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'[EE]: Looking for Piezo material'
2002\08\29@054727 by Morgan Olsson

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Me searching again

I am looking for a reliable way to detect water, although the sensor is covered with a layer of dirt, oil film, etc.  And the water may be pure or contaminated.
(=not possible to use electrodes or optics, and capacitive is not reliable for pure water, as the dirt may conduct better...  Other ideas are welcome! )

My idea is to place two small piezo crystals with some mm gap between.  The gap is to be wide enough to the dirt don´t bridge all over it.  Then when water arrives it fills the gap and acousticly couples the piezos.

Normal piezo speakers are too wide to fit, and probably to fragile.  I think a more cubic formed multilayer piezo would be better.

On my table i have a nice pamphlet from Philips showing exactly what i want :)
...but when i call them they say piezos was dropped or sold off years ago :/

So... anyone have an idea of where to find such cuties?

/Morgan
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2002\08\29@071142 by Marcelo Y.

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There are several manufacturers of ultrasonic level detectors. Have a look
at: http:http://www.endress.com
If you want to make your own, try to contact Morgan Matroc. Their ceramics
are not cheap, but are one of the best.
http://www.morganadvancedceramics.com

By the way, they bought Philips' facilities of piezoceramics.

Marcelo Y.

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2002\08\29@101215 by Russell McMahon

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> I am looking for a reliable way to detect water,
> although the sensor is covered with a layer of dirt,
> oil film, etc.  And the water may be pure or contaminated.

Nuclear Magnetic Resonance perhaps?



       RM

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2002\08\29@115247 by Roman Black

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Russell McMahon wrote:
>
> > I am looking for a reliable way to detect water,
> > although the sensor is covered with a layer of dirt,
> > oil film, etc.  And the water may be pure or contaminated.
>
> Nuclear Magnetic Resonance perhaps?


Thermal conductivity might be the easiest of all
methods. My motorcycle uses thermistors inside
metal tubes in the fuel tank to detect when the
liquid touches them (level).
:o)
-Roman

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2002\08\29@122237 by Brendan Moran

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> > I am looking for a reliable way to detect water,
> > although the sensor is covered with a layer of dirt,
> > oil film, etc.  And the water may be pure or contaminated.
>
> Nuclear Magnetic Resonance perhaps?

Have you worked with NMRI?

Cool idea, but it doesn't seem practical.  It's possible that for simply
detecting the presence of some water it would be far simpler than actual
NMRI, but I think the field strength wouldl be a concern

He said it had to be small...  That implies a low magnetic field,
considering that the magnets I work with weigh in at 17 metric tonnes, and
they're only .35 Tesla, though the gap is fairly large, and for this, your
gap could be quite small.  The higher the field strength, the higher the
output signal.  I assume a permanent maget is the only practical solution
for this application.

The field strength directly affects the frequency of the of the output
signal.  A low field strength means a low frequency, low amplitude signal.
I don't think that anyone makes high kHz low noise figure (>.1dB) high
sensitivity transistors.

The really big question is if there are any ferro-magnetic objects in the
surrounding area.  Anything ferromagnetic would cause some trouble in
keeping the magnet where it was supposed to be.

Actually, I guess this could be viable, as long as you could get the field
strength high enough.

Anyone know where I can get 1 Tesla bar magnets? ;o)

--Brendan

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2002\08\29@124127 by Alan B. Pearce

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>He said it had to be small...  That implies a low magnetic field,
>considering that the magnets I work with weigh in at 17 metric tonnes, and
>they're only .35 Tesla, though the gap is fairly large, and for this, your
>gap could be quite small.  The higher the field strength, the higher the
>output signal.  I assume a permanent maget is the only practical solution
>for this application.

Oh I dunno ..... :)

I was reading the obituary the just the other day, for a guy who had been a
pilot in WW2, flying bombers which were used to find magnetic mines. The
article had a photo of one of the bombers they used, which had a massive
coil of wire on a former, about 3/4 of the wingspan in diameter. Apparently
there was a 48kW generator driven by a diesel motor slung in the bomb bay to
provide the current for the coil.

From all accounts it could be fairly hairy flying this thing at about 50 to
100 feet above the water, if the mine was close to the surface it would
shower the plane with all sorts of shrapnel and debris when it exploded.

They do not say what the field strength of the coil was. :)

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2002\08\29@130344 by Brendan Moran

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> Cool idea, but it doesn't seem practical.  It's possible that for simply
> detecting the presence of some water it would be far simpler than actual
> NMRI, but I think the field strength wouldl be a concern

[snip]

> Actually, I guess this could be viable, as long as you could get the field
> strength high enough.

