Assignment: Venturi Channel

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Hands-on design training for MultiMEMS
D. Lapadatu and P. Ohlckers

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| 6. Assignment: Venturi Channel

The goal of this session is to learn how one can generate a pressure drop using a Venturi channel. The Venturi channel is fabricated using the front side recess etch (RECTH), which has a nominal depth of 10 microns. If a higher flow rate is desired, an additional DRIE process is offered by Sintef, offering 100 micron deep a recess etch.
The principles of a Venturi tube is described at Wikipedia, and is quite simple. If a flow passes an orifice, there will be a pressure drop through the orifice.
In the flow sensor here, the pressure drop is generated by the above described recess etch and the pressure drop monitored by the pressure sensor designed in Assignment 4 : Pressure sensor .
A cross section of the Venturi channel is shown above. The cell has of two main features:

- A backside etch the transport the fluid from the front side to the backside.
- A front side recess etch to generate the pressure drop.

Before we start designing the cell, we must decide on some dimensions. The pressure drop through a channel of rectangular cross section can be estimated by
where eta is the viscosity of the fluid or gas flowing through the channel, l is the length of the channel, Q is the flow rate, w is the width and h is the depth of the channel.
Assumptions:

- Linear operating point for pressure sensor: < 200 Pa.
- Nominal channel depth : 10 μm ±4μm.
- Flow -> 0.1 μl/min(for 100 mu etch depth: 8 ml/m)

To generate a 200 Pa drop, we then need

- Length: 1123 μm.
- Width: 1000 μm.

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Last updated: June. 2009 24.06.2009