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Unwanted Ultrasound in Data Loggers
| Almost all data logging equipment
in the field today uses a microprocessor based platform.
One of the more popular processors in use is the
MicroChip PIC. The PIC family of processors have a low
power operating mode that requires the utilization of a
32 kHz watch crystal as a timing element. The low
frequency crystal provides two benefits to the logger
design; it provides for greatly reduced battery drain -
and it provides a very stable timebase for time keeping. The problem that bat researchers need to be aware of, is the possibility of these crystals generating unwanted ultrasound. A crystal provides the timing function in an electronic circuit, much like a tuning fork. As it is stimulated by an electronic circuit, it physically vibrates. As it vibrates, it generates its own diminutive amount of electrical energy, which is fed back into the electronic circuit, and stabilizes it at a fixed frequency. Depending on the condition of the specific crystal in use, this mechanical vibration can be strong enough to generate considerable acoustic output. This is where we run into problems. |
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| I was first made aware of this problem
through email correspondance. It seems it has been noted
that bats were avoiding certain data loggers that were
placed in their roosting areas. ( It wasn't due to the
smell or any other physical trait, as bats have been seen
to hang and roost on these same types of loggers :-) The type of logger that was being used where this behavior was noted was an Onset HOBO-RH logger. One researcher sent me one of these "noisy" loggers to evaluate. |
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| NOTE: Though I am discussing one particular logger on this page, it is important to note that any logger, from any manufacturer, can develop this problem - as well as other types of electronic equipment ! | |||
| This is what the noise generated by the logger looked like on an Anabat display screen. This particular logger was detectable by the Anabat at over a meter away. Note that the ultrasound signal is a constant 32 kHz tone. This is the frequency of the crystal used by the microprocessor in the logger. |  |
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| I next visuallized the signal of the
ultrasound coming from the logger by connecting the high
frequency output of the Anabat to an oscilloscope. The
signal was clean and well defined - way above any level
of sound that could be considered background noise. ( NOTE: All of the other loggers I had at hand, Onset H-8 type loggers, were totaly undetectable by the Anabat - and only softly audible to a more sensitive heterodyne detector. ) |
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| Removing the cover of the logger, we see the
source of all this trouble. The crystal ( circled in red
) is a small cylindrical device with a metal case and two
wire leads. Replacing the crystal really appears to be the ONLY solution. I've tried silicon encapsulation, foam insulation, soft wire leads, etc. A noisy crystal is just too effective as an acoustic emitter. Simply removing the noisy crystal, and replacing it with a new device, solved the problem in this logger completely. |
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I have personally used Onset Data Loggers, and would never hesitate to recommend them to anyone. Onset has informed me that all crystals used in Onset loggers are of the same type from the same manufacturer. This would underscore that this type of crystal "failure" is likely to be remote, occurring only under very limited circumstances. The need for diligence on this matter rests on the shoulders of bat researchers ! Always try to be aware that any electronic device is a potential source of ultrasound. If you plan on deploying equipment to monitor bat activity, especially in a sensitive environment, fire up a bat detector - tweaked to it's most sensitive operating mode - and test the equipment for acoustic radiation. Other types of equipment to be concerned about include DC/AC converters, battery operated flourescent lights (such as those used in bug traps), tape recording equipment (internal recording bias oscillators), and any form of digital equipment from laptop computers to data loggers. Each of these has the potential to generate significant amounts of acoustic ultrasound if not operating correctly. Special thanks to Ray Miller and Bill Rainey, who first brought this issue to my attention ! |
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Tony Messina, Las Vegas, NV - page last updated 9/30/2004