It is possible that many people have seen a lamp grisumétrica on occasion and know something about its use as an old gas detector “firedamp” in coal mines and underground sewers. Although its origin was to be a light source, the device also could be used to calculate the level of combustible gases with an accuracy of about 25-50%, depending on the user’s experience, training, age, perceived colors, etc.. The modern combustible Gas Detectors to be much more accurate, reliable, reusable, and although there have been several attempts to overcome the subjectivity of the measurement of safety lamps (using, for example, temperature sensors called), have now been almost completely replaced by modern electronic devices.
However, the main device used today, the catalytic detector, is somewhat modern grisumétrica evolution of the lamp, since it also works by combustion of a gas and its conversion into carbon dioxide and water.
Catalytic sensor
Almost all gas detection sensors modern low cost fuel are of the electrocatalyst. They consist of a small sensor element sometimes called “pearl”, “Pellistor” or “Siegistor”, being the last two trademarks for these commercial devices. Consist of a coil of platinum wire electrically heated, covered by a ceramic base, eg alumina, and finally with an outer layer of palladium or rhodium catalyst dispersed in a substrate of thorium.
This type of sensor operates on the principle that when a mixture of fuel gas and air passes over the hot catalyst surface, combustion occurs, and the heat released increases the temperature of the ‘bead’. This in turn alters the resistance of the platinum coil may be measured using the coil temperature as a thermometer in an electrical bridge circuit. The resistance change is directly related to the concentration of gas in the surrounding atmosphere, and can be displayed on a meter or other indicating device similar.
Sensor output
To ensure temperature stability under changing environmental conditions, the best catalytic sensors adapted using heat beads. They are in opposite branches of a Wheatstone bridge, and the sensor “sensitive” (commonly called the sensor “s”) will react with any remaining fuel gas, while a balance sensor, “inactive” or “non-sensitive” (ns) will not. The idle operation is achieved by covering the bead with a glass film or deactivating the catalyst, thereby acting only as a compensator of any change in external temperature or humidity.
The stability of operation can be improved further by using poison resistant sensors. These have a greater resistance to degradation caused by substances such as silicones, sulfur and lead compounds that can quickly turn off (or “poison”) other types of catalytic sensors.
Response Rate
To achieve the security requirements in the design, catalytic type sensor must be mounted in a solid metal casing behind a flame arrestor. This allows the mixture of gas / air is dispersed into the casing and the hot sensor, but will prevent the spread of any type of flame to the outside atmosphere. The flame arrestor slightly reduces the speed of response of the sensor, but in most cases, the electrical output give a reading in seconds after the gas has been detected. However, as the response curve levels off considerably as it approaches the final reading, the response time is often specified in terms of time spent in up to 90 percent of its final reading and therefore value is called T90. T90 values for catalytic sensors are normally between 20 and 30 seconds.
Note: in the U.S.. States. and some countries, this value is often indicated with the lowest reading T60, and therefore must be careful to compare the performance of different sensors.
Calibration
The most common mistake in catalytic sensors is the reduction in performance due to exposure to certain poisons. Therefore, it is essential that any gas monitoring system is calibrated only at the time of installation, but should be checked regularly and recalibrated if necessary. Checks should be made using a standard gas mixture properly calibrated so that the zero level “span” can be adjusted properly in the controller.
Codes of practice and EN50073: 1999 may provide some guidance on the frequency of testing calibration and alarm level settings. Normally, in principle, the checks should be made weekly, but the period may be extended as more experience is gained with regard to performance. When it takes two alarm levels are generally set in the 20-25% LEL, to the lowest level, and 50-55% LEL, to the highest level.
Older systems (and less expensive) require two people to carry out the checks and calibrations, one to expose the sensor to a gas flow and the other to check the reading shown on the scale of the control unit. Adjustments are made in this case with the controller to zero and span potentiometers measured until the reading equals the concentration of the gas mixture.
Remember that when adjustments must be made within a flameproof enclosure, first disconnect the electricity and get permission to open the box.
Now available calibration systems “one-man” that allow calibration procedures are performed in the same sensor. This greatly reduces the time and cost of maintenance, especially when the sensors are located in less accessible locations, such as oil and gas platforms offshore. In addition, there are more sensors available that are inherently designed to safe standards, and thanks to them it is possible to calibrate the sensors in a comfortable place away from the facility (at a maintenance station, for example). Because they are intrinsically safe, you can change them without problems for sensors to be replaced on site without having to power down the system for security reasons.
Therefore, maintenance can be performed in a “hot” and is much faster and cheaper than the old conventional systems.
