There are a number of different types of sensors which may beutilized as essential components in different designs for machine olfaction systems.

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

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

The response from the sensor is a two part process. The vapour pressure in the analyte usually dictates how many molecules can be found inside the gas phase and consequently what number of them will be on the sensor(s). Once the gas-phase molecules are at the sensor(s), these molecules need so that you can react with the sensor(s) in order to generate a response.

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

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

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

MOS are subdivided into two types: Thick Film and Thin Film. Limitation of Thick Film MOS: Less sensitive (poor selectivity), it require a longer period to stabilize, higher power consumption. This kind of micro load cell is simpler to produce and therefore, are less expensive to buy. Limitation of Thin Film MOS: unstable, difficult to produce and therefore, higher priced to get. On the other hand, it offers much higher sensitivity, and far lower power consumption compared to thick film MOS device.

Manufacturing process. Polycrystalline is regarded as the common porous materials used for thick film sensors. It is almost always prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready inside an aqueous solution, which is added ammonia (NH3). This precipitates tin tetra hydroxide which is dried and calcined at 500 – 1000°C to produce tin dioxide (SnO2). This really is later ground and combined with dopands (usually metal chlorides) and then heated to recuperate the pure metal as a powder. For the purpose of screen printing, a paste is produced up through the powder. Finally, in a layer of few hundred microns, the paste will likely be left to cool (e.g. over a alumina tube or plain substrate).

Sensing Mechanism. Change of “conductance” in the MOS will be the basic principle in the operation within the sensor itself. A change in conductance occurs when an interaction with a gas happens, the conductance 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, as the p-type responds to “oxidizing” vapours.

Operation (n-type):

As the current applied in between the two electrodes, via “the metal oxide”, oxygen in the air begin to react with the surface and accumulate on the top of the sensor, consequently “trapping free electrons on rocdlr surface through the conduction band” [2]. In this way, the electrical conductance decreases as resistance during these areas increase due to absence of carriers (i.e. increase effectiveness against current), as you will have a “potential barriers” in between the grains (particles) themselves.

If the load cell sensor in contact with reducing gases (e.g. CO) then your resistance drop, because the gas usually react with the oxygen and for that reason, an electron is going to be released. Consequently, the release of the electron raise the conductivity as it will reduce “the potential barriers” and enable the electrons to start to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from the surface of the sensor, and consequently, due to this charge carriers is going to be produced.