BUOYANCY EFFECT CONSIDERATIONS IN THE USE OF WEIGHING BALANCE

-

 

  • 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:

1

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:

 2

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.

 

REFERENCE LINKS

 

International Standards Organisation (ISO)

https://www.nist.gov/sites/default/files/documents/calibrations/ac53-6.pdf

http://metrology.burtini.ca/grav_air.html

 

 

PharmaState Blog
This is a platform for people working in the pharmaceuticals industry for Discussions, Jobs, News updates, Professional Profile display space and company business pages. PharmaState DNA is having four pillars: Quality, Transparency, Innovation & Benchmarks.

Share this article

Recent posts

Popular categories

Leave a Reply

Recent comments

Business Alnavedic on History of Indian Pharma Industry
Ahmed Mohamed Elkhodragy on MACO Calculation
Jennifer Osunde on DEFECTS IN UNCOATED TABLETS
Vinod on DEVIATION
Rajib ghose on TOOLING FOR TABLETING
K. SRinivasula Reddy on Quality Agreements – PART 2
Sunil Kumar M on SITE ACCEPTANCE TEST (SAT)
Khalid on IQ Protocol
Khalid on IQ Protocol
Nagabooshanam on Calibration of Reactor
PharmaState Blog on SOP for Sampling
Swapnil Khodake on TOOLING FOR TABLETING
%d bloggers like this: