Coatings temperature sensors

GP 1] [R 10] By proper heater design, membrane-based reactors with internal heaters allowed one to reach quasi-uniform temperatures at the membrane, which determines the catalyst temperature [19]. This thermal uniformity was checked during reaction, i.e. when large heats were released in the oxidation of ammonia and needed to be transferred out of the reaction zone (Figure 3.29). Thin-film-coated temperature sensors in the center and at the edges of the membrane served to monitor the lateral temperature difference. 

In general, most of the available temperature sensors can be used in aqueous and gaseous media, but there are also Teflon-coated temperature sensors... 

The sensor element constitutes a palladium-nickel alloy resistor with a temperature sensor and a proprietary coating. The sensor has a broad operating temperature range and a sophisticated temperature control loop that includes a heater and a temperature sensor, which controls the die temperature within 0.1°C. 

In order to determine the thermal time constant of the microhotplate in dynamic measurements, a square-shape voltage pulse was applied to the heater. The pulse frequency was 5 Hz for uncoated and 2.5 Hz for coated membranes. The amplitude of the pulse was adjusted to produce a temperature rise of 50 °C. The temperature sensor was fed from a constant-current source, and the voltage drop across the temperature sensor was amplified with an operational amplifier. The dynamic response of the temperature sensor was recorded by an oscilloscope. The thermal time constant was calculated from these data with a curve fit using Eq. (3.29). As already mentioned in the context of Eq. (3.37), self-heating occurs with a resistive heater, so that the thermal time constant has to be determined during the cooHng cycle. 

The microhotplate with the transistor heater was electrothermally characterized similarly to the procedures presented in Sect. 4.1.3. Special care was taken to exclude wiring series resistances by integration of on-chip pads that allow for accurate determination of Fsg and sd- With the two on-chip temperature sensors in the center (Tm) and close to the transistor (Tt) the temperature homogeneity across the heated area was assessed as well. Both sensors were calibrated prior to thermal characterization. The relative temperature difference (Tj - Tm)/Tm was taken as a measure for the temperature homogeneity of the membrane. The measured thermal characteristics of a coated and an uncoated membrane are summarized in Table 4.6. The experimental values have been used for simulations according to Eq. (4.10). 

D. Briand, A. Krauss, B. van der School, U. Weimar, N. Barsan, W. Gopel, and N.F. de Rooij. Design and fabrication of high-temperature micro-hotplates for drop-coated gas sensors . Sensors and Actuators B68 (2000), 223-233. 

The dynamic response of most sensors is usually much faster than the dynamics of the process itself. Temperature sensors are a notable and sometimes troublesome exception. The time constant of a thermocouple and a heavy thermowell can be 30 seconds or more. If the thermowell is coated with polymer or other goo, the response time can be several minutes. This can significantly degrade control performance. 

Figure4.108 Coating module (processing from left to right 1, unwinding 2, coating 3, drying compartment with temperature sensor 4, additional driven roller pair 5, up-winding) and continuous etched foil (length 72 m, width 150 mm, thickness 300 pm, structure dimensions breadth x height x length = 300 x 200 x 10000 pm and 500 x 300 x 5000 pm) [109].

Since the adsorption of a gas is able to modify the dielectric constant of zeolites, chemical sensors based on interdigital capacitors (IDCs) using zeolites layers as sensitive coatings offer a wide field of applications depending on the type, modification, and working temperature of the coated IDC sensor. 

Fig. 15. Schematic presentation of the sensor (a) measuring electrode design coated with zeolite film (top) and heater electrodes (bottom), (b) cross section. The resistance of the Pt heater is also used as temperature sensor. [ 1271...

The use of polyvinylpyrrolidine (PVP) as a sensitive coating for the determination of ammonia in the range 6 ppb to 10 ppm was also reported (21). The PVP coated crystal was part of a multi sensor sysem containing a silver chloride coated crystal for water, and a temperature sensor. The optimum flow-rate was found to be 50 ml min", with equilibrium established after 15 min. 

Progress continues to be made toward the development of practical vapor detection instruments using coated SAW sensors. This paper reviews some of the characteristics of these versatile devices and their current capabilities. Factors affecting sensor performance, such as temperature and coating selection, are considered in terms of the sensitivity and selectivity of the SAW device. Special attention is given to a 4-SAW array instrument that is able to "fingerprint" and identify a variety of chemical vapors. 

Figure 1.5 Microreactor with permeable porous silicon membrane (squares of 350-750pm, thickness 70pm) coated with Pd (via immersion) with heating filament and temperature sensor for CO measurements up to 140°C. Reprinted from [37], Copyright 2002, with permission from Elsevier.

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