Hot-wire anemometers have proven useful for macroscale measurements. For example, hot-wire anemometers are used to monitor air flow within automobile engines, ventilation and heating ducts, and the like. A hot-wire anemometer uses a heated wire positioned within the flow of media, such as gas, liquid, particle-laden liquid, or the like. As the media flows past the hot wire, heat is transferred from the hot wire to the media, cooling the hot wire. Flow can be determined from the temperature variation effects on the hot wire.
Less success has been achieved in using hot-wire anemometers in a microscale environment, such as microfluidics. Conventional macroscale hot-wire anemometers cannot easily be scaled into microscale dimensions. For example, as the scale of components is reduced, the components can become fragile and be easily damaged during the fabrication process. While micromachining techniques can be used to form small-scale structures, micromachining techniques are typically complex and time consuming.
Hewlett Packard inventors Douglas Snell and Theodore Kamins succeeded in creating a nano-anemometer by growing a nanowire to span an open area between a pair of electrodes. The nanowire is coupled to a sensing apparatus to form a hot-wire nano-anemometer. According to U.S. Patent 7,591,193, multiple nanowires can be used to increase the sensitivity of the nano-anemometer or to provide for multiple points of measurement. Various units of measure for flow rate can be obtained. For example, flow rate may be expressed as a velocity (e.g. distance/time), mass times velocity, mass per unit time, volume per unit time, etc. For example, from a flow velocity measurement, flow volume rate can be calculated by taking into account the dimensions of the open area through which the media is flowing. Various other measures, such as total flow over a given time interval, can also be obtained, as will be appreciated. While high resolution measurements of flow rate or flow volume can be performed, coarse measurements can also be performed. For example, the nano-anemometer can be used to determine presence or absence of a flow through the open area. Measuring flow velocity of fluids is helpful in a variety of application, including for example research, metering, monitoring, and similar applications.
