• Weighing machines are calibrated by accredited laboratories on a conventional mass basis.
• If the true mass of an object is to be found, or if conventional mass is required but the air density is not 1.2 kg m-3, then an air buoyancy correction must be made.
• This correction will vary depending on the density of the object weighed and of the air at the time of weighing. In air of density 1.2 kg m-3, no corrections would be required to give conventional mass.
• However, the correction for true mass would be zero for stainless steel (density 8 000 kg m-3), -7 ppm for brass (density 8 400 kg m-3), + 1 050 ppm for water (density 1000 kg m-3) and + 1350 ppm for organic solvents (density 800 kg m-3). For a 1 kg load these corrections would be 0 mg, -7 mg, +1.05 g and +1.35 g, respectively.
• If the air density is different from 1.2 kg m-3, then other correction values will be needed for both true and conventional mass.
• To make buoyancy corrections, the measured values may be multiplied by the factors given in the following formulae:

1. To obtain conventional mass:

Where:-

Da = density of air during the weighing in kg m-3

Ds = density of the reference weight (usually 8000 kg m-3)

Du = density of the material being weighed in kg m-3

1. A reasonable approximation of the air density (uncertainty ±5000 ppm of the calculated air density) may be obtained from the following formula:

Where:

Da = air density in kg m-3

p = air pressure in mbar

h = relative humidity of the air in %

t = air temperature in 0C

For best results, the weighings to which these corrections should be applied should generally be either by comparison with a calibrated standard weight, or (where the facility is available) by direct weighing after the weighing machine has been ‘spanned’ by use of a calibrated weight.