(268d) Low-Power Microflow Sensor Using Thermal Capacitance

There are many different flow sensors commercially available, and the application of each depends on the properties of the fluid such as viscousity, pressure, particle concentration, and density. For example, high viscousity fluid or slurries are usually measured by turbine and fluid trapped meters, and can be cost effective. Other meters, such as ultrasonic and magnetic meters, are designed to be contactless, usually for the purpose of medical or food safety and can be relatively expensive. Cheap meters such as float gauges  are typically difficult to integrate into control strategies.

 A micro thermal-capacitance flow sensor is relatively simple and, if mass-produced, can cost less than a dollar to produce. These sensors measure the energy transfer change associated with flow over a solid surface. Thermal flow meters fall into three categories: time of flight, hot wire, and calorimetric flow meter. Hot wire meters sense the flow rate directly from the heater resistance change. Time of flight methods sense the time for a heated bolus of fluid to travel a set distance. Calorimetric sensors measure the temperature distribution around the heater and previous research has focused on low flow rates (e.g., microliter per minute). Though flow rates of several gallons per minute of liquid are much more common in industrial and home use, microflow sensors are rarely studied in this region.

Therefore, thermal flow rate sensor with higher sensitivity is required. Etching substrate for thermal isolation or high TCR resistance are two chief methods to increase calorimetric and hot wire sensor sensitivity. Nevertheless, thin substrate or cantilever design entail increasing fabrication cost. Most importantly, both can not sustain flushing water. In contrast, glass substrate is much cheaper and has excellent thermal isolation property. With higher yield strength, it is also more robust than silicon. Glass substrate can also be used without any pre-deposition, which silicon wafer needs for surface electrical isolation. Our goal is to fabricate a cheap but robust flow rate sensor suitable in water of high velocity. Glass wafer is therefore chosen for its convenient properties.  

In this talk, we will describe the design and construction of microflow sensors for laminar and turbulent water flow rates ranging from 0.5 to 2.0 GPM -- more than one thousand times the typical current use. The sensor is easy to manufacture, requiring only two masks for fabrication. Influence of the sensor surface area, water temperature, and sensor substrate is analyzed experimentally and theoretically (i.e., COMSOL simulation). Thermal capacitance sensors from 0.5 to 2.0 gallon per minute (GPM) in a one inch diameter pipe is demonstrated.. Reynolds number range from 1600 to 6300, thus spanning laminar, transition, and turbulent flow. We found that the sensitivity rises with decreasing sensor surface area and increasing power input. Further, the sensor can function with only 2.75mW of applied power. The sensor can work at different temperatures, and the temperature shift is normalized in both simulation and experiment results.