Re: Milligram digital scales- Inexpensive vs. expensive scientific

From: ray_harlan <rbharlan_at_comcast.net>
Date: Thu, 29 Jan 2009 18:05:55 -0000

Ah, I just can't resist responding to the digital scale crowd. My
beam scales are not sensitive to static electricity or humidity and
give consistent readings. They are a bit sensistive to temperature,
in that the beams are not equal, but they can be quickly zeroed out.
Of course they don't fold up as well and are only for people with
deep pockets.

I needed a good digital scale for weighing balsa strips for SO kits
and, after looking for a couple of years on eBay, I found a Mettler
PG802 top loader that is just magnificent. It weighs to .01 g, is
self-calibrating, and always comes back to zero after removing
weighed material. It settles in less than two seconds!. It has a 6"
square pan. It cost $300, but originally sold for $2300, so it was
quite a bargain. Mettler is about the best you can get, but, of
course, very expensive.

Ray



--- In Indoor_Construction_at_yahoogroups.com, "Bruce McCrory"
<hbm55_at_...> wrote:
>
> 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 Thu Jan 29 2009 - 10:06:00 CET