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NNTP-Posting-Date: Mon, 28 Oct 2002 22:15:57 -0600
From: "Big John"
References: <6EGu9.email@example.com> <6oadnSBXeP61vSCgXTWcrg@netlabs.net>
Subject: Re: vibration measurement circuit (vibration gurus please help)
Date: Mon, 28 Oct 2002 23:14:05 -0500
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150 degrees C! Yeah that's hot! If you found electronics that does the
job by all means stick with it. In years past, I worked for a company that
made electric motors and we used accelerometers for checking out bearing
generated vibrations among other things. The motors were mainly for Naval
applications where vibration measurement is a major concern. Fortunately,
things never got that hot.
For your info only, there are piezoelectric accelerometers that can
operate up to 250 degrees C. I've never had to use them and I wouldn't
recommend changing the Analog devices accelerometer because you know it
works, but they may be an option if you have to go hotter or have other
problems. The best made, that I know of, are made by Bruel & Kjaer
(http://www.bkhome.com/). They are expensive but their applications
engineers are great. There is one concern with piezoelectric accelerometers
that I should mention. When they are designed for high temperatures they
normally do not have integral amps. By themselves, piezo accels are charge
sources with a very high impedance output. It's not easy to build a charge
amplifier at room temperature. At high temperatures and with limited
components it's a major task.
Given Op-Amps, resistors, and caps that can handle it building custom
active filters at audio frequencies isn't that bad. Drift is a possibility,
so I'd soak them in a hot oven for a few hours to be sure they don't go too
far. If you haven't done it before pick up a copy of "Active-Filter
Cookbook" by Don Lancaster. It's a good place to start.
Good luck with your project,
mook johnson wrote in message ...
>Thanks Big John,
>This limited IC availability is due to the environment. This thing will be
>stuck down a oil well about 3 to 4 miles down. The temperature down there
>can be as high as 150 degrees CO. We have to be really selective about
>which components we put down there or they won't operate or last for a
>significant length of time. We've used the Analog Devices accelerometers
>150 degrees C before and they do work with somewhat degraded performance.
>Temperature correction in the software takes care of most of this.
>The data will be extracted from the device once per second transmitted to
>the surface with our long distance telemetry system at 600 bps. This
>is replacing another sensor that has 9 bytes allocated to it in the bit
>stream going back the surface. This is why the data needs to be processed
>downhole. Otherwise I'd send the sampled waveform up and FFT on the
>I'll probably have to resort to custom built band pass circuits with known
>good Op-amps that work at temperature.
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