Allow me to correct myself.  I just mentioned the idea to the resident
physicist, and we discussed it a bit.

Apparently, this scheme is used in oil wells (which I didn't know).  The
operating frequency for those is around 2MHz, which implies something around
0.05T.  And I was forgetting that to test the amplitude of the signal, you
really don't need to look at the noise figure much.

The scheme that would work well here is something called "inside-out NMR,"
where the magnet, transmitter and reciever coils are all mounted outside the
tank.  I'm not sure on this, but I think that a rare-earth diske drive
magnet should be getting close to the required field strength.  And if you
did it really well, you should be able to use the magnet to hold the probe
onto the tank ;o).

I know a bit about NMRI reciever coil construction, so if you want some
suggestions about how to build the coils... I can probably help out a little
unless someone else here is more experienced :o)

--Brendan

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2002\08\30@075010 by Morgan Olsson

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Thank you for all suggestions.

NMR seem too expensive even if using the proposed inside-out thing.
Then there is EMC and power constraints too.

I think i´ll head for piezoacoustic methods.

/Morgan
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2002\08\30@092311 by Russell McMahon

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> > > I am looking for a reliable way to detect water,
> > > although the sensor is covered with a layer of dirt,
> > > oil film, etc.  And the water may be pure or contaminated.
> >
> > Nuclear Magnetic Resonance perhaps?
>
> Have you worked with NMRI?

No. But I was introduced to the concept in 1973 (or 1972?) when it was an
idea in its infancy with no major applications. I still have my original
university notbook showing a coil around a bottle of water and using the
earth's magnetic field! If I had played with that idea then ....

> Cool idea, but it doesn't seem practical.  It's possible that for simply
> detecting the presence of some water it would be far simpler than actual
> NMRI, but I think the field strength wouldl be a concern.

The resolution is dependant on field strength and to get the 1mm odd
resolution of medical MRI you need the field strengths concerned. For
grossly low resolutions (ie the water sample is or isn't there) far lower
field strengths would suffice.

Also the medical units need to produce a uniform (or well controlled) field
over a very large physical distance by normal measures. When your sample can
be all but in contact with your magnet and coils the field strength
requirement is lower AND/OR the magnetics required to produce the field is
smaller. I suspect that a physically small rare earth magnet with the sample
essentially in contact is liable to be in the order of right. (Is that
enough "maybe" words? :-) ).

I have an idea for a simple sensor (not for water) based on this principle
which I hope to get around to trying "one of these days".



                   Russell McMahon

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2002\08\30@112634 by Morgan Olsson

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Hej Russell McMahon. Tack för ditt meddelande 14:22 2002-08-30 enligt nedan:
>> > > I am looking for a reliable way to detect water,
>> > > although the sensor is covered with a layer of dirt,
>> > > oil film, etc.  And the water may be pure or contaminated.
>> >
>> > Nuclear Magnetic Resonance perhaps?
>>
>> Have you worked with NMRI?


>I still have my original
>university notbook showing a coil around a bottle of water and using the
>earth's magnetic field! If I had played with that idea then ....

So very high frequency is not needed?

I´ll better use a rare earth metal magnet, as the unit is mobile, and i also want to keep power requirement down.

But i don´t know anything about this really... how does it work?
Can i use one sending coil, one magent and a reciever coil placed around a tube in which i want to detect water?  How?

/Morgan

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2002\08\30@121130 by Roman Black

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Morgan Olsson wrote:
> I´ll better use a rare earth metal magnet, as the unit is mobile, and i also want to keep power requirement down.
>
> But i don´t know anything about this really... how does it work?
> Can i use one sending coil, one magent and a reciever coil placed around a tube in which i want to detect water?  How?


Why not use one cheap thermistor attached to
the tube, (as I mentioned in a previous post)
this system is used with great reliability in
my motorcycle fuel tank to detect the difference
in thermal loss from the thermistor when in contact
with the liquid.

It is possible to get SMT thermistors that would
detect tiny quantities of water, and match your
low power requirements.
-Roman

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2002\08\30@124449 by Brendan Moran

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> > > > I am looking for a reliable way to detect water,
> > > > although the sensor is covered with a layer of dirt,
> > > > oil film, etc.  And the water may be pure or contaminated.
> > >
> > > Nuclear Magnetic Resonance perhaps?
> >
> > Have you worked with NMRI?
>
> No. But I was introduced to the concept in 1973 (or 1972?) when it was an
> idea in its infancy with no major applications. I still have my original
> university notbook showing a coil around a bottle of water and using the
> earth's magnetic field! If I had played with that idea then ....

