Detection of gases and vapours
A.D.S. is a system designed to detect and continuously measure concentrations of toxic and explosive gases and vapors ranging from tens to thousands of ppm. It consists of detectors, concentration elements, power supply and processing units, and accessories, and uses semiconductor, electrochemical, catalytic or infrared sensors for the detecting elements.
Semi-conduction sensors respond to changes in the concentration of the detected substance by changing the conductivity of the semiconductor layer, heated to a specific temperature which - together with the layer composition - determine properties and selectivity of the sensor. The long-term development of these sensors enabled a wide range of substances to be detected and ensured the long life and stability of the sensors. Their disadvantage is - in most cases - the limited selectivity, which has to be considered for practical applications.
Electrochemical sensors behave similarly as fuel cells, in which molecules of the detected substance are oxidized or reduced at the working electrode, while oxygen is generated or consumed at the other electrode. Their main advantages are high sensitivity, excellent selectivity and linear output dependence on the concentration. A disadvantage can be seen in their limited operating life.
Infrared sensors operate on the principle of the non dispersive infrared spectroscopy and on the contrary to the other sensors they are usable in a very wide concentration range.
Catalytic sensors burn the component detected on a catalyst, the temperature changes of which are proportional to the concentration measured. This sensors react to all combustible gases and are applicable only for the measuring in air or mixtures with sufficient content of oxygen.
The system uses as detection elements detectors and concentration transmitters. Several types of central units are used for the treatment of their signal.
Detectors are delivered with the logical output of two adjustable concentration levels. As the measuring element the semi-conduction or electrochemical sensors are used. All types of detectors are fed by the direct-current voltage 12 - 18 V. The logical output is made by the open collector of an NPN transistor. In the resting state the detectors provide the current 10 - 15 mA which at reaching the adjusted concentration level decreases to zero. Detectors are delivered in the normal, rugged and Ex version.
Concentration transmitters provide the analogue output 4-20 mA which is proportional to the concentration of the component measured. They are equipped with electrochemical, less with semi-conduction or infrared sensors. The transmitters require the feeding 15 - 18 Vdc. The workmanship both normal, rugged and Ex is possible.
Central units are used for the power supply of detectors and transmitters and for the treatment of their output. In case of detectors they transfer their signals to a relays output, in case of the transmitters they display the concentration and can perform some other functions (e.g. filtration, linearization and storage, control functions, transfer to PC etc.).