5 - Measuring parameters

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

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.

Calorimetric gas sensor

ZrO₂ based amperometric O₂ sensor

Clark-type oxygen sensor

Dissolved oxygen concentration (partial pressure) can be measured by means of the Clark-type amperometric sensor (Clark, 1958). The anode is the reference Ag/AgCl electrode itself. The cathode is generally made of platinum or gold (sometimes silver or graphite), and the electrolyte solution usually contains potassium chloride with a buffering agent. The whole electrochemical cell is separated from the sample liquid with a diffusion membrane that is permeable to oxygen but impermeable to water, ions, proteins, and blood cells.The cathode reaction is the basis of operation: The cell current is theoretically a linear function of the external oxygen concentration

Thick-film gas sensor

H⁺Ion Selective FET - ISFET

pH sensitive glass electrode (left)

pH sensitive glass electrode: The electrode is used in electrochemical cell for potentiometric measurement to detect the pH changing. Inside the reference solvent and the metal electrode have constant potential, the transition potential of the glass membrane created by the hidrogen ions of the outer agent.

Ag/AgCl reference electrode (right)

Ag/AgCl reference electrode: The electrode is used in electrochemical cell for potentiometric measurement as a potential reference. Inside the concentrated KCl solvent and the AgCl electrode have constant potential. The cations dissolved in the electrolyte changes the potential of the cell according to the following equation:

"An optrode (or optode) consists of two (maybe one or more than two) fibers for the input and output light. The operation is based on spectral change of the output light caused by different physical and chemical effects at the tip of the sensor:

• Colorimetric detection is based on the color changes of indicators entrapped in the material at the sensor tip. Practically the reflected light spectra are to be measured. A special case of that method is the simple reflectance/absorbance measurement using a monochromatic light source or a special wavelength region. Sometimes the spectral change is caused directly by the color change of the analyte of interest.
• Fluorimetric detection is based on the phenomenon of fluorescence quenching. The absorbed incident light excites a secondary light wave, the spectrum of which differs from that of the incident light spectrum. The intensity of the emitted light may vary according to environmental changes.
• Phosphorescence behavior is also a secondary light emission as an answer for the excitation, however, the emission decays slowly and is present for a given period of time even when the excitation has already been switched out. Not only spectral changes, but decay time measurements can be used for analytical purposes.
• Catalyzed light emission caused by chemiluminescence or bioluminescence reaction can also be used for sensor purposes. There is no need for incident input light in this case.

Optrodes can be used for the detection of gases, ions, enzyme substrates, as well as macromolecules. The active material and the membrane at the tip may contain immobilized ionophores, indicators, fluorescent dyes, chemi- or bioluminescence enzymes, organic adsorbents, etc., according to the application purposes and operation principles. Immobilizing selective reagents and/or applying permselective membranes can increase chemical sensitivity and selectivity."