I suspect that while this is *possible* it is impractical outside a nicely
shielded lab.  The earth's magnetic field strength would likely cause too
low a signal strength for picking anything up outside a good shielding room.

Incidentally, I took my cell phone inside a 15MHz shielding room, and tried
it out.  Clear signal.  Clearer, maybe, than outside the shielding room.

> > Cool idea, but it doesn't seem practical.  It's possible that for simply
> > detecting the presence of some water it would be far simpler than actual
> > NMRI, but I think the field strength wouldl be a concern.
>
> The resolution is dependant on field strength and to get the 1mm odd
> resolution of medical MRI you need the field strengths concerned. For
> grossly low resolutions (ie the water sample is or isn't there) far lower
> field strengths would suffice.

I know, I forgot the difference in resolution when I posted the original
message, but I corrected myself in the second.

> Also the medical units need to produce a uniform (or well controlled)
field
> over a very large physical distance by normal measures. When your sample
can
> be all but in contact with your magnet and coils the field strength
> requirement is lower AND/OR the magnetics required to produce the field is
> smaller. I suspect that a physically small rare earth magnet with the
sample
> essentially in contact is liable to be in the order of right. (Is that
> enough "maybe" words? :-) ).

As I said, a .04T magnet, with a small transmitter and reciever coil would
likely do the job.

> I have an idea for a simple sensor (not for water) based on this principle
> which I hope to get around to trying "one of these days".

--Brendan

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2002\08\30@130113 by Brendan Moran
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>> So very high frequency is not needed?

No, as I mentioned in my second post, you can likely get away with 2MHz

> I´ll better use a rare earth metal magnet, as the unit is mobile, and i
also want to keep power requirement down.

> But i don´t know anything about this really... how does it work?
> Can i use one sending coil, one magent and a reciever coil placed around a
tube in which i want to detect water?  How?


As to the power requirement, Roman Black's suggestion is likely better for
you there, but here's how it works:

You line up the magnet and the receiver coil so that the magnetic field of
the magnet is perpendicular to the normal vector of the reciever coil.
(I.e. make it so that if you were to run a current through the coil, the
magnetic field inside the coil would be perpendicular to the magnetic field
from the magnet at that point)  This is because the nucleii of the water
will be oscillating at 90deg. from the magnetic field.

Now, take the output from the receiver, and ampify it enough to make it
detectable (probably at least 40dB).  At this stage, I would suggest
rectifying it, integrating it (using an opamp integrator) and reading the
output as a DC level.

Now, your receiver coil can be comnimed with your transmitter coil, but if
this is done, you need a T/R (transmit/receive) switch, which couples the
coil to the RF amplifier and signal generator during transmit, and to the
preamplifier/receiver during the receive stage.  To avoid this complexity,
you can use two seperate coils, perpendicular to each other.  This also
prevents the need for decoupling, although putting protection diodes on the
pramplifier would be a good idea.

The sequence is this:  (switch to transmit) transmit a bunch, (switch to
receive) receive a bunch, read the level of the output from the integrator,
then reset the integrator and repeat.

Russell, how's that sound to you?

Regards,
--Brendan

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2002\08\30@142027 by Peter L. Peres

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On Fri, 30 Aug 2002, Morgan Olsson wrote:

>But i don´t know anything about this really... how does it work? Can i
>use one sending coil, one magent and a reciever coil placed around a tube
>in which i want to detect water?  How?

Search the web for proton precession magnetometer. What you want is the
same instrument but with constant field and you put in or remove the
water.

Peter

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2002\08\30@160646 by Peter L. Peres

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>> university notbook showing a coil around a bottle of water and using the
>> earth's magnetic field! If I had played with that idea then ....
>
>I suspect that while this is *possible* it is impractical outside a nicely
>shielded lab.  The earth's magnetic field strength would likely cause too
>low a signal strength for picking anything up outside a good shielding room.

That's why you do not use the earth field, instead you swamp it using a
strong field of your own from a pm, besides the magnetisation/readout
coil. This causes the precession frequency to be higher than usual for
these instruments.

Why would anyone try to use this expensive and complex method to detect
water I don't know.

You could try to use microwave absorbtion for that, it is mucho easier,
but it depends on what kind of dirt you have.

Peter

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2002\08\30@170513 by Brendan Moran

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> That's why you do not use the earth field, instead you swamp it using a
> strong field of your own from a pm, besides the magnetisation/readout
> coil. This causes the precession frequency to be higher than usual for
> these instruments.

Yeah, I know.  In fact, the post that followed gave a full description of
how to implement this.

> Why would anyone try to use this expensive and complex method to detect
> water I don't know.

