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advice on pressure sensors

Discussion in 'Electronic Design' started by amorphia, Oct 13, 2005.

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  1. amorphia

    amorphia Guest

    Hi all,

    I'd much appreciate any advice on my search for a cheap and simple
    pressure sensor. I need to make a device which can measure how hard a
    baby is squeezing a silicon tube, around 2cm thick. I figure the best
    way is to connect the tube via a pneumatic tube to a pressure sensor.
    So what I need is a pressure sensor that does not have to be accurate
    in absolute terms, but needs to be relatively sensitive, so it can
    measure changes in grip strength. I reckon if it went up to 1000 kPa
    that would be more than enough. Preferably it would come with an
    integrated USB interface and drivers which provided an API.
    Alternatively, if you know a good one but it just outputs a voltage,
    perhaps you could also recommend a good cheap and cheerful USB A to D

    So far the best thing I've found is this, which you can combine with
    a USB interface from the same supplier:
    But I think it will only interface using their proprietary software -
    it's for classroom use, after all. But this gives you a basic idea of
    what I'm after.

    Other solutions to the basic problem also gratefully received!


  2. Guest

    Sounds reasonable to me. Another way to do it is probably with a strain
    gauge, but that might require that the baby hold something in a specific
    orientation, which isn't going to happen.

    I wonder if it might be helpful to have a short section of relatively
    soft tube for the baby to squeeze, and then a longer section of stiffer
    tube linking the squeeze tube to the measurement device. This way, the
    pressure increase provided by the baby's squeeze isn't offset by the
    long tube to the sensor swelling up.
    Maybe babies are stronger than I think, but that seems like a lot of
    pressure (145 psi). I just did an experiment in my garage with some
    automotive fuel line and a pressure gauge. The fuel line is braid-
    reinforced rubber hose, about 15 mm outside diameter and 8 mm inside
    diameter - about 3.5 mm wall thickness. I used a piece about 7 cm
    long, plugged one end of it, and connected the other to a pressure
    gauge. Even squeezing the hose with fingers from both hands, the best
    I could develop was about 5 to 6 psi (34 to 42 kPa) over atmospheric.

    The silicone tubing that you propose is very likely much more flexible
    and easier to squeeze, and has a larger internal volume. However, I
    still think you won't get much pressure this way. If you assume that
    there is a 8 cm total length of tubing, it starts at atmospheric
    pressure, the baby can completely flatten the last 4 cm of it, and
    the remaining 4 cm don't swell up at all, then the pressure in the
    tube will be about twice atmospheric.
    You might look at the current Digi-Key catalog, pages 1601 to 1610.
    Sensors that might do what you want with a 0-5 V output are around
    US$17 to $35 in quantity 1. I know that USB A/D boxes exist but I
    don't have a favorite. Depending on how fast you need to make readings,
    an alternative might be a digital multimeter that has RS-232 serial
    output, and a USB-to-serial adapter cable. This is probably only good
    for up to a few readings a second, though.

    Matt Roberds

    PS I very much enjoyed reading about the results of the crow experiments
    back when they came out.
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