Electrochemical Sensors

Principles of Operation: Electrochemical sensors usually contain anacid electrolyte, sensing electrode, counter electrode, third reference

electrode, and a gas-permeable membrane. As the air diffuses into the cell, certain gases oxidize on thesensor and a voltage differential is produced. The current produced by the chemical reaction is propor-tional to the concentration level of the reacting gas. The sensing electrode is designed to catalyze aspecific reaction.

Disadvantages: While this technology is somewhat specific, other common gases will react at differentlevels and be detected, resulting in false positives and false alarms. These sensors have a limitedlifetime and deplete over a period of time. The depletion rate is primarily determined by the sensor’sexposure to the reactant gases. Deciding when to recalibrate these sensors to maintain a specific accu-racy can be a problem. On average, most equipment manufacturers using electrochemical sensorsrecommend recalibration every three months, but this is influenced by the sensor’s reactant gas expo-sure and the required accuracy level. Electrochemical sensors will also degrade when exposed to highhumidity conditions.

Metal Oxide Semiconductor (MOS) Sensors

Principles of Operation: MOS sensors consist of a metal oxide semiconductor such as tin dioxide, onsintered alumina ceramic located inside a flame arrestor. Sensitivity to specific gases may be altered bychanging the temperature of the sensing element.

Disadvantages: MOS sensors will detect gases at lower ppm levels than electrochemical sensors.These sensors are less gas specific than electrochemical sensors and react to many types of gases,producing many more false positives and false alarms. This can shut down equipment and, when usedto measure toxic gases, can cause needless evacuation of personnel. MOS sensors are also extremelysensitive to humidity, which can affect the monitor’s performance.

Photoacoustic Sensors

Principles of Operation: Photoacoustic sensors are commonly used in many types of refrigerantmonitors. Gas levels are measured by using the momentary heat produced by a pulsed light source(usually infrared in gas monitors) that illuminates the gas, exiciting acoustic waves and producing anoise. These noises or acoustic waves are detected using a sensitive microphone. Fourier transformanalysis divides the acoustic wave into its constituent frequencies, which are compared against ameasurement of the background noise.

Disadvantages: Background noise, harmonics, or vibrations can cause problems with thephotoacoustic sensor. Monitors that include a sample pump and sequencing manifolds can producebackground noise that can yield undesirable results. Some monitormanufacturers use smaller pumps or a batch/purge procedure tointroduce a sample, while some units turn off the pump completelywhen evaluating the gas. This is one reason why most photoacoustic

Comparison of Gas Detection Technologies