A gas detector is a device which detects the presence of various gases within an area, usually as part of a safety system. This type of equipment is used to detect a gas leak and interface with a control system so a process can be automatically shut down. A gas detector can also sound an alarm to operators in the area where the leak is occurring, giving them the opportunity to leave the area. This type of device is important because there are many gases that can be harmful to organic life, such as humans or animals.
Gas detectors can be used to detect combustible, flammable and toxic gases, and oxygen depletion. Gas detectors are usually battery operated. They transmit warnings via a series of audible and visible signals such as alarms and flashing lights, when dangerous levels of gas vapors are detected, but there are also new systems for remote monitoring. As detectors measure a gas concentration, the sensor responds to a calibration gas, which serves as the reference point or scale. As a sensor’s detection exceeds a preset alarm level, the alarm or signal will be activated. As units, gas detectors are produced as portable or stationary devices.
Gas detectors can be classified according to the operation mechanism (semiconductors, oxidation, catalytic, infrared, etc.).Gas detectors come in two main types: portable devices and fixed gas detectors.
Catalytic bead gas sensors
The sensor is composed by two platinum spirals, both plated with a ceramic coating (alumina). One of the pellistor is soaked with a special palladium catalyst that causes oxidation - detector (sensing bead), while the other one is not treated in order to forbid oxidation - compensator (reference element). Those two filaments and their supports are fixed in a flameproof body of cell.
The working principle of these sensors is based on flammable gas oxidation on the surface of a catalytic element with electric heating. The current passes through the spirals in order to reach 450°C temperature that allows gas oxidation. When fuel gas has burned in the detector, oxidation causes a temperature increase only in the treated pellistor and not in the non-treated one (reference), causing unbalance in the bridge circuit.
Flammable gas oxidation must be used in environments containing a concentration of oxygene (O2)>15%. The sensor can be poisoned so that it cannot respond to a flammable gas if exposed to lead, silicone or certain other gases. The presence of inhibitors or poisons is the most common cause of problems in gas detection systems and, for this reason, it ’s necessary to pay attention in order to avoid any contamination.
Inhibitors (H2S, SO2, halogenated compounds) cause a temporary sensitivity loss of the sensor. Poisons affect catalytic sensor response & longevity and cause a permanent reduction of the sensor sensitivity that may be completely damaged.
Known catalytic sensor poisons: silicone oils, greases, resins (RTV adhesive), halogens ( halon, chlorine, fluorine, bromine, freon), phosphate esters, tetraethyl lead, trichlorobenzene, acid and pvc vapors, other corrosive materials.
Advantages: The principle is simple, it uses a real phenomenon, valid for all flammable gases, very short response time ( <15s.), very good repeatability, very good reproducibility, low cost, easy to calibrate and service, can be replaced easily, small, long life (usually 2-4 years), versatile in a wide range of gases.
Disadvantages: Catalyst can be poisoned by certain compounds (e.g. silicon), loss of sensitivity when exposed to high gas concentrations, frequently miss small leaks, requires constant supply of oxygen.
Here a few links about catalytic bead sensors: Catalytic Combustible Gas Sensors
The thermal conductivity gas sensors
Has been used in instruments for measuring gases above the % LEL (Lower Explosive Limit) range and for leak detection. Measuring the thermal conductivity of gases was one of the earliest forms of gas detection and it’s suitable for % volume levels of certain binary mixtures: two different gases, one of which can be air. TC gas detectors operate by comparing the thermal conductivity of the sample with that of a reference gas (usually air). This principle of detection, without chemical reaction, can be used in an atmosphere with or without oxygen. The sensor consists of two elements, both comprised of a wire coil. One element (detector) is exposed to the atmosphere, whereas the other element (reference) is sealed in a standard gas atmosphere such as nitrogen or air. The reference element compensates for changes in temperature. The elements are heated to an operating t° of approximately 250°C.
Advantages: High concentrations measurement (100% v/v), with or without oxygen, possibility of detection: helium, no poisoning, long life time, resistant filaments.
Disadvantages: This technique is only suitable for gases and vapors whose thermal conductivity is significantly different from air, thermal conductivity sensors are used primarily in portable gas leak detectors.
More about this gas sensors at following links: Thermal conductivity gas sensors
Infrared gas sensors
The non-dispersive infrared sensor, commonly referred as the infrared sensor, is based on the principle that gases absorb light energy at a specific wavelength, typically in the infrared range. Infrared sensors are used in hydrocarbon gas monitors and detectors. In addition, flame detectors commonly utilize an IR sensing mechanism. Gases that contain more than one type of atom absorb infrared radiation, so Gases such as carbon dioxide (CO2), carbon monoxide (CO), methane (CH4) and sulphur dioxide (SO2) can be detected by this means. But gases such as oxygen (O2), hydrogen (H2), helium and chlorine (CL2) cannot.
When gas passes between the source and detector, the gas absorbs infrared radiation and a lower intensity is registered at the detector. The gas concentration is directly proportional to the amount of energy absorbed and this absorption is illustrated by the "BEER LAMBERT" formula. Infrared sensors measure two wavelengths, a reference and a sample wavelength. The ratio of the sample wavelength energy to the reference wavelength energy indicates gas concentration.
Non-dispersive Infrared (NDIR) sensors are simple spectroscopic devices often used for gas analysis. The key components are an infrared source (lamp), a sample chamber or light tube, a wavelength filter, and an infrared detector. The gas is pumped or diffuses into the sample chamber, and gas concentration is measured electro-optically by its absorption of a specific wavelength in the infrared (IR).
