Gas station without pumps

2012 September 3

PC board for pressure sensor

In Pressure sensing lab possibilities I mused about a lab based on modeling water in a tube using electrical circuit techniques:

Two reservoirs of water connected by a thin hose, with a pressure sensor near the bottom of each reservoir.

I decided that it might be too much setup and expense to do a sinusoidal excitation (though I might revisit that idea), but a step response from a bursting membrane seems simple to set up.  I’ll start with a simpler setup with just a single reservoir and sensor, to see how well the bursting membrane works for a step response.

I ordered some “biomedical” pressure sensors (Freescale’s MPX2300DT1) which have a differential output of 5µV/V/mmHg, and will need an instrumentation amp to get usable signals.

One downside of these cheap sensors is that the die and wirebond is exposed on the ambient side of the pressure sensors, so I designed a breakout board to protect the chip and allow easy wiring:

Capacitor and screw-terminal side of the breakout board.

Sensor side of the breakout board

The breakout board has a bypass capacitor on board, but no other circuitry. Since the sensor needs to be glued to the reservoir and it has only 4 rather fragile surface-mount wires to connect to the board, I’ll be supergluing the package to the board and the board to the reservoir, to avoid any strain on the leads. (I learned my lesson from the failure of a pressure sensor on the underwater ROV.)  I’ve ordered this board from OSH Park, since $3.65 for 3 prototype boards (including shipping) is a hard price to beat.

I’m providing screw terminals for the connections to a power supply and amplifier, as that is a more robust connection than a header, and less likely to put much stress on the glue joints when connecting or disconnecting.

I’ll try using the instrumentation amp prototyping board to build the amplifier circuit, most likely with a gain 0f 700 or 1000, to get a full a full-scale range of 286 mm Hg (resolution 0.28 mmHg) or 200 mm Hg (resolution 0.2 mm Hg). Note that 200 mm Hg is only 3.9 psi, so a bicycle pump would be overkill.  We could use a tiny Lego pump, or just breath (SensorWiki reports maximum exhalation pressure as around 2.1 psi—that web page also lists several low-pressure air pressure sensors, though they list $25–$30 parts, not $5 parts like the one I’m using, and the ones they list are designed for dry air, not saline solution).

I’ll need DC coupling, since I want to record the static pressure of the column of  water.  The high frequency response will probably be limited by the response of the sensor. Since I’ll be doing downward step responses for essentially full-scale steps, the relevant parameter is the 1msec 10%-to-90% rise time of the sensor.  I think that a 1kHz or 2kHz cutoff would be fine.  I may not even need to do any filtering, if there isn’t much high frequency noise. If I use the Arduino and laptop to record the step response, I may be limited to sampling at 3msec, with a Nyquist frequency of 167Hz.  I’ll look at the traces on the oscilloscope first, to see if that upper limit is good enough, or if we need to write another program to do single-shot recording into the Arduino RAM (which is really too tiny for that task).

I’ll probably build the reservoirs out of ¾” PVC pipe fittings, since that is an easy, familiar technology for me, and I have some screw-in connectors with hose barbs for tubing that I can use.  I’ll probably use a plug with male pipe threads for the sensor, so that it can be easily reused in different configurations.

I’ve not figured out what to use for the bursting membrane yet—Saran wrap? plastic bag? toy balloon?  I suspect that the plastic wrap is designed not to tear rapidly, so it might not give a very good burst with only a couple psi driving the tear.  A toy balloon may be more fragile, so may work better.

 

3 Comments »

  1. […] Pressure sensing lab possibilities  and PC board for pressure sensor, I outlined a lab based on modeling water in a tube using electrical circuit techniques: Two […]

    Pingback by Pressure sensor assembly « Gas station without pumps — 2012 October 16 @ 13:54 | Reply

  2. […] PC board for pressure sensor […]

    Pingback by Rethinking the pressure sensor lab « Gas station without pumps — 2012 October 23 @ 17:27 | Reply

  3. […] On my way to work today, I was thinking a bit about redesigning the PC boards for the circuit lab.  I have 2 boards designed so far: an instrumentation amp protoboard and pressure-sensor breakout board. […]

    Pingback by Thinking about PC boards and parts kits for circuits lab « Gas station without pumps — 2012 October 24 @ 22:52 | Reply


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