Huiwen offers a wide range of gas sensors. It contanins four categories based on sensor technology.
Electrochemical Type Sensor
Electrochemical gas sensors are gas detectors that measure the concentration of a target gas by oxidizing or reducing the target gas at an electrode and measuring the resulting current.
Principle
The gas diffuses into the sensor, through the back of the porous membrane to the working electrode, where it is oxidized or reduced. This electrochemical reaction results in an electric current that passes through the external circuit. In addition to measuring, amplifying, and performing other signal processing functions, the external circuit maintains the voltage across the sensor between the working and counter electrodes for a two-electrode sensor or between the working and reference electrodes for a three-electrode cell. At the counter electrode, an equal and opposite reaction occurs, such that if the working electrode is an oxidation, then the counter electrode is a reduction.
Catalytic bead sensor
A catalytic bead sensor is a type of sensor that is used for combustible gas detection from the family of gas sensors known as pellistors.
Principle
The catalytic bead sensor consists of two coils of fine platinum wire each embedded in a bead of alumina, connected electrically in a Wheatstone bridge circuit. One of the pellistors is impregnated with a special catalyst which promotes oxidation whilst the other is treated to inhibit oxidation. Current is passed through the coils so that they reach a temperature at which oxidation of a gas readily occurs at the catalysed bead (500-550 °C). Passing combustible gas raises the temperature further which increases the resistance of the platinum coil in the catalysed bead, leading to an imbalance of the bridge. This output change is linear, for most gases, up to and beyond 100% LEL, response time is a few seconds to detect alarm levels (around 20% LEL), at least 12% oxygen by volume is needed for the oxidation.
Semiconductor sensor
Semiconductor sensors, also known as metal–oxide–semiconductor (MOS) sensors, detect gases by a chemical reaction that takes place when the gas comes in direct 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. The resistance of the tin dioxide is typically around 50 kΩ in air but can drop to around 3.5 kΩ in the presence of 1% methane. This change in resistance is used to calculate the gas concentration. Semiconductor sensors are commonly used to detect hydrogen, oxygen, alcohol vapor, and harmful gases such as carbon monoxide. One of the most common uses for semiconductor sensors is in carbon monoxide sensors. They are also used in breathalyzers. Because the sensor must come in contact with the gas to detect it, semiconductor sensors work over a smaller distance than infrared point or ultrasonic detectors.
MOS sensors can detect different gases, such as carbon monoxide, sulfur dioxide, hydrogen sulfide, and ammonia. Since the 1990s, MOS sensors have become important environmental gas detectors. MOS sensors although very versatile, suffer from the problem of cross sensitivity with humidity. The cause for such behaviour has been attributed to interaction of hydroxyl ions with the oxide surface. Attempts have been made to reduce such interference using algorithmic optimizations.
Nondispersive infrared sensor
A nondispersive infrared sensor (or NDIR sensor) is a simple spectroscopic sensor often used as a gas detector. It is non-dispersive in the fact that no dispersive element (e.g a prism or diffraction grating as is often present in other spectrometers) is used to separate out (like a monochromator) the broadband light into a narrow spectrum suitable for gas sensing. The majority of NDIR sensors use a broadband lamp source and an optical filter to select a narrow band spectral region that overlaps with the absorption region of the gas of interest. In this context narrow may be 50-300nm bandwidth. Modern NDIR sensors may use Microelectromechanical systems (MEMs) or mid IR LED sources, with or without an optical filter.
Principle
The main components of an NDIR sensor are an infrared (IR) source (lamp), a sample chamber or light tube, a light filter and an infrared detector. The IR light is directed through the sample chamber towards the detector. In parallel there is another chamber with an enclosed reference gas, typically nitrogen. The gas in the sample chamber causes absorption of specific wavelengths according to the Beer–Lambert law, and the attenuation of these wavelengths is measured by the detector to determine the gas concentration. The detector has an optical filter in front of it that eliminates all light except the wavelength that the selected gas molecules can absorb.
Ideally other gas molecules do not absorb light at this wavelength, and do not affect the amount of light reaching the detector however some cross-sensitivity is inevitable. For instance, many measurements in the IR area are cross sensitive to H2O so gases like CO2, SO2 and NO2 often initiate cross sensitivity in low concentrations.
The IR signal from the source is usually chopped or modulated so that thermal background signals can be offset from the desired signal.
NDIR sensors for carbon dioxide are often encountered in heating, ventilation, and air conditioning (HVAC) units.
Configurations with multiple filters, either on individual sensors or on a rotating wheel, allow simultaneous measurement at several chosen wavelengths.
Fourier transform infrared spectroscopy (FTIR), a more complex technology, scans a wide part of the spectrum, measuring many absorbing species simultaneously.