Load Sensor – Common Questions..

There are a number of several types of sensors which can be used essential components in different designs for machine olfaction systems.

Electronic Nose (or eNose) sensors belong to five categories [1]: conductivity sensors, piezoelectric sensors, Metal Oxide Field Effect Transistors (MOSFETs), optical sensors, and these employing spectrometry-based sensing methods.

Conductivity sensors may be made up of metal oxide and polymer elements, each of which exhibit a modification of resistance when subjected to Volatile Organic Compounds (VOCs). Within this report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) will be examined, because they are well researched, documented and established as important element for various machine olfaction devices. The application, in which the proposed device will be trained to analyse, will greatly influence the option of load cell.

The response in the sensor is really a two part process. The vapour pressure from the analyte usually dictates how many molecules exist inside the gas phase and consequently what percentage of them will likely be in the sensor(s). If the gas-phase molecules are in the sensor(s), these molecules need to be able to interact with the sensor(s) to be able to generate a response.

Sensors types used in any machine olfaction device could be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. according to metal- oxide or conducting polymers. In some instances, arrays may contain both of the aforementioned two types of sensors [4].

Metal-Oxide Semiconductors. These compression load cell were originally manufactured in Japan in the 1960s and utilized in “gas alarm” devices. Metal oxide semiconductors (MOS) have already been used more extensively in electronic nose instruments and therefore are easily available commercially.

MOS are made from a ceramic element heated by a heating wire and coated with a semiconducting film. They can sense gases by monitoring modifications in the conductance through the interaction of the chemically sensitive material with molecules that need to be detected within the gas phase. Away from many MOS, the material which was experimented with all the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Several types of MOS might include oxides of tin, zinc, titanium, tungsten, and iridium, doped using a noble metal catalyst including platinum or palladium.

MOS are subdivided into 2 types: Thick Film and Thin Film. Limitation of Thick Film MOS: Less sensitive (poor selectivity), it require an extended period to stabilize, higher power consumption. This type of MOS is a lot easier to generate and thus, are less expensive to buy. Limitation of Thin Film MOS: unstable, difficult to produce and therefore, higher priced to buy. On the contrary, it has higher sensitivity, and a lot lower power consumption compared to the thick film MOS device.

Manufacturing process. Polycrystalline is the most common porous material used for thick film sensors. It will always be prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is prepared inside an aqueous solution, which is added ammonia (NH3). This precipitates tin tetra hydroxide that is dried and calcined at 500 – 1000°C to generate tin dioxide (SnO2). This can be later ground and blended with dopands (usually metal chlorides) then heated to recuperate the pure metal as being a powder. With regards to screen printing, a paste is made up from your powder. Finally, in a layer of few hundred microns, the paste is going to be left to cool (e.g. on the alumina tube or plain substrate).

Sensing Mechanism. Change of “conductance” in the MOS is definitely the basic principle of the operation within the sensor itself. A modification of conductance takes place when an interaction having a gas happens, the lexnkg varying depending on the concentration of the gas itself.

Metal oxide sensors fall into 2 types:

n-type (zinc oxide (ZnO), tin dioxide (SnO2), titanium dioxide (TiO2) iron (III) oxide (Fe2O3). p-type nickel oxide (Ni2O3), cobalt oxide (CoO). The n type usually responds to “reducing” gases, whilst the p-type responds to “oxidizing” vapours.

Operation (n-type):

Because the current applied in between the two electrodes, via “the metal oxide”, oxygen inside the air commence to interact with the top and accumulate on the top of the sensor, consequently “trapping free electrons on the surface from your conduction band” [2]. This way, the electrical conductance decreases as resistance during these areas increase because of absence of carriers (i.e. increase potential to deal with current), as you will have a “potential barriers” involving the grains (particles) themselves.

If the torque sensor in contact with reducing gases (e.g. CO) then this resistance drop, because the gas usually interact with the oxygen and therefore, an electron will likely be released. Consequently, the discharge from the electron raise the conductivity as it will reduce “the possible barriers” and enable the electrons to start to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons through the top of the sensor, and consequently, due to this charge carriers will likely be produced.

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