A US Patent Issued to the Present Author Was Published in December 15, 2015 

Tu Xiang Zheng

The present author is happy to speak out that a US patent issued to me was published in December 15, 2015. The title and the patent number of the US patent are “Vacuum cavity-insulated flow sensors” and 9,212,940, respectively. This invention is related to thermal mass flow sensors, which are for sensing the mass flow rate of fluid flow. In the same field the present author already hold two US patents. One is the US patent 6,139,758 with a title as “Method of manufacturing a micromachined thermal flow meter” and the other is the US patent 6,378,365 with a title as “Micromachined thermal flow meter having heating element disposed in a silicon island”. All these US patents utilized the porous silicon micromachining technology proposed in 1988 and since then continuously improved by the present author.

Flow can be measured in a variety of ways. One way is thermal mass flow sensors.

Thermal mass flow sensors generally use combinations of heated elements and temperature sensors to measure the difference between static and flowing heat transfer to a fluid and infer its flow with the fluid's specific heat and density. If the density and specific heat of the fluid are constant, the sensor can provide direct mass flow readouts, and does not need any additional pressure and temperature compensation over their specified range.

With the powerful porous silicon micromachining technology MEMS thermal mass flow sensors have been explored extensively for their simple structure and implementation in POSIFA Microsystems. The micromachining technology is amenable to creating micro-heaters and thermal sensors with no moving parts, thus simplifying fabrication and operational requirements. Other advantages of thermal mass flow sensors is small size, short response time, low power consumption, higher sensitivity to low flow rates.

POSIFA thermal mass flow sensors can be used to measure the flow of gases in a growing range of applications, such as chemical reactions or thermal transfer applications that are difficult for other flow measuring technologies. This is because thermal mass flow sensors monitor variations in one or more of the thermal characteristics (temperature, thermal conductivity, and/or specific heat) of gaseous media to define the mass flow rate.

POSIFA thermal mass flow sensors can satisfy many industrial and laboratory applications that require the detection or precise measurements of liquid flows. Commercially available liquid flow sensors, mostly are constituted by turbines equipped of an optical or magnetic pick-up, are generally very expensive devices, especially if reasonable precision and reliability are requested. Other factors that limit the extensive use of flow sensors for liquids are the difficulty of matching low flow measurement ranges with low insertion loss, the compatibility with corrosive or unfiltered liquids and the possibility to plug the sensors directly on the conducts. Such requirements are typical of the biomedical and environmental monitoring fields where the cost is also a crucial factor.

Another application of the POSIFA thermal mass flow sensors is for micro-pump controllers. Micro-pumps are the essential components in the liquid handling system, micro analytical instrumentation, genetic engineering, protein synthesis, portable sampling systems, environmental monitoring and drug delivery. Various mechanical micro-pumps with different actuating principles have been developed, such as thermo-pneumatic, electrostatic, shape memory alloy (SMA), electromagnetic as well as piezoelectric. All micro-pump controller needs to incorporate a high quality flow sensor for sensing any malfunctions that lead to an accuracy loss or accident to take place. The malfunctions generally include bubble, leakage, degradation, and failure. The small size, low power consumption, good reliability, and fast response of the POSIFA thermal mass flow sensors are preferred for this application.