<P> Whenever a high - precision scale (or balance) in routine laboratory use is calibrated using stainless steel standards, the scale is actually being calibrated to conventional mass; that is, true mass minus 150 ppm of buoyancy . Since objects with precisely the same mass but with different densities displace different volumes and therefore have different buoyancies and weights, any object measured on this scale (compared to a stainless steel mass standard) has its conventional mass measured; that is, its true mass minus an unknown degree of buoyancy . In high - accuracy work, the volume of the article can be measured to mathematically null the effect of buoyancy . </P> <P> When one stands on a balance - beam - type scale at a doctor's office, they are having their mass measured directly . This is because balances ("dual - pan" mass comparators) compare the gravitational force exerted on the person on the platform with that on the sliding counterweights on the beams; gravity is the force - generating mechanism that allows the needle to diverge from the "balanced" (null) point . These balances could be moved from Earth's equator to the poles and give exactly the same measurement, i.e. they would not spuriously indicate that the doctor's patient became 0.3% heavier; they are immune to the gravity - countering centrifugal force due to Earth's rotation about its axis . But if you step onto spring - based or digital load cell - based scales (single - pan devices), you are having your weight (gravitational force) measured; and variations in the strength of the gravitational field affect the reading . In practice, when such scales are used in commerce or hospitals, they are often calibrated on - site and certified on that basis, so that the mass they measure, expressed in pounds or kilograms, is at the desired level of accuracy . </P> <P> In the United States of America the United States Department of Commerce, the Technology Administration, and the National Institute of Standards and Technology (NIST) have defined the use of mass and weight in the exchange of goods under the Uniform Laws and Regulations in the areas of legal metrology and engine fuel quality in NIST Handbook 130 . </P> <P> NIST Handbook 130 states: </P>

Relationship between weight and mass on the moon