Oscillator circuit for 32.768 kHz quartz tuning fork crystal element immersed in water

That's not the proper matching. You need a "pi", cap-to-gnd, xtal, cap-to-gnd, and a series R. Look up a data sheet.

It can probably be made to oscillate in water, but who knows at what frequency.

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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You'd have to measure it to see what the Q is like. I'm guessing uou could get it to oscillate.

AT-cut crystals oscillate fine underwater. One analytical chemistry technique is to coat a crystal with a chemical reactant and expose it to something else that bonds to it. The presence of minute amounts of the sensed stuff changes the frequency.

I bet a tuning-fork crystal could detect nanograms of stuff deposited on the tips of the forks.

Google tuning fork Quartz crystal microbalance

John

It won't oscillate at all, unless it's mind-bogglingly overdriven (which is likely to break the crystal) and you'd be lucky to get 3 KHz out of it even then.

Check the viscosity of water vs. the viscosity of air.

An ordinary bimorph might be made to oscillate, but then you'd have a very expensive ultrasonic cleaner. ;-P

Sorry, Rich

Rich Grise gives better technical advice than John Larkin! Miracles do happen.

32kHz crystals look like small tuning forks, and the tines vibrate the same way

If the crystal could be persuaded to oscillate under water, the frequency would indeed be lower than 32kHz - the mass of the water coupled to the forks would drop the frequency, and the vicosity of the water would lower the Q enormously.

The mass-sensing oscillators that John Larkin was talking about don't work in water, but in gas phase environments.

I don't think the device would be much use as an an ultrasonic cleaner. It might allow you to measure the viscosity of the water involved, but the circuit that you'd use to do it would be tolerably complicated.

-- Bill Sloman, Nijmegen

You are in the running for the new honorary AlwaysWrong award. It comes with a handsome engraved and signed certificate.

I know, and sometimes help, people who are doing research in quartz microbalance instruments, and they are definitely working in water.

"The QCM200 uses a 5 MHz, 1" diameter, AT-cut quartz crystal wafer with circular electrodes on both sides. Crystals are available in a variety of materials. The crystal holder is a rugged, compact, easy to use fixture. The holder and all crystals may be used in liquid or gas environments."

These are usually AT-cut crystals. As I said, one would have to try a tuning-fork crystal to see what happens.

John

Measuring viscocity changes of liquid solutions when needle-like crystals begin to form is exactly what I am attempting to do with the tuning fork. I am aware that the frequency will decrease relative to air and Q will drop. It seems that these viscocity changes must happen right on or at the crystal surface in order to be sensed as a change in resonant frequency.

John - I read that 32.768 kHz crystals are typically AT cut since those are the easiest to make. I've been using the circuit below and changed component values in an attempt to compensate for changes in crystal parameters when immersed in water but have not been able to get it to oscillate in water. Maybe I'll try a higher frequency crystal

Thanks for the references John. It'd be nice to make this work - a $0.30 crystal, 4069 gate and a few capacitors and resistors is a financially attractive alternative to a $2500 commercial instrument.

Steve

[snip]

Try reducing the 330K first. You probably don't have enough drive to support the "load".

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

now

r m

when you put the crystal in the water do you somehow insulate the contact pads from the water?

I would think the electrical changes due to the water contacting the contacts would be a problem... come to think of it the electrical contact of the water to the crystal itself would be a problem...

Mark

John- from your cited reference :

"Until recently, its was believed that excessive viscous loading would prohibit use of the QCM in liquids. In fact, operation in liquids is indeed possible(12), and the response of the QCM is still extremely sensitive to mass changes at the solidsolution interface. For many years, QCMs have been used in direct contact with liquids and/or viscoelastic films to assess changes in mass and viscoelastic properties during chemical and electrochemical surface processes. When the QCM comes in contact with a solution, there is a decrease in frequency that is dependent upon the viscocity and the density of the solution. A quantitative understanding of the resonator behavior is a prerequisite for proper interpretation of experimental results under total liquid immersion. This problem was first treated by Glassford (13), and later by Kanazawa and Gordon( 14)."

12 T. Nomura and M. Okuhara, Anal. Chim. Acta 142(1982) 281.

13 A. P. M. Glassford, ?Response of a Quartz Crystal Microbalance to a liquid Deposit?, J. Vac. Sci. Technol., 15(6)(1978) 1836

14 K. Keiji Kanazawa and Joseph Gordon II, ?Frequency of a Quartz Microbalance in Contact with Liquid?, Anal Chem. 57(1985) 1770 K. Keiji Kanazawa and Joseph G. Gordon II, Analytica Chimica Acta, 175(1985) 99-105.

Thanks,

Steve

On Fri, 20 Feb 2009 09:38:01 -0500, Steve wrote: (top posting fixed)

What does the crystal look like? A tuning fork crystal will _not_ be a little disk or square.

Before you go struggling with getting the oscillator to work blind, why don't you measure the crystal properties in and out of water? A network analyzer would be nice here, if you have one that works at that frequency, but you can do more or less the same thing with careful arrangements of signal generator, loading resistors and oscilloscope.

Once you know the crystal properties, you can design your oscillator.

--
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Come to think of it, perhaps at 32kHz you'd do better to just run frequency sweeps into the crystal and analyze it's impedance directly, instead of doing it all indirectly through an oscillator? Things would be much more controlled, and you'd have more information for figuring out the fluid mass _and_ velocity.

--
http://www.wescottdesign.com

The guys I know (not SRS) are using shear-mode AT-cut crystals. I can look up the guy who makes them. They won't have anything like the coupling to liquid that a tuning fork will.

I'd suggest you set up a simple circuit to characterize the vector impedance of the wet crystal, and understand that before frobbing oscillator circuit values.

John

There are (free?) programs that turn a sound card into a vector network analyzer.

John

That's what I meant by "an ordinary bimorph." I read the term about

50 years ago, in a chapter about quartz crystals.

Don't hate me because I'm beautiful. ;-)

Cheers! Rich

Steve,

I have an academic friend who is working with this stuff, using Homeland Security money to make quartz microbalances to detect chemical agents in solution. Email me (jjlarkin//highlandtechnology//com) and I'll get you in touch.

John

From my university years, I remember the project where they used a similar sensor to detect the alcohol content in breathing. It was pretty sensitive and quick, however not very specific. Fun to play with, though.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

yes, frequency goes way down and drive went way up.

tried that in my former university days.

Steve Roberts

I'll settle for the occcasionally partly wrong award that I can earn

Tuning fork crystals?

Or briefly engage one's brain.

I'd be a bit interested to see what an adsorbed monolayer would do to an AT-cut crystal immersed in water; as opposed to the adsorbed monolayer of water that you'd find if you just immersed the crystal in water.

It's nice that the QCM200 electronics can still keep the crystal oscillating even when it is immersed in water. It's less obvious that the oscillation frequency is going to change in a useful way if you dope the water (or whatever) with other molecules.

But the ingenious people who made the electronics don't have to find a useful effect, they just have to supply some gear that optimistic customers can use to look for such a useful effect.

--
Bill Sloman, Nijmegen