The IR light is directed through the sample chamber towards the detector. The detector has an optical filter in front of it that eliminates all light except the wavelength that the selected gas molecules can absorb. Other gas molecules do not absorb light at this wavelength, and do not affect the amount of light reaching the detector. The IR signal from the source is usually chopped or modulated so that thermal background signals can be offset from the desired signal.
For greater optical efficiency, a reflector assembly can surround the lamp used for the NDIR sensor. The reflector is usually parabolic in shape to collimate the IR light through the sample chamber towards the detector. The use of a reflector can increase available light intensity by two to five times. The reflector surface can also be gold-coated to further enhance its efficiency in the infrared.
The intensity of IR light that reaches the detector is inversely related to the concentration of target gas in the sample chamber. When the concentration in the chamber is zero, the detector will receive the full light intensity. As the concentration increases, the intensity of IR light striking the detector decreases. Beer's Law describes the exact relationship between IR light intensity and gas concentration:
I = I0* ekP
I = the intensity of light striking the detector
I0 = the measured intensity of an empty sample chamber
k = a system dependent constant
P = the concentration of the gas to be measured
Try Beer Lambert calculator: http://www.changbioscience.com/calculator/BeerLambert.html
NDIR sensors can be used to measure practically all inorganic and organic gases, but are most often used for measuring carbon dioxide because no other sensing method works as simply and reliably for this gas. Calibration gases of specific concentration are available for determining the system constant k for any particular sensor design.
Applications for NDIR Gas Sensors: indoor air quality, cycle regulation in self-cleaning ovens, automotive and flue gas emissions, greenhouse farming, hazardous area warning signals, gas leak detection, landfill gas monitoring, alcohol breathalyzers, patient monitoring for anesthesiology.
Advantages: Can be made specific to a particular gas, require less calibration than other sensors, relatively maintenance free, does not require oxygen, freedom from poisoning, no loss of sensitivities, quick response.
Disadvantages: Dust and dirt can coat optics and impair response, not well suited for multiple gas applications, can be affected by humidity and water, high initial cost, and cannot monitor all gases.
More information: Infrared Gas Sensor
Semiconductor (metal-oxide) gas sensors
Semiconductor sensors detect gases by a chemical reaction that takes place when the gas comes in contact with the sensor. Tin dioxide is the most common material used in semiconductor sensors, and the electrical resistance in the sensor is decreased when it comes in contact with the monitored gas.
A semiconductor gas sensor (called device hereafter) possesses an electrical resistance made with a porous assembly of tiny crystals of an n-type metal oxide semiconductor, typically SnO2, In2O3, or WO3. The crystals are often loaded with a small amount of foreign substance (noble metals or their oxides) called a sensitizer. When operated at adequate temperature in air, the resistor changes its resistance sharply on contact with a small concentration of reducing gas or oxidizing gas, enabling us to know the concentration from the resistance change.
A filament inside sensor is heated by an electric current; the substratum increases its temperature until it reaches 300 to 500 °C. The sensitivity of SnO2 sensor to different gases varies with the temperature. This temperature will be chosen to work with the maximum operation sensitivity.
Advantages: High sensitivity, very good stability of the signal, long life time (~5 years), low cost, used to measure a wide range of gases and vapors, commonly used in low cost, hard-wired gas detection systems.
Disadvantages: Wide range of sensitivity (interference) to different gases, after exposure to high gas concentrations the sensor may need a recovery time of several hours and may have irreversible changes to its zero gas reading and sensitivity, exposure to basic or acidic compounds, silicones, organo-lead, sulphur compounds and halogenated compounds may have a significant effect on sensitivity, oxygen concentration, humidity and temperature may have a significant effect on sensitivity.
More information: Metal Oxide Semiconductor (MOS)
Electrochemical gas sensors
These sensors are widely used for the gas detection of toxic gases at the ppm level and for oxygen in levels of % of volume. In its most simple form, the electrochemical sensor has two electrodes, “Sensing” and “Counter: which are divided by an electrolyte thin coat. This may be in a liquid state or in a gel state and recently also in a solid state. The electrolyte is isolates towards the outside through a membrane permeable to gas. Gas enters in the sensor by diffusion, through the membrane, and there is an oxidization reaction (reduction that causes an electrical current directly proportional to gas concentration) if a polarization tension is applied to electrodes.
While electrochemical sensors offer many advantages, they are not suitable for every gas. Since the detection mechanism involves the oxidation or reduction of the gas, electrochemical sensors are usually only suitable for gases which are electrochemically active, though it is possible to detect electrochemically inert gases indirectly if the gas interacts with another species in the sensor that then produces a response. Sensors for carbon dioxide are an example of this approach and they have been commercially available for several years.
Considering oxygen electrochemical sensors there are two fundamental variations in fuel-cell oxygen sensor designs:
• Partial atmospheric pressure, is that fraction of the total atmospheric pressure due to oxygen
• Capillary-pore, these sensors are much less influenced by changes in presure than partial pressure oxygen sensor designs.
Oxygen sensors may be affected by prolonged exposure: to acid gases, to carbon dioxide (CO2), most oxygen sensors should not be used continuously in atmospheres containing more than 25% CO2, limitation of operations in extreme cold or excessively hot temperatures.
More information: Electrochemical Gas Sensors
Photoionization detector (PID)
The photoionization detector (PID) utilizes ultraviolet light to ionize gas molecules, and is commonly employed in the detection of volatile organic compounds (VOCs). The heart of the photoionization detector is an ultraviolet source, which is essentially a lamp. Requirements for the monitoring of underground storage tanks to help prevent ground water contamination began to emerge, which required monitoring of VOCs. These events led to the design of small, portable PIDs that have proven to be both practical and reliable, and which offer fast response and the ability to detect low gas concentrations. To this day, PID sensors are the preferred choice for the detection of VOCs.
More information: PID Gas Sensors