by "Accuracy"?
The term "accuracy" is routinely used to describe a wire 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.

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 to total deviation (+/- 0.5 ppm) or, does he/she want to reach his target within 1.0 ppm (+/- 2.0 ppm)?

Do they require consistency by having repetitions with ±1.0?

Is the user looking for resolution of 1.0 ppm?

Is a bi-directional measurement required?

How linear must the output be?

Out put variations with changes in temperature be of concern?

What effect will EMI and RFI have on the signal?

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 he does not need and may pay a price when it comes to response time or stability. Generally specified as a percentage of Full Scale (F.S.), resolution is usually indicated using the term "less than or equal to" (=).

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 (F.S.), with ± understood.

Non-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 an catalytic sensors, outputs are far from linear, but may be linearized (output is modified to compensate for the response curve of the sensor). Non-linearity is generally specified as a percentage of F.S.

Temperature Drift - the variation in output readings for 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.

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3. What should I be monitoring for in diesel exhaust applications?
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 as ventilation system to begin increased ventilation at 1.0 ppm NO2, with further increased ventilation and/or and alarm at 3.0 ppm, a reasonable, workable compromise can be made, to best ensure air quality.

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4. Solid state vs. Electrochemical for CO monitoring - which is best?
When specifying or supplying a carbon monoxide monitoring system, there are many factors to consider when choosing the sensor that will best suit your project. The Armstrong Monitoring line includes both electrochemical and solid state sensors, each with their own advantages for particular applications.

The most popular application for carbon monoxide sensors is that of parking garages, in which ventilation is controlled, depending on carbon monoxide concentration. For this application, solid state sensors (packaged in the AMC 1022 standard, AMC 3700 remote, or AMC 3701) will often fit the bill. The long life and economy of these systems usually outweigh the cross sensitivities and lack of accuracy, which is inherent in solid state technology.

If however, a more demanding application is encountered, in which a more specific and precise sensor is required, the electrochemical sensor (AMC 1040 monitor, AMC 2701 sensor/transmitter or AMC 1228-20 multidrop) is more appropriate. For circumstances in which very accurate concentration readings are required, such as those involving workplace safety or analytical applications, or projects in which cross sensitivities will be of major concern, as with most industrial or maintenance facility applications, the electrochemical approach is more appropriate.

One other factor to consider is the tightening of standards and lowering of allowable limits in various jurisdictions. A low alarm of 50 PPM was traditionally the standard, until the limits began creeping downwards toward 35 PPM. Currently, standard CO systems are calibrated with low and high alarms at 35 and 100 PPM respectively.

We have however, had requests for low alarms at 25 and even 10 PPM. In the case of 25 PPM, the limits of solid state technology are being taxed to the maximum and electrochemical sensors are a better choice. Any trip point below 25 PPM absolutely requires electrochemical sensors. Be sure to confirm your local, state or provincial regulations, when recommending or supplying any gas detection system.

With recent developments in electrochemical technology, lifespan has increased substantially and price has decreased substantially, making electrochemical sensors a more attractive solution for many applications formerly reserved for solid state.

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5. Where should sensors be located?
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 - Vapour density

Vapour Density	Ideal Sensor Location
Heavier than air*	Near the ground (no lower than 18 inches, and in some cases, no higher than 36 inches)
Lighter than air*	Near ceiling, roof or outtake fan

* 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.

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