Frankly, as this list has many hobbyists on it, why not?  It's not as
complex as all that... A magnet, two coils, an oscillator, a power amp, a
preamp, an opamp integrator, and a micro...

process is: send; receive; measure; reset

I doubt that this is too much more complex than either sonic coupling or
microwave absorbtion...

The clear winner from my perspective would have to be Roman's thermal
conductivity.  The NMR method just interests me.  I work with it, and am
surrounded by people who understand it well, so it's an interesting idea to
me.

--Brendan

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2002\08\30@170727 by Morgan Olsson

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Roman Black wrote:

>Why not use one cheap thermistor attached to
>the tube,

That is also a good idea i might try.

Problem might be wind might blow, and that combined with moist dirt might have as good cooling effect as the warmest water it has to detect.
Or greasy dirt isolating the thermistor from detecting water reliably.  Or...

Abour power need:
For any sensor i choose i may pulse it on just enougth to make it detect, for one second intervals.

I guess i need *much* (thousand) longer on periods for NTC heating, than for NMRI exitation pulse.

Also piezo pulsing can use very short on-times.

Another problem i am sure i wil encounter is that there might be radio sources within meters, or somebody using a GSM phone...  There are also often radar nearby, sometimes a gasoline engine that might have noisy ignition system.  And a PIC of course and tiny switched power supply within a couple centimeters.  Why do´nt i just drop this project...? ;)
Then, thermistor is best as i can discard any high freq.  I guess piezo-acoustic may give good readout, be reasonalby EMC tolerant, and low drive (short pulses)

Peter:
Good idea, the microwave absorption might also be a solution.
If we could make a cheak microwave osc and reciever.
I have no RF experience so i have no ideas.

BTW, both microwave and NMRI solutions will probably make it different for certification.  2,45(?) GHz is free for low power (and spread spectrum?) IIRC.  But precise microwave do sound expensive.

Also moist dust with salt might damp much too.

And it might conduct too much to use a capacitive method to differ it from water sucessfully.

I still believe most in acoustic method, where i can demand a large part of the cavity to be filled before detection.  Water is the only matter nearby that is plenty enough to fill it. (within reason)

/Morgan

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2002\08\30@173259 by Peter L. Peres

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On Fri, 30 Aug 2002, Brendan Moran wrote:

>> That's why you do not use the earth field, instead you swamp it using a
>> strong field of your own from a pm, besides the magnetisation/readout
>> coil. This causes the precession frequency to be higher than usual for
>> these instruments.
>
>Yeah, I know.  In fact, the post that followed gave a full description of
>how to implement this.
>
>> Why would anyone try to use this expensive and complex method to detect
>> water I don't know.
>
>Frankly, as this list has many hobbyists on it, why not?  It's not as
>complex as all that... A magnet, two coils, an oscillator, a power amp, a
>preamp, an opamp integrator, and a micro...

Run some numbers, will you. Esp. wrt the amplitude of the output signal
(as in output signal, in normal house/industrial environment ...). You
will need a LOT of protons to have a decent signal even with a very good
preamp.

The only advantage of the constant pm field is, you can use a narrow band
preamp, unlike in magnetometers. This helps with s/n.

>process is: send; receive; measure; reset
>
>I doubt that this is too much more complex than either sonic coupling or
>microwave absorbtion...

All the principles are simple, the devil is in the details.

Thermal conductivity is a well known method for doing this. National even
had (has?) a chip to interface with the wire thermistor kind of gauge
(the kind often used in fuel tanks). I DIYd such a circuit a long time ago
and it worked ok but was not linear. I used a 20 ohm piece of wire (2
meters of 10R/meter).

Peter

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2002\08\30@174113 by Peter L. Peres

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On Fri, 30 Aug 2002, Morgan Olsson wrote:

>Peter:
>Good idea, the microwave absorption might also be a solution.
>If we could make a cheak microwave osc and reciever.
>I have no RF experience so i have no ideas.

Obtain one of those radar door openers (not the IR kind), put a paper
napkin in front of it, see what happens when you soak it with water. You
may have to adjust the sensitivity control. Make sure you get one with an
adjustment. You may have to move the napkin to have it react (put it on
cardboard and move the cardboard). You can duplicate this with a sample of
your dirt. Small modifications to the detector will remove the requirement
to move the napkin. This kind of sensor is non-contact (can be mounted on
a pole looking down etc).

>BTW, both microwave and NMRI solutions will probably make it different
>for certification.  2,45(?) GHz is free for low power (and spread
>spectrum?) IIRC.  But precise microwave do sound expensive.

What 'precise' ? ;-)

>Also moist dust with salt might damp much too.