Semiconductor sensor
Sensors made of semiconductor materials gained considerable popularity in the late 80′s. His emergence allowed to offer the opportunity to purchase a gas detector and low-cost universal. Just as catalytic sensors, are powered by gas absorption on the surface of a heated oxide. In fact, is a thin film of metal oxide (often oxides of transition metals or heavy metals such as tin) deposited on a piece of silicon, a process very similar to that used in the manufacture of “chips “computers. The absorption of the sample gas on the surface oxide, and subsequent catalytic oxidation results in a change in electrical resistance of the oxide material and may be related to the concentration of the gas sample. The sensor surface is heated at a constant temperature of about 200-250 ° C, to accelerate the rate of reaction and to reduce the effects of changes in ambient temperature.
Semiconductor sensors are simple, fairly robust and can be very sensitive. Have been used with some success in the detection of hydrogen sulfide gas, and also widely used in the manufacture of inexpensive detectors domestic gas. However, they have proven to be quite unreliable for industrial applications because they are not specifically indicated for specific gases and can be affected by variations in atmospheric humidity and temperature. May be taken to check more often than other types of sensors, since it is known that “sleep” (ie, lose sensitivity), unless it is checked regularly with a mixture of gas and are slow to respond and to recover after exposure to a gas explosion.
Thermal conductivity
This detection technique is suitable gas for measurement of high concentrations (% V / V) of binary gas mixtures. Is used primarily for gas detection with a much higher thermal conductivity than air, for example, methane and hydrogen. The gas thermal conductivities near the air can not be detected, for example, ammonia and carbon monoxide. The gas thermal conductivities of less than the air are more difficult to detect because the steam can cause interference, for example carbon dioxide and butane. Mixtures of two gases in the absence of air can also be measured using this technique.
The sensing element is exposed to heat the sample and the reference element is introduced into a closed compartment. If the thermal conductivity of the gas is greater than the reference temperature sensing element decreases. If the thermal conductivity of gas is less than the reference, the temperature of the test element increases. These temperature changes are proportional to the concentration of gas present in the sample element.
Infrared gas detector
Many fuel gases have absorption bands in the infrared region of the electromagnetic spectrum of light, and infrared absorption principle has been used as an analytical laboratory tool for many years. However, since the 80′s, electronic and optical advances have made it possible to design equipment with sufficient low power consumption and small size for this technique can also be used in detection products for industrial gases.
These sensors have several important advantages over the catalytic type. Include a very fast response speed (typically less than 10 seconds), low maintenance and a very simplified check by the auto-proven modern microprocessor-controlled equipment. Can also be designed to not affect any “poison” known, have an intrinsically safe and work properly in inert atmospheres, and under a wide range of ambient temperature, pressure and humidity.
This technique works on the principle of dual infrared absorption wavelength, whereby light passes through the mixture at two wavelengths, one of which is adjusted to the absorption peak of the gas to be detected, while the other not. The two light sources are pulsed alternately and are guided along a common optical path to go through a “window” with protection flameproof and then through the sample gas. Subsequently, a retroreflector reflects the beam again, back again through the gas to return to the unit. Here a detector compares the signal strength of the sample and reference beams and, by subtraction, there is provided a measurement of gas concentration.
Such molecules can only detect diatomic gases and, therefore, not suitable for detection of hydrogen.
Infrared Gas Detector Flammable Open Road
Traditionally, the conventional method for detecting gas leakage was fixed by spot detection, using individual sensors to cover an area or perimeter. However, more recently, you have a number of instruments using infrared and laser technology in a broad beam (or open road) that can cover a distance of several hundred meters. The old designs were used normally open road to complement the fixed detection point, but now used as a method of priority instruments latest 3rd generation. Typical applications in which they have had considerable success include FPSOs, as well as jetties, loading and unloading terminals, pipelines, perimeter monitoring, offshore platforms and storage areas of LNG (liquefied natural gas).
The ancient designs using double beam wavelength, the first coinciding with the peak of the absorption band of the gas in question and a second reference beam which is near an area without absorbing. The instrument continuously compares the two signals that are transmitted through the atmosphere, using both the scattered radiation behind a retroreflector, or more commonly, in more recent designs by a separate transmitter and receiver. Any change in the proportion of both signals is measured as a gas. However, this design is susceptible to interference from fog, as different types of fog can affect positively or negatively to the ratio of the signals, and thus falsely indicate a read / gas alarm over the scale or a read / error below the scale. The design of newer 3rd generation uses a dual-pass filter strip having two reference wavelength (one at each side of gas) that fully compensates the interference of any kind of fog or rain. Other problems associated with old designs have been overcome by the use of a coaxial optical design to eliminate false alarms caused by partial obstruction of the beam, and the use of xenon flash lamps and reliable state detectors that make the instruments are totally immune to interference from sunlight or other radiation sources such as fireplaces burning, arc welding or lightning.
