– Syed Taqi Hussain

Syed Taqi Hussain

Carbon monoxide (CO) is one of the most dangerous gases known to humankind. It is highly poisonous in nature. The symptoms of its toxicity include headache, mental dullness, dizziness, weakness, nausea, vomiting, loss of muscular control, increased, and then decreased pulse and respiratory rate, collapsing, unconsciousness and death (CO > 1,500 ppm).

What makes the gas dangerous, though, is that it is odourless, colourless and tasteless. And that is precisely why CO sensors or detectors are indispensable in enclosed car parks, where the CO levels could be high. New and existing parking garages require CO-based ventilation systems designed for car park applications to provide safe ventilation rates. Indeed, using CO sensors to control ventilation fans in enclosed parking garages is a widely recognised method for not only controlling CO levels but also for reducing the energy usage by activating fans only when automobile exhaust is present.

Sensor spacing and fan selection are among key factors to be considered when designing a CO monitoring system. Parking problems of the gaseous kind.

Checklist during design phase
In terms of human safety and energy efficiency, there are quite a few factors to be considered during the design phase for installing a CO monitoring system.

Fan selection is a key factor. It is essential that proper minimum exhaust rates need to be selected. As per ASHRAE, the minimum exhaust rate is 0.75 cfm/ft² and the outdoor air requirement is 1.5 cfm/ft².

Sensor spacing and fan selection are among key factors to be considered when designing a CO monitoring system

Sensor spacing is another factor for consideration. Sensors do not have what are termed a “capable radius” or “area of coverage”. In order for a sensor to detect a gas concentration, the gas must migrate from the source location to the sensor location. As the distance from the source to the sensor increases, the migration time also increases. Subsequently, so does the time for detection and time for action via the DDC (Digital Data Control).

As a general rule, a minimum spacing of 5,000 square feet or 500m² can be considered, based on the design of the car park. Further spacing needs to be reduced, considering the layout of the car park.

Sensor properties constitute another key factor for consideration. An electrochemical sensor can replace the conventional solid state (SOS) types of devices, also called metal oxide semiconductors. One of the main advantages of an electrochemical sensor over an SOS is high accuracy at 0 – 50 ppm range. Also, a lack of interference of temperature or humidity eliminates seasonal drift of sensors, in the case of an electromechanical sensor.

The location of the sensor is yet another important factor for consideration. It is a good idea to locate sensing points on walls and columns at 1.5 to two metres above the floor level, which is assumed to be ideal by most of the manufacturers of CO sensors.

Control logic is another key factor. It is ideal for the logic to be programmed as per required air changes/hour, based on the area of the car park. Switching matrix should be based on the CO set-point. In other words, in case of an increase in the level of CO above the set-point (adjustable), the jet and extract fans switch from low to high speed.

Ideally, CO detectors need to be recalibrated at least annually but preferably every six months

Fire interlock can be hardwired through the panel or the input can be taken through the DDC to increase the fan speed and to switch off the fresh air fan, irrespective of the set-point, considering only the affected level.

These days, programmable DDC offers several logic options. Truly, what an engineer thinks can be programmed, provided the proper inputs and outputs at the DDC are available.

Maintenance is the mantra
After installing the sensor system, the major challenge is to maintain it, as the sensors require periodic calibration. Since the sensor element is electrochemical, there is constant chemical reaction in the sensor and, therefore, after a few months of installing a sensor, the sensor element loses its efficiency and stability. That is, if the CO level is high, it will still show the reading as low. This means that if the set-point is set as 20 ppm, the fans will be switched on, when the actual is above 20 ppm. In fact, it could be anything from above 20 to 300 ppm, based on the chemical reaction of the sensor element.

To reiterate the point, in order to avoid inefficient performance of the sensors, and to keep them working as per the requirement, they need to be periodically calibrated.

Ideally, CO detectors need to be recalibrated at least annually but preferably every six months in order to ensure accurate and proper operation. This needs to be stringently followed, given the potential detrimental or fatal consequences that could occur without the required monitoring or indication of harmful CO concentrations present. The duration of the recalibration process will depend upon the amount of target gas the sensor is exposed to, or the extent of deterioration of the sensor, whereby one sensor can have its calibration re-established quicker than another, depending on the installation location.

If recalibration schedules are met regularly, a CO detector should have an operational lifespan of five to seven years. Typically, regular calibration takes care of under-performance, as well as offer energy saving.

Some sensors show the reading as high, irrespective of the reading being low and, consequently, all the exhaust fans in the basement get switched on to high speed. However, the calibration procedure will fix the sensors and make them work as per their requirement.

Unfortunately, there are car parks that do not have a sensor system in place, or are not maintained properly. Both situations are dangerous, considering the health hazards, life-threatening situations or general risk to safety they pose. These can easily be avoided by providing the correct sensor systems and maintaining them scrupulously.

Electrochemical sensing technology
Gas sensing devices are available in several technologies or principles of operation. The most common are the electronic, electrochemical and metal oxide semiconductors. The selection of a specific sensing technology is usually determined by its purpose – to sense a toxic gas or to sense a combustible gas. Here, I will deal specifically with electrochemical sensing technology used for the purpose of measuring CO.

Typically, electrochemical CO sensors consist of chemical reactants (electrolytes or gels) and two terminals – an anode and a cathode. The anode is responsible for an oxidisation process, and the cathode is responsible for a reduction process. Consequently, current is created by way of positive ions flowing to the cathode, and the negative ions flowing to the anode. The output is directly proportional to the concentration of CO.

A more superior three-terminal configuration consists of the anode or a “working” electrode, the cathode or a “counter” electrode and a third, “reference” electrode. The reference electrode maintains a healthy operation of the cell. As it is surrounded by electrolyte, it sees that no gas and no current is allowed to be drawn from it. Therefore, its electrochemical potential remains constant at a level referred to as the “rest air potential”. It is used to regulate the potential of the working electrode, regardless of the current generated during operation. Some of the benefits achieved with a three-electrode sensor are extended range, improved linearity and other performance advancements. (See Figure 1)

As with all gas detectors, their operation and accuracy will deteriorate over time, depending on their installation and environment. The life expectancy of an electrochemical CO sensor depends upon several factors, such as the specific gas detected, the total amount of target (CO) gas the sensor is exposed to, temperature, humidity and pressure.
Typically, an electrochemical gas sensor should operate as designed for one to three years, as the electrolyte within the sensor cell will eventually get used up. Regularly scheduled manufacturer’s recommended maintenance (calibration) procedures should extend the lifespan of the sensor, if properly followed.

Syed Taqi Hussain is Managing Partner, Green Tech FZC. He can be contacted at syedtaqi@greentechfz.com.

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