Hot film sensor

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

Hot-wire (hot-film) sensors measure the effect of the flowing fluid on a hot body. At a constant heat power, the forced convection causes a decrease in the temperature of the heater. 

Drag reduction can be achieved by direct injection of microbubbles through slots or porous skin (193-196) or the generation of hydrogen by electrolysis at the wall (197). The primary parameters, independent of gas type and Reynolds number, appear to be the actual gas flow rate referenced to injector conditions of temperature and pressure (198-200) and the location of the bubbles in the turbulent boundary layer (198,199,201-203). Merkle and Deutsch (196) have provided a comprehensive review on skin friction reduction by microbubble injection. Mahadevan and co-workers (204) postulated that microbubbles like polymer solution destroy turbulence production by selectively increasing the viscosity near the buffer region. They increase the local dynamic viscosity. Pal and co-workers (205) demonstrated that microbubble and polymer solution shear stress statistics as measured by flush moimted hot film sensors are similar at equivalent value of drag reduction. 

The operating modes for the microthermoresistive flow sensor are similar to their conventional bulk ones. For traditional research applications, cylindrical sensors which take the form of either a fine wire (typical diameters of 1-15 xm) or a cylindrical film (fypical diamefers of 25-150ixm) are the most common. The wire sensor is mainly used for clean air or gas measurement due to its fragility, while hot-film sensors can be used in both liq-... 

Thermoresistive Row Sensors, Figure 2 Principle for a single hot-wire or hot-film sensor to measure microchannel flow, (a) Sensor configuration (b) Sensor temperature... 

For the present study, a general purpose, hot film sensor was purchased from TSI Incorporated (model no 1210-20). The manufacturer certified the probe for single component air velocity measurements at ambient temperatures up to 150°C with a recommended minimum velocity of 0.15 m/s and a maximum velocity of 350 m/s. 

Fig. 13.1 The Hot Film Anemometry (HFA) method [54, 199]. a The HFA measuring system, b Hot-film sensors, c Flow-direction measurements. The figures b and c are reproduced from Doebelin [54] (pp. 608-609) with permission from McGraw-Hill Education...

This section describes a methodical procedure that allows reliability issues to be approached efficiently. MEMS reveal specific reliability aspects, which differ considerably from the reliability issues of integrated circuits and macroscopic devices. A classification of typical MEMS-failure modes is given, as well as an overview of lifetime distribution models. The extraction of reliability parameters is a Tack of failures situation using accelerated aging and suitable models. In a case study, the implementation of the methodology is illustrated with a real-fife example of dynamic mechanical stress on a thin membrane in a hot-film mass-airflow sensor. 

The approach diagrammed in Figure 5.9.2 has been used in a real-life reliability problem concerning the dynamic mechanical stability of a thin-film membrane. The membrane is part of the sensing element of a hot-film mass-airflow sensor. It consists of several dielectric layers with an overall thickness of about 1.5 pm and a thin-film metallization that forms heating elements and thermosensors on a silicon chip. The schematic layout of the sensor is shown in Figure 5.9.13 its principle of operation is described in more detail in Section 7.6. 

The resistive type of anemometer (hot wire or hot film) described earlier relies on the temperature dependence of the hot sensing element to monitor the convective heat loss. The resistance is maintained at a constant value by adjusting the Joule heating to maintain a constant temperature. That power input is used as a measure of the convective or other heat losses. This type of anemometer is in widespread use today. However, the energy loss of the resistive anemometer depends on the temperature of the fluid ambient in which it is immersed. Changes in the fluid ambient temperature are reflected in the power required to maintain the anemometer s resistance and this is the reason why temperature compensation is needed for this type of sensor. 

There are three types of thermal flow sensors that combine the evaluation methods and heater modes hot wire (hot film), calorimetric, and time of flight. 

The highest flow measurement resolution for a thermoresistive flow sensor has been in the order of nano- or microliters. To illustrate the basic method of characterizing the sensitivity of a thermoresistive flow sensor, we consider here only a single sensor element for a lab-on-a-chip application, i.e., the hot-film case. For a single thermoresistive element, it utilizes a miniaturized adaption of the anemometric principle. The combination of an established detection principle with the benefits of microtechnology has resulted in a sensor with very high sensitivity [9]. 

Thermal hot-wire and hot-film anemometers (HFA) have been available for measuring instantaneous single phase fluid velocity for about a century. The small size of the sensor and good frequency response makes them especially suitable for turbulence investigations. These techniques are thus commonly used in many engineering disciplines. 

Bruun HH (1996) Hot-film anemometry in liquid flows. Meas Sci Technol 7 1301-1312 Burgess JM, Calderbank PH (1975) The measurement of bubble parameters in two-phase dispersions-1 the development of an improved probe technique. Chem Eng Sci 30 743-750 Cao Z, Xu L, Fan W, Wang H (2010) Electrical capacitance tomography with a non-circular sensor using the dbar method. Meas Sci Techn 21(1-6) 015502... 

Temperature control is one of the longest established and most important functions in household appliances. One example of modern thin film fabrication technology of platinum temperature sensors with application examples in the kitchen in hot plates and ovens is given in Chapter 5.1. 

As opposed to inertial sensors, micromachined anemometers are often based on micro hot plates, that is, combinations of heaters and thermometers on thermally insulating membranes or multilayered thin films [4]. Functional sensor parameters are obviously determined by thermal conductivities and heat capacities, which can be monitored to reject faulty chips. Until recently, suitable wafer-level testing methods have not been available, and most sensor designs were based on literature values, despite the fact that these values depend strongly on fabrication processing parameters. 

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