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Name: Lori R. M.
Status: Educator
Age: 30s
Location: N/A
Country: N/A
Date: August 2004


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.



Replies:
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


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


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


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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