Which is the right balance for weighing my sample?

In day to day analysis your weighing requirements will depend on quantity of sample available, sensitivity of weighing balance and the sensitivity of the analytical technique. The three types of digital display balances in common use in most of the laboratories are

 Top Pan balance Range: g – Kg Analytical Balance Range : 0.1mg – 220g Micro Balance Range: $10^-^6$– 5g
Top Pan Balance

A Top Pan Balance is used mainly for rough weighing for preparation of reagents and stock solutions where slight weight variations do not have significant influence on the result of analysis.

Electronic Analytical Balance

Majority of times you will find an electronic analytical balance suitable for weighing your samples and reference standards. The readability or least count of such balance is 0.1mg or 0.0001gm.

Micro Balance

A Micro Balance provides the ultimate in terms of accuracy for weighing of very small amounts of samples. The readability of a micro balances 1 µg or $$10^ -^6$$ g or $$10^-^3$$mg and for more sensitive weighing an ultra micro balance can be used which has readability of 0.1 µg or $$10^-^4$$ mg or $$10^-^7$$g. It would be indeed very rare that you will have requirement of weighing below these values. Use of micro balance requires a cabinet that is dust free, wind draft free, anti-vibration and anti-accoustic so that no external disturbances affect your weighings.

Suitability of a balance

The suitability of a balance is governed by the amount of sample that is required to be weighed as the same balance cannot be used to weigh samples ranging from grams to micrograms. As per US Pharmacopoeia Section 41 the measurement uncertainty (random plus system systematic errors) does not exceed 0.1% of the reading. This means that if you need to weigh a mass of 10 mg the combined percent uncertainty should not exceed the 0.01 mg. As the readability of electronic analytical balance is 0.1 mg it is advisable that masses below 10 mg are not weighed using such balance for acceptability of results.

Having decided on the suitability of the balance it is important to follow correct weighing practices which will be covered in a subsequent article.

Hope the guidelines provided will help you choose the balance for the required application.

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Responses

1. Wind and vibrations affect readings significantly on analytical balances and are of negligible significance in top pan balance weighings.For analytical range weighings it is advisable to use vibration proof tables or work benches and take readings after closing the covers.