October 2011
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Gas Detection (2 of 2)

Technologies & Applications

Steven R Calabrese

Steven R. Calabrese
Control Engineering Corp.

Contributing Editor


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Welcome to part two of this short series on gas detection. If you missed part one, please go back now and read it, as it’s a prerequisite for this! Joking aside, it helps to understand the basics, as what was covered last time, in order to appreciate what we cover in this installment. So now that you’re up to date and have completed your required reading, on to the next part!

Technologies

Non-Dispersive Infrared (NDIR): Using the principle of infrared absorption, this sensing technology is based on the premise that certain gases absorb light at specific bandwidths. The sensor is “tuned” for the gas to be detected, and the concentration of gas is measured and converted into an electrical signal. This sensing technology is suitable for a wide range of refrigerant gases.

NDIR exhibits a high degree of accuracy and has a very fast response time. With minimal maintenance and a long life cycle, gas detectors incorporating this sensing technology are suitable candidates for applications requiring those features, however not to say that you can “set and forget” these things for years to come! As with all gas detection systems, proper maintenance and lifecycle replacement are critical.

Solid State: A somewhat older technology, this semiconductor-based sensing method gained its popularity in the 1980’s, and is still a viable option today. The sensing technology entails the absorption of the gas by the sensor, thereby affecting a change in the electrical resistance of the solid state component.

These are low cost sensors, are physically robust, and are able to withstand a high level of humidly. The average life cycle is reported to be in the range of 5-10 years, of course depending (on what??). They do need to be checked out and calibrated frequently, so maintenance is an issue.

Electrochemical: This sensing technology consists of passing the gas through a pair of electrodes and an electrolyte, thereby generating a current between the electrodes that’s proportional to the concentration of the gas. A popular choice, as these sensors can detect most of the “common” toxic gases.

Electrochemical sensors are low in cost, small in size, and consume very little power. They have the ability to measure relatively low concentrations, exhibit a very linear response, and have a high degree of accuracy. Average life span for these is reported to be 5 years or less.

Systems

Single-point systems are used in applications that require only a single gas detector, or in applications that require no more than a handful of detectors, spread out over a large sensing area and controlling different ventilation systems. You may find that a specification calls for a single CO detector to be located in a boiler room, to turn on an exhaust fan if the concentration rises above the TWA setpoint (see part one of this series for a refresher on setpoints).

Multi-point systems, on the other hand, are used in applications for which a single sensor is not adequate. These systems consist of a central control panel, with inputs for sensors, and relay outputs for engaging ventilation systems and annunciating alarm conditions. They’re made to accommodate 2, 4, 8, or more sensors, and depending on your application, multiple panels may be able to be “cascaded” to provide even more sensor capacity.

Recently emerging technologies have given way to the “communicating sensor buss”, which allows for the daisy-chain networking of sensors via a single twisted pair. Even slicker is the “wireless mesh network”, whereby wireless (battery powered) sensors communicate their readings from one to another, all the way back to the central controller.

Sensor Locations & Densities

Reliable Controls Sensor location is extremely important in properly and consistently detecting the intended gas. Common sense dictates that, to detect gases lighter than air, you should mount the sensors at a high level, and for gases heavier than air, mount them at a low level. While this holds true for most gases, note that CO is heavier than air, although only slightly so, and only in a completely “still” environment. Any amount of circulation will disperse the gas, resulting in the concentration of the gas to be quite consistent within the immediate space. The lesson here is, for carbon monoxide detecting, put the sensor where your mouth is (4-5 feet above the floor).

For these gases that are lighter than air, mount the sensors 1 foot below the ceiling: Nitrogen Dioxide (NO2), Methane, Hydrogen, Ammonia. For these gases that are heavier than air, mount them 1 foot above the floor: Propane, most refrigerants.

With regard to sensor densities, and what that term actually means, for single-point sensing applications, the term’s not applicable seeing as you’re only using one sensor. The location of the sensor should be readily apparent: place the sensor where the “leak” is most likely to occur. For multi-point sensing, there are rules of thumb and guidelines out there, however it’s not always that cut ‘n dried.

A popular rule of thumb in laying out a parking garage CO/NO2 detection system states that you should have a sensor for every 5,000-10,000 square feet of floor space. When figuring in terms of radius, using a 50’ radius per sensor puts us smack dab in the middle of that range. Of course in using sensors that measure concentrically, there will be areas of overlap, as well as areas of theoretical “non-coverage”. Consult the product specifications, and solicit the advice of an expert, when laying out and physically installing a multi-point gas detection system.

Practical Applications

Following are some typical applications in which you’re likely to find a gas detection strategy. Bear in mind that these are only a few of the more common scenarios, and are presented here to give a “bird’s-eye view” of the application and implementation of gas detection technology.

•    Parking Garages (CO, NO2)
•    Boiler Rooms (CO, Methane – for natural gas)
•    Chiller Rooms (Refrigerant)
•    Battery Rooms (Hydrogen)
•    Warehouses (Propane)
•    Food processing (ammonia)

Tip of the Month: Have your local expert come in and give you and your associates a Lunch ‘N Learn on the topic of Gas Detection. A gas detection system reseller is your best bet, and although he/she may be partial to the particular product they rep, you’ll still walk away from the meeting with a more thorough understanding of the general topic (and a satisfied hunger to boot!).



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