5 - Measuring parameters
5.8 - Chemical sensors
Humidity sensors
Photoelectric-dew point detector
The photoelectric mode is used most frequently for dew-point detection. The condensation surface is polished to mirror-quality reflectivity. A light beam is aimed at the mirror and one (or more) light sensors receive the light reflected from the mirror; condensation is accompanied by an abrupt change in the amount of light reflected to the sensor(s).
Relative humidity detector
Gas sensors
Fluorescent oxygen optrode
Fluorescent optrodes utilize the fluorescence quenching of special indicator dyes. In this cases, the incident light excites a secondary light emission with a different wavelength. Several sensor types were developed for measuring the partial pressure of molecular oxygen in liquids, for instance, in blood. The fluorescent dye, such as perylene dibutyrate, is absorbed to organic beds contained within a hydrophobic gas permeable membrane, such as porous polyethylene tubing. The dye is excited with blue light (468 nm) and it emits radiation at 514 nm (green). The oxygen partial pressure can be calculated according to the Stern-Volmer equation:
where A and m are empirical constants. Fluorescent dyes can also be applied for pH measurements and in other ion-selective optrode types.
Pd-gate FET
The structure of this sensor is similar to the conventional well-known MOSFET transistors, except for the different type of the metal control (gate) electrode. Hydrogen molecules will dissociate on the Pd surface and then the hydrogen atoms will be adsorbed. The next step is absorption and permeation of the atoms to the Pd-oxide interface. The absorbed hydrogen atoms will shift the characteristics of the FET, thus varying the threshold voltage VT.
Taguchi gas sensor
The sensing material in Taguchi gas sensors is typically SnO2, either in think-film or thin-film form. When a metal oxide crystal such as SnO2 is heated at a certain high temperature in air, oxygen is adsorbed on the crystal surface with a negative charge. This results in a negative shifting of the resistance by different mechanisms. In the presence of a deoxidizing gas, the surface density of the negatively charged oxygen decreases, so the resistance of the sensing material will increase towards its original (when heated-up) value.