Thermal Mass Flow Sensor (MFS)
Thermal mass flow sensors can be used in various applications for gaseous media: in medical technology (artificial respiration, obstructive sleep apnue), in process technology (mass flow controller) and air conditioning, to name a few. For the application in fluidic media, this sensor can also be used after modification and customized electronic signal evaluation. This senssor is planned as a low cost solution for (wireless) flow monitoring in extend networks in municipal water supply and irrigation systems.
The primary element of this flow sensor is a heated wire (Rh) that is exposed in a media flow. As the flow passes over the hot wire, it carries away heat. The heat loss depends on the mass flow rate, the heat capacity of the medium and the temperature difference between the wire and the medium. A second sensor Rt is used to measure the media (gas or fluid) temperature. Since the heat capacity of the medium is known and the temperatures are monitored in real-time, the mass flow rate can be determined from the heat loss (related to the electric resistance of the wire) and the temperature coefficient α of the wire. In an operation in a constant temperature mode the temperature of the heated wire relative to the ambient temperature is kept constant by a Wheatstone bridge. Therefore the current (or voltage) which is flowing, is proportional to the mass flow. However, there are deviations from a linear dependence according to the “Kings Law”. This nonlinearity must be compensated by a special signal conditioning. For the measurement of the direction of a flow in gaseous media the heating resistors are arranged twice on a chip in a way that they are adjoined closely in parallel. Together with the respective reference resistors they are constituting Wheatstone half-bridges, which together with the evaluation circuit form two complete Wheatstone bridges for measurement. Each bridge is providing a change in voltage (according to a change in resistor temperature in response to a flow). The generated voltage for each bridge is different due to different cooling effects on the two resistors and thus different currents in each Wheatstone bridge to keep the temperature constant. The final signal conditioning is done by creating the difference of the two signals: Δ (Δ U1 - Δ U2). This signal has to be calibrated due to a flow, and the sign of this signal is indicating the flow direction. In addition, this method assures an independence from environmental temperature to a large extend. For the application in water only one heater is needed, however, the second heater is usefull for occasional prove of the flow direction.
The thin wires are deposited on a thin membrane, which is a stack of three layers – Siliconnitride, Silicondioxide and Siliconnitride. These layers are formed in LPCVD (Low Pressure Chemical Vapour Deposition) processes, which generate highly stoichiometric layers without contaminants and high long-time reliability. The membrane is formed by anisotropic KOH etching and shows only slightly tensile stress, which results in high mechanical stability. Beyond this the membrane enables a high thermal isolation of the heated wires to the chip edges and a fast signal response. The membrane with the wires is subsequently covered with a Siliconnitride layer formed in a Plasma Enhanced CVD process. This final passivation is known to be inert against most environmental detrimental effects and is also biocompatible. The resistors are made of Titanium and completely covered with a nanolayer of Titaniumnitride to eliminate any reaction of Titanium with adjacend media. The Ti/TiN layer show no drift due to electrical or temperature stress. The design of the MFS sensor is based on standard MOS semiconductor batch processes, available for high volume, low cost production.