# Can a poled PVDF sheet be used as a pressure transducer?

Hello,
I believe that poling a PVDF sheet gives it piezoelectric properties, which makes it useful for pressure transducer applications (acoustics,
etc). However, my understanding of piezoelectric materials is that they generate a voltage as they are in the process of being stressed or unstressed. I have always thought of this behaviour as being dynamic (i.e. a varying pressure will cause a varying voltage to be generated). I would like to confirm if a poled PVDF sheet is also useful for measuring *static* pressure. For example, could it be used as a sensor for an electronic weighing scales, where the pressure would be constant during the measuring process? My current understanding of energy conservation laws leads me to guess that a voltage would only be generated as the weight is being applied or removed, and not under static pressure (else we could build an 'infinite' power source by sitting a weight on top of a PVDF sheet). I would be grateful for any help in clarifying this point.
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John wrote:

Transducers have input variables and output variables. Assuming power is applied, a strain gauge takes a displacement and produces a voltage, with some significant drive capability due to its low impedance. A photodiode takes optical radiation and produces electric current; over a reasonable bias range (say +0.3V to -25V for a typical silicon device) the photocurrent is independent of bias voltage--a high impedance.
Piezoelectric and pyroelectric devices (PVDF is both) produce a *charge* as their output, not a current or a voltage. Of course, since the device looks like a capacitor, the charge will produce some open circuit voltage, but it produces no dc current whatever--once the charge flows away (either by leakage or through an external circuit) it's gone.
That means that piezo and pyro detectors intrinsically detect *changes* in position or temperature, not absolute values. This takes care of your conundrum, I think.
Cheers,
Phil Hobbs
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John wrote:

There will be a decay time inherent in such a measurement. So if you want to measure pseudo-static pressure, PVDF will work. PVDF would be a bad choice for static measurements, such as weighing a rail car as it is filled.
--
Dave
dvt at psu dot edu
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John wrote:

Any polymer material will undergo cold flow (a.k.a. creep or compression set). Therefore, it is inherent in these materials that application of stress will result in a time-dependent change in their properties.
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