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There is an array of different sensors for different applications. Jörg Kühn explains the benefits and disadvantages of each kind and how to use them safely.
Each of the following sensors – electrochemical, catalytic bead, solid state, infrared and photoionisation detectors – must meet certain criteria to be practical for use in area air quality and safety applications. Some of the basic requirements are:
1. The sensor should be designed for a housing that is small and rugged. The sensor should be suitable for use in hazardous locations, harsh environments and should also be explosion proof. The sensor should be cost-effective, designed for installation and use in industrial production areas, and installable at a reasonable cost.
2. For portable applications, the instruments should have reasonable energy consumption and the option of powering the instruments with batteries should be available. The instruments should be small and compact so they can be carried easily. They should be safe for use in industrial environments. Preferably, the instruments should be certified as intrinsically safe for use in hazardous areas.
3. The operation and maintenance of the instruments should be easily performed by regular plant personnel with minimal special training requirements.
4. In stationary installations the sensors should be able to function continuously and reliably for a period of time, preferably longer than 30 days. The sensor should be able to function in an industrial environment for at least two years or longer and should be replaceable or renewable at a reasonable cost. It should be easy to install into a multipoint system and be managed by a controller or a computer controlled distribution system.
5. The cost of the instruments should be reasonable so that multiple sensors can be installed to effectively protect the area.
Four of the five sensors discussed in this article all meet the above criteria. The exception is the photo-ionisation detector. The PID is a good detector for portable applications but is limited by the lamp because it has a relatively short life expectancy and the frequency of maintenance required may not be practical for stationary applications. However, there are PID stationary instruments available that can be useful as long as users are aware of their limitations.
There are other types of sensors which meet the above criteria, but most have limitations. For example, thermal conductivity sensors are mostly used for high concentration applications and are not widely used as gas monitors.
Factors to Consider When Selecting Sensors
One of the most frequently asked questions regarding sensors is: “Which sensor is the best?” Of course, there is no simple answer to this question. Each sensor has certain capabilities and limitations, and thus the suitability of a given sensor depends largely on the application in which it is to be used. Thus, to choose the correct sensor, one must first properly define the application. It is common for manufacturers to exaggerate the capabilities of the sensors that they offer and downplay sensors that they do not offer.
In determining which sensor to use for a given application, the following factors/observations should be considered:
A. Realistically define what objective one is trying to accomplish and define an instrument specification that meets the minimum requirements. The specifications should define the gases and ranges of the sensors. The detection ranges of the sensors for the gases to be measured should be 3 to 5 times the actual (expected) monitoring concentration. As with a voltmeter, one should always select a range higher than the actual voltage to be measured. For example, select a 50-volt range to measure a 12-volt battery.
B. Determine the background gases in the monitoring area. In cases where the background gases cannot be determined a representative sample should be analysed. A major cause of sensor failure is the presence of background gases that the instrument’s manufacturer did not take into consideration and which might lead to wrong readings or even destroy the sensorcell. The selectivity or specificity of the sensor must be acceptable for the application.
C. The temperature ranges in which the sensor is to be installed should be within the sensor specifications and should be suitable for the gases to be monitored. For example, jet engine fuels have very low vapour pressure. It is useless to install a sensor to measure the combustible range in a hanger if the temperature will never exceed 100 degrees F because the vapour concentration cannot reach combustible levels. In this case, it is more appropriate to measure in ppm ranges. The temperature changes between day and night, and during summer and winter should also be considered.
D. A wide temperature change can cause moisture condensation. This is particularly important in confined spaces such as closed containers where air circulation is poor. A typical specification for humidity is 95% non-condensating. The occurrence of condensation is a function of temperature change, as seen on wet windows and car windshields in the morning. Normally, there is no problem in ordinary industrial background environments, even during the hot summer months in coastal areas such as the Gulf of Mexico, as long as the air circulation is normal. Areas with poor air circulation can often suffer from condensation. Both solid state and catalytic sensors have heated elements.
In addition, their transmitters are designed to operate at 14-24 VDC, which generates heat. Therefore, the sensor transmitters are always a few degrees warmer than the environment in order to minimize the possibility of condensation. Electrochemical sensors normally require relatively much less power; therefore, the temperature of their transmitters is similar to the surrounding temperature. In this case, it is easier for condensation to occur.
E. In applications requiring the sensors to be constantly exposed to gas special considerations are required and therefore not only the sensor specification sheet but also the supplier of the sensors may need to be consulted because of their experience and knowledge. A properly designed sampling system may make difficult or apparently impossible applications easy to handle.
It is difficult to mention every consideration needed, but a carefully evaluated and studied application can yield savings in both time and money. With the specifications of the sensors one can decide which sensor best meets one’s requirements. There is no general consensus that establishes which sensor is the best for a given application. Hence, the information that follows contains some guidelines that may be helpful in making the proper selection of a sensor.
Toxic versus Combustible Gas Monitoring
Gas monitoring applications are generally classified into toxic or combustible range monitoring. Toxic gas monitors are generally used for human health protection and the ranges of the monitors are 3 to 5 times higher than the permissible exposure limits displayed in ppm. For most combustible gases, ranges are typically 100% lower explosion limit (LEL) or a fraction of this range, such as 50% LEL. The gas concentrations are high and are generally in the range of several percent (Vol.).
In other words, for toxic gas applications a sensor must be able to measure gases at lowest concentrations while, for combustible monitoring, a sensor must measure high gas concentrations.
Summary
Electrochemical Sensors: Except for oxygen metering applications, electrochemical cell sensors are designed to be used as toxic gas monitors. These sensors are only suitable for low concentration ppm ranges. For portable applications, the electrochemical sensor life expectancy is two years; however, depending on the application, it can be much shorter. The cost of replacement sensors is high, especially when the number of instruments in use is large. The annual budget and labour to keep the instruments functioning need to be considered. There are approximately 20 gases that can be monitored by electrochemical sensors. For the rest of the gases (for ppm ranges), solid state sensors or PIDs need to be used.
Catalytic Sensors: In portable monitors for combustible gases in the LEL ranges a catalytic sensor is good for normal, simple applications. The sensors can last for a long time because they are used sporadically. Catalytic combustion sensors are relatively inexpensive but you have to make sure they are made by reputable suppliers.
Infrared and Solid State Sensors: For gases that can poison the catalytic sensors and make them impossible to use the choice is between infrared sensors and solid state sensors. Depending on the gases to be detected, infrared sensors have the better performance but detection of gases is limited. On the other hand, solid state sensors can detect most chemicals in the LEL ranges.
In stationary applications, the sensors are constantly exposed to environmental background gases. For toxic gas applications, it is generally favourable to use solid state sensors, especially when the number of sensors is sizable. In applications where interference can be a problem, it is best to study the sensor specifications and to consult the supplier.
For stationary combustible gas applications, the choice is between catalytic sensors, solid state sensors and infrared sensors.
Published: 10th Oct 2002 in Health and Safety International