Open path detectors actually measure the total number of gas molecules (ie, the amount of gas) that is in the beam. This value is different from the usual concentration of gas given in a single point and, therefore, is expressed in terms of meters LEL.
Electrochemical sensor
Electrochemical sensors can be used to detect specific gas most common toxic gases, including CO, H2S, Cl2, SO2 etc. in a wide variety of security applications.
Electrochemical sensors are compact, require little energy, show a great linearity and repeatability, and generally have a long life, usually one to three years. Response times, indicated T90, ie, time to reach 90% of the final response is typically 30 to 60 seconds and interval limits of detection ranged from 0.02 to 50 ppm depending on specified gas.
Many commercial designs of electrochemical cells, but they share many common features are described below:
Electrodes immersed three active gas diffusion in a common electrolyte, often a concentrated aqueous acid or a salt solution, for efficient conduction of ions between the active electrodes and the counter electrodes.
Depending on the particular cell, the gas is oxidized or reduced at the active electrode surface. This reaction alters the active electrode potential relative to the reference electrode. The main function of the electronic driver circuit associated with the cell is to minimize this potential difference passing current between the active electrodes and the counter electrodes, the measured current being proportional to the gas concentration specified. The gas enters the cell through an outer diffusion barrier which is permeable to gas but impermeable to liquid.
Many designs incorporate a capillary diffusion barrier to limit the amount of gas that comes into contact with the active electrode and therefore to keep the operation of cell “amperometric”.
Requires a minimum oxygen concentration to the correct functioning of all electrochemical cells, making them unsuitable for certain applications of process monitoring. Although the electrolyte contains a certain amount of dissolved oxygen, allowing the detection in the short term (minutes) of gas specified in an oxygen free environment, we strongly recommend that all the calibration gas flows incorporate air as the diluent component or principal.
The specificity for the gas is achieved by optimizing the electrochemical, ie the choice of catalyst and the electrolyte, or by incorporating filters which absorb in the cell physically or chemically react with certain gas molecules that interfere to increase the specificity of the gas. It is important to consult the appropriate product manual to understand the effects of potential interfering gases in the response of the cell.
The necessary inclusion of aqueous electrolytes in electrochemical cells results in a product sensitive to environmental conditions of both temperature and humidity. To address this, the patented design features two tanks Surecell electrolyte taking into account the “absorption” and “loss” occurring electrolytes in high temperature and high humidity and low temperature and low humidity .
The life of the electrochemical sensor is usually guaranteed for two years, but the real life time often exceeds the above values. The exception is the oxygen sensors, ammonia and hydrogen cyanide, wherein the cell components necessarily eaten as part of the reaction mechanism sensitive.
Chemcassette Sensor
Chemcassette is based on the use of an absorbent strip of filter paper serving as a reaction substrate in dry, acts both as a gas collection means as a means of gas analysis and can be used in a continuous operation. The system basa in conventional colorimetric techniques and is capable of a very low detection limits for a particular gas. Can be used with success for a variety of highly toxic substances, including diisocyanates, phosgene, chlorine, fluorine and several hydride gases used in semiconductor manufacturing.
The specificity and sensitivity of detection are achieved by using specially formulated chemical reagents which react only with the gas or gas sample. As the gas molecules are transferred to Chemcassette with a vacuum pump, the reagents react with dry chemicals and form a colored spot specific only the gas. The intensity of this spot is proportional to the concentration of the reactive gas, ie the higher the gas concentration, the darker the stain. Carefully regulating both the sampling interval as the flow velocity with which the sample gas reaches to Chemcassette, are readily detect very low levels of parts per billion (ie 10 -9).
The intensity of the stain is measured with an electro-optical system reflecting light from the surface of the substrate in a photocell located at an angle of the light source. Subsequently, as the spot develops, this reflected light is attenuated and the photodetector detects the intensity reduction in the form of an analog signal. This signal in turn is converted into digital format and then appears as a gas concentration, using a calibration curve generated internally and an adequate software library. Chemcassette formulations provide a means of detecting only one not only fast, sensitive and specific, but is also the only system available that leaves physical evidence (ie, the spot on the cassette tape) that has taken place a leak or a gas leak.
