What capacity and readability do I need for a laboratory balance?
The weighing specification of a balance is determined by its capacity and readability.
For example a specification for a typical analytical balance would be: 200g (capacity) x 0.1mg (readability)
As the capacity of the balance increases the readability decreases
For example on a top pan: 4000g (capacity) x 0.01g (readability)
Readability
Readability is the smallest increment of mass that a laboratory balance can display. It represents the fi nest resolution of measurement and is typically expressed in grams (g), milligrams (mg), or micrograms (μg). This does not indicate the smallest sample weight you can weigh out on a balance ( see below)
Capacity
Determine the maximum weight you need to measure. Ensure you take into account any tare vessels / container that may be used.
A tare vessel (or tare container) is a container used to hold a substance while being weighed in a laboratory. The term tare refers to the weight of the container itself, which is subtracted from the total weight to obtain the net weight of the substance.
Most modern laboratory balances have a tare function, which allows the user to place the empty container on the balance and press the tare button. This resets the displayed weight to zero, so when the substance is added, only its weight is shown.
Common Examples:
- Weigh boats (small plastic or aluminium dishes for powders)
- Beakers or flasks (for liquids or bulk solids)
- Filter paper (for weighing precipitates)
- Centrifuge tubes (for biological samples)
You need to take into account the weight of the tare vessel / container when selecting a balance. For example if you are weighing a sample with a weight of 150g and you use a tare vessel / container weighing 50g – you will need a balance with a minimum capacity of 200g