Thermoresistive Flow Sensors

A thermoresistive flow sensor refers to the flow measurement element based on monitoring the thermal states of an electrically heated sensor whose dependence on the heat loss represents the flow rates of the surrounding fluid. The thermoresistivity of a sensor reflects its variation in electrical resistivity with temperature. A sensor element should usually have a sufficiently high-temperature coefficient of resistance (TCR). Typical sensor materials are platinum, nickel, or polysilicon with a specific TCR in the... 

The electrical resistance of a thermoresistive flow sensor can be calibrated as a change in the... 

Thermoresistive Flow Sensors, Fig. 1 Wheatstone bridge circuit for control and evaluation of hot-wire or hot-fihn sensor (a) constant-current bridge (b) constant-temperature bridge (Modified from [3])... 

When an electrical current /q is applied on a thermoresistive flow sensor and its self-Joule heating is maintained at a constant level, any fluid flow through the sensor will cause a shift in its temperature and thus resistance R, respectively. As a result, a decrease in the voltage V (=1qR) of the sensor will be detected. The aim of the thermoresistive flow sensor is to evaluate such changes caused by variations of the flow rate alone. 

The most typical electrical circuits for control and evaluation of a thermoresistive flow sensor are based on the well-known Wheatstone bridge using either the constant-current or the constant-temperature mode, respectively [4]. In a constant-current circuit (see Fig. la), resistors R and R4 at the two arms are much larger than the sensor... 

In each of the above thermoresistive flow sensors, the sensor elements are powered by an... 

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]. 

Thermoresistive Flow Sensors, Fig. 4 Flow measurement by time-of-flight principle, (a) Sensor configuration (b) sensor signal (Modified from [V, 9])... 

Modeling the electrical, thermal, and fluidic behavior of a microfluidic flow sensor and its surrounding structures will provide comprehensive information on many of the physical and dynamic behaviors of the thermoresistive flow sensors located within microfluidic environments [2]. An accurate device model and simulation offer the designer many advantages such as reducing the time needed for the development cycle and providing the possibility for device optimization through software instead of iterations on physical devices. This, in turn, helps to reduce the end cost of the device. Here, we will... 

Thermoresistive Flow Sensors, Fig. 5 Side view geometry for a typical calorimetric flow sensor with a middle heater and upstream and downstream temperature sensor (Modifled from [2])... 

A complete simulation of the above coupled equations and boundary conditions would help to give a complete picmre of the detailed working process of the thermoresistive flow sensor including the electrical, thermal, and fluidic phenomena [2]. It would also provide a useful way to optimize the geometry and material of the whole fluidic device. 

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