Brian Whatcott encouraged me to try a test with some electronic
balances. I did not do the test the way he suggested because there is
a much, much simpler way. So thanks, Brian, for the prodding, but
listen to this method, because it's simple, obvious, and pretty darn
cool.
I went to my Ohaus E400 balance and set in to calibration mode. It
asked for zero mass, which I did. Then it asked for a 200 gram mass.
But I gave it 100 g instead. It promptly read 200 g and indicated it
was calibrated. I then placed a 25 g mass on it and it read 50 g. It
worked beautifully, always giving a reading exactly twice the value of
the mass actually placed on the pan. Hence, this balance will
calibrate just fine on any planet with g between 4.9 and 9.8 m/s^2.
But how low will it go? I re-calibrated it, and when it asked for a
200 g mass I gave it 50 g. No problem. I tried it again and gave it a
20 g mass. No problem. One more time with a 10 g mass... ERROR.
Obviously the programmers foresaw the need to confirm that some
standard mass had been added to the pan, and 10 g does not pass their
test, but 20 g does.
When I calibrate this with a 20 g mass rather than 200 g, I am
simulating g of 0.98 m/s^ rather than 9.8 m/s^2. It works just fine.
After said calibration, if I put a 5 g mass on the pan, the balance
reads 50g. If I put a 40 g mass on the pan, it reads 400 g. No
problem.
The Ohaus E400 balance will definitely calibrate and read mass properly
on Earth's moon, and even some smaller planets/moons. Ah...
vindication feels good!
Note, however, that I tried this with other balances and some are
programmed to tighter tolerances. For example, our Mettler AE-200 has
a built-in 100 g calibration mass, but you can substitute your own. I
substituted 50 g and got an error. But it calibrated without error
with 80 g up to 120 g. Hence this balance will calibrate properly and
work on any planet where g = 9.8 m/s^ +- 20%. It will not calibrate
properly on the moon.
One balance, a Mettler PE3600, seems to have almost no tolerance. It
seems to want a calibration mass within about 1% of what it requests,
else it indicates and error. But some other balances behaved similarly
to the Ohaus E400 and would take almost anything. I maintain this
difference is clearly a software difference and not any mechanical
difference.
Before this I had never intentionally tried fooling the calibration
routine of any balance. What a wonderful trick and a wonderful lesson.
This definitely will go into my cool demonstrations collection.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817