Types of Scales and What They Measure
Technically, whenever someone stands on a balance-beam-type scale at a doctor’s office, they are truly having their mass measured. This is because balances ("dual-pan" mass comparators) compare the weight of the mass on the platform with that of the sliding counterweights on the beams; gravity serves only as the force-generating mechanism that allows the needle to diverge from the "balanced" (null) point. Balances can be moved from Earth’s equator to the poles without spuriously indicating that objects became over 0.3% more massive; they are immune to the gravity-countering centrifugal force due to Earth’s rotation about its axis. Conversely, whenever someone steps onto spring-based or digital load cell-based scales (single-pan devices), they are technically having their weight (force due to strength of gravity) measured. On force-measuring instruments such as these, variations in the strength of gravity affect the reading. As a practical matter, when force-measuring scales are used in commerce or hospitals, they are calibrated on-site and certified on that basis so the measure is mass, expressed in pounds or kilograms, to the desired level of accuracy.
Read more about this topic: Mass Versus Weight
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