Ask A Scientist© Environmental Earth Science Archive

name         Lori R. M.
status       educator
age          30s

Question -   I am interested in finding out exactly how an electronic
balance measures mass (meaning what's under that smooth cover and how
does it work?)  It seems as though the instrument measures how much
downward force is exerted upon the platform of the balance, but that
would be a measurement of weight, not mass.  It almost seems as though
weight must be what is measured by this instrument.  I teach high school
chemistry and cover the difference between weight and mass with my
students.  Each time I prep my lab that introduces common laboratory
measurements and equipment, I wonder about this.
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All scales and electronic balances we use in the ordinary lab setting are 
measuring
"weight" not "mass". It does not matter what type. To obtain the "mass" one
needs to divide the "weight" by the local acceleration due to gravity.
That is: Weight = Mass * g, where "g" is the local acceleration of gravity.
However, because under usual conditions we are measuring differences, the
"g" cancels out. That is: W1/W2 = M1/M2. In careful measurements of fairly
large objects -- say ~ kg the local value of "g" must be taken into account
as a "gravitational correction" to the measure weight.
     Regarding the mechanism of what is inside a top loading electronic
balance, I would have to look up what electro-mechanical device is used.

Vince Calder
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Electronic balances use a device called a "load cell."  The full name is
"strain gauge load cell."

First a little background: a "strain gauge" is a thin device, smaller than a
fingernail, which changes electrical resistance when it is stretched or
compressed.  If you glue a strain gauge onto a metal bar, you can determine
how much the bar is bent by measuring changes in electrical resistance of
the strain gauge with a meter.

Typically, several strain gauges are used in Wheatstone bridge arrangement
and they are glued onto the load cell in a protected location.

A load cell is usually in the shape of a beam or plate.  When you push on
the beam or plate with a force, it bends a tiny amount, and this tiny
bending is detected by the strain gauges.  The amount of bending might be
only a thousandth of an inch, but that is enough for a strain gauge to
measure.

Fancy load cells can measure forces in three directions and also torques
around three axes.

A load cell can only measure force.  An "electronic balance" can thus only
measure weight.  If you take your electronic balance to the moon it will not
measure right.

  (If you take a beam balance, the kind of one with weights that you put on a
pan, to the moon, it will measure mass correctly because the mass weights on
both pans weigh proportionately less.)

If you do an Internet search on "load cell" you can learn all about them.

Bob Erck
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Quite correct, an electronic balance generally measures weight, not mass.
If it has many digits of resolution and a stable reference-weight,
it's readings would vary if Earth's gravity changed or the scale tilted, 
or its electromagnet changed constants.

It is merely a pan, a lever, an electromagnet, and an electronic feedback 
loop trying to keep the lever exactly horizontal..
The lever must have double-arms to add parallelogram reinforcement
so the effective lever-arm length remains the same regardless of where on 
the pan the weight sits.

On the other hand,  a digital chain-counterweight balance can be 
considered to measure mass directly,
because it is always comparing with an equal mass deposited on an opposing 
pan.
Perhaps there are electronic versions of that, also with smooth 
(concealing) covers.
They would be expensive and uncommon.

Many electronic balances have a calibration weight built-in.
If you weigh it just before or after weighing your unknown object,
that would eliminate some of the differences between weight and mass 
measurements.

Jim Swenson
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Yes, you are measuring weight, and inferring mass.  The thing would
need to be re-calibrated if it were on the moon.  Same for your bathroom
scale.  But its generally a safe bet that you are on Earth, so they get
a bit sloppy in the naming of things.

Steve Ross
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