I think that you need to define to yourself what kind of water you need to
detect. You seem to have no clear ideas yourself. Do you mean water, as
in, 3 feet under ? 3 cm under ? Just a little wet ? Rain ? Spray ? How
slowly can it accumulate ?

>And it might conduct too much to use a capacitive method to differ it
>from water sucessfully.

Conduction and loss at RF are different things.

Peter

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2002\08\30@182208 by Morgan Olsson

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Peter L. Peres wrote:
>I think that you need to define to yourself

I have, sorry for not communicating it cleraly before

>what kind of water

Pure, or:
with or without salt.
and with or without dirt.
shouldmake same result

> you need to
>detect. You seem to have no clear ideas yourself.

sure i do:

>Do you mean water, as
>in, 3 feet under ? 3 cm under ?

YES
as soon as the device is *covered* more or less with water.
But it must not have anything sticking out.
Therefor i will use a cavity to measure in, or a hole right through the device.
the device is approx 20mm thick, max alowable hole diameter 30mm.
Shell is plastic.

> Just a little wet ? Rain ? Spray ?

NO

> How
>slowly can it accumulate ?

max 1 minute.

/Morgan

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2002\08\31@092657 by Peter L. Peres

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On Sat, 31 Aug 2002, Morgan Olsson wrote:

>> How
>>slowly can it accumulate ?
>
>max 1 minute.

I think that you will be happy with a self heated thermistor and a
pic-based high pass filter to determine when the event happens. You could
use something as simple as a 555 oscillator or a PIC AD. Some
experimenting will be necessary.

Peter

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2002\08\31@145300 by Morgan Olsson

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Hej Peter L. Peres. Tack för ditt meddelande 08:27 2002-08-31 enligt nedan:
>On Sat, 31 Aug 2002, Morgan Olsson wrote:
>
>>> How
>>>slowly can it accumulate ?
>>
>>max 1 minute.

Clarification:

Water can accumulate as slow as one minute.
But normally within a second, and i need reliable detection whithin 2 seconds, so i´ll sample at about 1Hz.

/Morgan

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2002\08\31@145314 by Roman Black

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Morgan Olsson wrote:

> I still believe most in acoustic method, where i can demand a large part of the cavity to be filled before detection.  Water is the only matter nearby that is plenty enough to fill it. (within reason)


Ok, maybe the acoustic method can be simplified?
With ultrasonic piezos etc you get ringing after
the initial pulse train is sent. Maybe you only need
one piezo element, and can use something cheap and
common like a commercial buzzer? Ping it at it's
(dry) resonant frequency and use the PIC ADC to
examine the ringing. I'm sure once it has any
significant amount of water on the piezo the
resonance (and ringing) at the test freq will
vary noticeably.
-Roman

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2002\08\31@173204 by Morgan Olsson

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Hej Roman Black. Tack för ditt meddelande 18:36 2002-08-31 enligt nedan:
>Morgan Olsson wrote:
>
>> I still believe most in acoustic method, where i can demand a large part of the cavity to be filled before detection.  Water is the only matter nearby that is plenty enough to fill it. (within reason)
>
>
>Ok, maybe the acoustic method can be simplified?
>With ultrasonic piezos etc you get ringing after
>the initial pulse train is sent. Maybe you only need
>one piezo element, and can use something cheap and
>common like a commercial buzzer?
I´m thinking about that :)

Also, using hearable frequency also make it serve as audible function indicator :)

>Ping it at it's
>(dry) resonant frequency and use the PIC ADC to
>examine the ringing.

Or count cycles in a short defined time.

> I'm sure once it has any
>significant amount of water on the piezo the
>resonance (and ringing) at the test freq will
>vary noticeably.
>-Roman

Yes, but i think it will be damped too quickly, both by water taking away power, and by the silica gel (or whatever) sealing.

Some standard piezos have a third terminal that is used to drive itself a drive transistor at exact self resonanze.  I thinkt that will give a more clear response.  Just turn the power on for a short time, wait until decent amplitude, then count cycles during a few ms, and shut down to rest a while.

I just have not seen a small standard piezo with that third terminal, but i think it is easy to etch or mechanically carve out a third terminal on a small piezo.

Thank you all for your input.
-Even more ideas are welcome :)

/Morgan

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'[EE]: Looking for Piezo material'
2002\09\01@031430 by Nelson Hochberg
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> I am looking for a reliable way to detect water,
> although the sensor is covered with a layer of dirt,
> oil film, etc.  And the water may be pure or contaminated.

Two cheap and quick methods:

Manual reset:  aspirin under micro switch

Auto reset: Go to toy store.  Buy monster that expands when wet.  Cut off
small piece.  Place under micro switch.

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