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.

Thanks in advance,
John.

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.

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.

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.

Piezoelectrics respond to dP/dt, not to sustained P. If you want
static measurement, use a load cell. They are both sensitive and
survivable. Their cost is very reasonable and they are commercial
items from sub-atmospheric to deep sea, microgram balances to truck
stops.

Thank you all very much for your help, which clarified my understanding of PVDF.

Where can i buy poled pvdf sheets.

Also what are techniquess avaibale for poling pvdf fibers (micro or nano)?