Milligram digital scales- Inexpensive vs. expensive scientific

From: Bruce McCrory <hbm55_at_comcast.net>
Date: Wed, 28 Jan 2009 22:20:40 -0000

After seeing the autrageously low price for the milligram scale being
discussed, I checked to learn why there was such a variation in cost;
then found more information about the current discussions. The
digital scales, for all their ease of use, seem to have some problems.

There appears to be two basics types of digital scale. The less
expensive strain gauge type– currently in 2009, a low of $25 for
0.001 gram – and analytical scales using electromagnetic force
sensors with a $300 starting price tag.

The following was copied from a scientific scale sales site, and
seems to account for some of the puzzlements we face when using
digital weighing scales. Puzzlement over drifting readouts and
constant recalibration are a couple.

Strain gauge-
"…Unfavorable features of a strain design include susceptibility to
instability or drift (when the numbers on the display crawl) due to
hysteresis and temperature coefficient characteristics. The reason
why some manufacturer's select a strain gauge for a milligram scale
is cost."

Force coil, or electromagnetic force measuring sensor-
"…Mechanically the sensor …..is a simple lever & fulcrum. One end of
the lever holds the weighing pan where an unknown weight is placed.
On the opposite end of the lever is a FORCE COIL suspended in a
magnetic field (much in the same way a speaker operates). The
displacement detector, and power amplifier produce an appropriate
current to hold the lever balanced in the null position for any
weight placed on the pan. The amount of current required to do this
is proportional to the weight on the pan.

"The MICROPROCESSOR monitors the current produced in response to the
applied load to determine the magnitude of the weight…."

The force coil type scales are generally sensitive to temperature and
humidity.

All scales of centigram and lower resolution should have breeze
shields, to minimize external interference. There is more. Besides
the above noted issues, a level vibration free surface, and
particularly static electricity will affect the accuracy of both
types. Static electricity will cause erratic readings. Instruments
should be operated on a static dissipating surface (antistatic mat)
Avoid the use of plastic containers for items being weighed. Never
replace broken glass doors on instruments with plastic ones.
Maintain humidity at 65% or more.

Of course, until I read all these cautions, weighing as part of model
building was actually fun to do; especially after converting to
digital scales. Now, I wonder if it is really worth the trouble. … :(

Bruce
Received on Wed Jan 28 2009 - 14:20:52 CET