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Calculation for Weighing Range of Balances

Determination of the operating range of the balances used for pharmaceuticals including the analytical balances.
Balances are used to weigh different things as raw material, dispensed material and samples. Sometimes very low quantity is weighed for analysis. An error in the weighed material can cause a big variation in analytical results.

Following circumstances may cause the error in the weighing:
  • Balance for analytical use must be closed; an open balance may cause inaccurate weighing.
  • Hygroscopic material gains weight when it comes in contact of the air causing unstable weighing.
  • Air flow or current in the balance room may cause disturbance in accurate weighing.
  • Unleveled surface or balance may lead to inaccurate weighing.
  • Vibrating surface can also cause inaccurate weighing.
Assay, related substances and other quantitative analytical tests require accurate weighing for better results. Near the lower and upper capacity of the balance, weighing may be inaccurate. Therefore, a weighing range must be defined for all balances.

Related: Calibration of Analytical Balance

Weighing Range for Balances:

Some manufacturers direct in their operating manual to weigh minimum 1.0 mg on balance having 0.01 mg least count i.e. least count X 100. But the thumb rule followed in pharmaceutical industries for lower limit is least count X 50 and upper limit is 80% of capacity of the balance. If any balance has operating range from 0.01 mg to 110 gm then it should be used for weighing the material from 0.01X50 = 0.50 mg to 110X80% = 88.00 gm. Detailed guidance on the balances can be learned through OIMLR76-1 Non-automatic Weighing Instruments.

But according to USP <41> Weights and Balances, a simple theoretical calculation can be used to calculate the minimum weight for any balance. To weigh 10 mg sample you have to use 10X0.1% = 0.01 mg least count balance. It shows that if you have a balance with least count 0.01 mg, you can weigh minimum 10 mg sample on it. Other weights can be calculated accordingly.

Update: Simply you can calculate the minimum limit by multiplying least count with 1000.
Minimum Weighing Limit = Least Count x 1000

Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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1 comment:

samhitha a said... on 27/8/16 08:50

thank you so much it is help full.

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