Analytical balance · Calibration procedure
Repeatability
Weighs the same test weight ten times and calculates the standard deviation. This is the most important check for small weighings.
PURPOSE
Why this is checked
For small samples, the random scatter of the balance (its repeatability) is the largest source of error. USP <41> sets the requirement on repeatability compared with the smallest net weight the lab intends to weigh.
Frequency
Typically every 6 or 12 months, and after repair, relocation or a failed daily check, as defined in your calibration SOP.
Learn how the Analytical balance worksYOU WILL NEED
Standards & tools
- One calibrated test weight (e.g. a 20 g OIML class E2 weight). The exact value is not critical for repeatability.
BEFORE YOU START
Prerequisites
- Balance level, clean and warmed up
- No draughts or vibration
- Test weight at room temperature
PROCEDURE
Step by step
- 1Zero the balance with the doors closed.
- 2Place the test weight in the centre of the pan, close the doors and record the stable reading.
- 3Remove the weight, let the balance return to zero (re-zero if needed).
- 4Repeat until you have at least 10 readings.
- 5Calculate the mean and standard deviation (s) of the readings.
- 6If s is smaller than 0.41 × d (d = readability), use 0.41 × d in place of s, as USP <41> describes.
- 7Check the result against the smallest net weight the lab intends to weigh on this balance.
RECORD
Example observation table
| Reading | Observed (g) |
|---|---|
| 1 | 20.0001 |
| 2 | 20.0002 |
| 3 | 20.0001 |
| 4 | 20.0000 |
| 5 | 20.0001 |
| 6 | 20.0002 |
| 7 | 20.0001 |
| 8 | 20.0001 |
| 9 | 20.0000 |
| 10 | 20.0001 |
CALCULATION
How to calculate the result
Repeatability is acceptable when 2 × s ÷ m ≤ 0.10%, where s = standard deviation and m = smallest net weight to be weighed
- Mean = 20.0001 g; s = 0.0000667 g = 0.0667 mg
- Readability d = 0.1 mg, so 0.41 × d = 0.041 mg. s is larger, so use s = 0.0667 mg
- Smallest net weight the lab weighs on this balance: m = 150 mg
- 2 × 0.0667 ÷ 150 × 100 = 0.089% (≤ 0.10%, passes)
ACCEPTANCE CRITERIA
Pass if
- 2 × s ÷ m not more than 0.10% for the smallest net weight to be weighed (USP <41>).
IF IT FAILS
What to do
- Check the environment (draughts, vibration, temperature) and repeat.
- If it still fails, raise the minimum weight for the balance, or remove it from use and call service.
- Assess any weighings below the new minimum weight since the last pass.
MORE
Other Analytical balance calibrations
In LimsPulse · Calibration Management
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.