Resistive sensors

Recent developments are leading toward other materials like silica gel or polymers. Certain types of semiconductors are also used as resistive probes. The measurement range of resistive sensors varies depending on materials used. It can be as wide as 0-99% RH. The dynamics are fast enough for normal ventilation applications and the stability of good resistive sensors is high. This does not reduce the need for calibration, but the intervals of successive calibrations can be extended. 

The observation may be by a lamp illuminating the surface and a photocell to detect the scattered light due to the water droplets on the surface. The accurate measurement of the surface temperature, which is the dewpoint temperature, is critical. If a coolant is used, a close approximation for the surface temperature is the fluid temperature otherwise a small thermocouple or resistance sensor can be attached to or embedded into the surface. 

A lock-in present in a bridge for cryogenics has usually a reference at frequencies between 10 and 1000 Hz. Electrical power delivered to the resistive sensors are between 10-16 -7-10-14 W. The maximum measuring rates are usually less than 10 readings for second. The latter fact poses severe limits in dynamic measurements as, e.g., the measurement of heat capacity (see Section 12.6.4). 

Among the cryogenic sensor, we will describe only the example of resistive sensors (Section 15.2.1) and their use in detectors for nuclear physics (Section 15.3) and astronomy (Section 15.4). 

However the carbon specific heat was not as low as the crystalline materials employed later. Also important, the resistor material exhibits an excess low-frequency noise. Nowadays, two types of resistance sensors are used to realize LTD NTD Ge sensors and TES. 

The ambient temperature sensor in the IRT 3000 is a KTY type spreading resistance sensor. The resistance of the sensor can be written as a second order polynomial function... 

CO Resistive sensors pellistors, metal-oxide sensors Optical sensors micro-spectrometer, IR-sources, IR-detectors, IR-filters Hybrid or integrated, surface micromachining Sn02 sintered thick film (Figaro, FIS,. ..), Sn02 thin and thick film on silicon (MiCS, Microsens) IR spectroscopy (Vaisala, Honeywell,. ..)... 

Exhaust gases Smoke Resistive sensors pellistors, oxygen-sensors, metal-oxide sensors Hybrid or integrated Hybrid or integrated... 

T. Takada. Temperature drop of semiconductor gas sensor when exposed to reducing gases - simultaneous measurement of changes in sensor temperature and in resistance . Sensors and Actuators B66 (2000), 1-3. 

Passive types of leak detection such as observation wells and collection sumps where product is collected and analyzed directly should work effectively with methanol. Active leak detection systems that rely on thermal conductivity and electrical resistivity sensors will not work with methanol because its properties are so different from gasoline. Another type of active leak detection system that will work with methanol or any other type of fuel relies on changes in impedance in a sensor wire as it becomes wetted with the fuel [4.5]. These leak detection systems also have the advantage that they can pinpoint the location of the leak along the length of the sensor wire. 

Although the temperature measurement ranges of thermistors are fairly wide, -200 to 300°C (-300 to 600°F), their spans can be rather narrow 1°C (2°F). These are high-resistance sensors 5,000-1,000,000 (1, with 250 ft/degree sensitivity and about 2% of span error. 

Both kinds of resistive sensors are summarized as unstructured gas sensors. Gas-sensitive field effect transistors (gas FET) are called, however, structured gas sensors. The gate of the FET is sensitized by a gas-sensitive layer (e.g., Pd for H2 FETs, Fig. 9). The gas FET operates preferably in the so-called constant charge mode, in which current and potential are kept constant. 

Also organic compound like metal phthalocya-nines [xxvi] and electronic conducting polymers [xxvii] are used as gas-sensitive layers in resistive sensors, POSFETs (polymer oxide silicon field-effect transistors), heterojunction diodes (e.g., polyrrole on... 

Nanosized FezOs-SiOz composites are considered to be promising optical and magnetic materials, catalysts, and magneto-resistive sensors. To obtain the above composites most of authors [1,2] used the combined hydrolysis of Fe salt and tetraethylorthosilicate (TEOS) or introduction of Fe salt into silica sol prepared by TEOS hydrolysis. 

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