Analytical weighing balances
Analytical Weighing balances are most critical part of pharmaceutical industry as it directly impact the final product quality.
Q1. What is the principle of analytical weighing balance ?
Analytical weighing balances are based on the principle of load cell.
Types of load cells are – Strain gauge type,Frequency modulated and Electromagnetic force compensation type.
Strain gauge type load cell is most widely used weighing balance in pharmaceutical industry based on the principle that it contains electrically deformable body which upon stress application change its resistance and changed resistance is digitally converted into weight. Amount of stress applied is directly proportional to resistance.
Excessive stress during cleaning procedure could impact the sensitivity of load cell so cleaning procedure must be validated and followed as per GMP.
Q2. What are the guidelines associated with weighing balance calibration ?
- 21 cfr part 211.160 USFDA guidelines.
- ISO guidelines EN ISO 9001:2008 are associated with weighing balance calibration.
Q3.How to perform Calibration of analytical weighing balances ?
Before looking deep into calibration we must be aware of few common terms associated with weighing balances.
- Standard deviation – How much far away from mean are the values in group.
- Calibration – Degree of accuracy.
- Residual standard deviation – Residual standard deviation is big term to be defined in few lines However to summarize RSD is variation between Standard deviation.
To perform calibration Accuracy , Reproducibility and Eccentricity must be measured.
- Accuracy – To measure accuracy weights are taken of capacity 5% , 10%, 20% , 50% and 90% of balance capacity and it should be within ±2%.
- Reproducibility – Take weight of 1 gram and subsequently take 10 readings from center of balance and it should be within ±2%.
- Eccentricity – Take readings from 5 different locations i.e Left , Right, Front, Back and center of weighing balance and calculate Mean , % Residual standard deviation and Standard deviation.
- Standard deviation = √∑(Xi-X)²/n-1
- Residual standard deviation = SD *100/mean.
- Where Xi = Individual value.
- X = Mean of values
- n = number of values.
- Limit of RSD is Not more than ±2%.
Q4. What are recent updates in pharmaceutical guidelines for weighing balances and what do FDA auditors inspect in weighing balances in recent trends ?
- USP chapter 41 “Weights and balances” has been revised recently in 2013 for repeatability and accuracy of weighing balances This USP chapter 41 makes it mandatory for pharmaceutical manufacturers to use use a test weight between 5% and 100% of balance capacity .
- As this guideline has been revised by USFDA after 20 long years FDA investigators are looking at weather it has been implemented in the facilities or not.
Q5. What is FDA perspective on Auto calibration feature of weighing balance calibration ?
- As per statement by FDA site FDA do not rely on auto calibration feature of balances for calibration purpose However , if Pharmaceutical manufacturer is using this feature it must be used along with manual calibration.