Experiments in InfraSonics Notebook

14/08/00: DCAL401DN Ultra Low Pressure Sensor and Pneumatics:

In order to carry out experiments with IFK, AFK and FDK modulation techniques in the future as applied to InfraSonic communications I need to gain experience in the basics of the pressure sensor and associated pneumatic components.

The first setup is just the pressure sensor vented on one side to the free atmosphere while the other side is connected to an enclosed volume.   The enclosed volume was implemented by capping both ends of a 1m long by 90mm diameter plastic pipe. The principle is that the enclosed volume provides a fixed pressure reference against which atmospheric variations can be measured.  

It became immediately obvious that this setup had problems as any long-term shifts in atmosphere pressure would drive the sensor into its limits.   Additionally, the enclosed volume was extremely sensitive to temperature changes - even a body approaching the pipe as far away as about 2-3 metres would be sufficient to drive the sensor into a limit.   The solution is to limit the DC pressure response by providing a slow "leak" back to the atmosphere - effectively a high-pass filter response.  A second blind tube was attached to the enclosed volume which was pierced by a pinhole.   While this reduced the DC component, there was still a sensitivity to body approaches within a metre or so which would drive the sensor into the measurement limits (+/-400mV).

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