Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Mass



I agree w/ the usual definition for balance [I even made one using a d
'Arsonval meter and called it a balance.]

However, in the context I thought it important to point out that modern
balances are strain gauges or current meters.

"The sensitivity of the electronic balances does decrease as the
gravitational field decreases because the current needed to balance the
calibration mass is smaller, and that means you are using a smaller
portion of the range of the analog to digital converter that is
measuring the current."

The above was why I questioned the use of the word "balance".



"The beam balance can also lose sensitivity in a smaller gravitational
field if there is any friction or imperfections at the knife edge."

Probably why those balances are limited to ~? 10 micrograms resolution.
The first current balance I used was in 1970.




bc


Edmiston, Mike wrote:

* * * quoting Bernard Cleyet * * *

Unless, I'm confused, this means those balances are not balances. i.e.
e.g. use an electric current.

W/o much thought, I think both an equal arm balance and a substitution
balance are not sensitive to the value of the G field. Incidentally,
the cheap (relatively) analytic balances assume sine (displacement
angle) = angle.

* * * end quote * * *

Whether they are balances or not depends on your definition of balance.
If your definition of balance requires a mass comparison on a beam
balance, then these are not balances. However, if your definition of
balance requires that one mass is compared to another mass by some
suitable means, then they are balances.

The metrologists have adopted the second definition as far as I can
tell. As I have said before, you can compare the gravitational force on
two mass by using both masses at the same time (beam balance), or you
can use one mass first and then the other (typical electronic balance).
There are very few beam balances in use today. Almost all balances are
the electronic type. I believe all the Mettler-Toledo balances and mass
comparators are not beam balances.

The sensitivity of the electronic balances does decrease as the
gravitational field decreases because the current needed to balance the
calibration mass is smaller, and that means you are using a smaller
portion of the range of the analog to digital converter that is
measuring the current. I suppose you could counteract this by
increasing the gain of the linear electronics portion of the circuit as
long as you have sufficient signal to noise ratio. This adjustment is
not available to the user of the balance.

The beam balance can also lose sensitivity in a smaller gravitational
field if there is any friction or imperfections at the knife edge.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu