Flow rate influencing effects of micropumps

Current micropump technology features bubble tolerance and self-priming ability, achieved with high pressure ability at shallow pump chambers for low dead volume and high compression ratio. Though, the pump dynamics are strongly affected by a pump chamber height dependent viscous squeeze film damping. Therefore, we investigated all major flow influencing effects for micro diaphragm pumps, being liquid resistive damping, cavitation based gaseous capacitive damping and reactive valve volumes. Analytical models were developed that are able to describe and predict the transient actuator displacement, derive the frequency dependent displacement and estimate the valve loss effect in order to determine the flow rate performance of micropumps. In addition, a new oscillator model is able to simulate the inception of cavitation and the time course of both actuator and liquid. The individual effect models were combined into a single-cycle based, frequency dependent, flow rate model, validated for frequencies of up to 100 Hz. Compared to the maximum achievable flow rate, the liquid resistive damping reduces dynamic stroke volume and is responsible for a reduction of 23-57 % at 100 Hz. Valve induced flow losses reduce the effectively redirected volume towards the outlet. The effect is enhanced by the strong damping, but also feeds back on the actuator by reducing initial damping significantly. Reactive valve volumes amount for 6.2-12.4% flow rate reduction at 100 Hz. Full cavitation, generated at the supply mode and affecting the pump mode, can cause gas bubble generation and lead to a drop of flow rate down to 65% at 80 Hz. By placing an in-line degasser the flow stability was improved to ±2.2% over 10 h. After reducing the frequency to 20 Hz, cavitation only affected the supply mode and in combination with a degasser a flow stability of ±0.15% over 2 h was achieved. By reducing voltage levels, cavitation was avoided completely, featuring a stability of ±0.08%. The understanding of the flow rate limiting effects enables the application specific design to meet all performance and reliability requirements for micropump based liquid dosing systems.