How to Test Repeatability of a Laboratory Scale
Scientists who use lab balances for performing measurement tasks know the importance of having efficient and reliable equipment. Good repeatability is essential for laboratory balances due to the delicate nature in which they are used. Laboratory balances come in a range of designs including moisture analysers, precision balances and analytical balances used in scientific experiments from density determination to the handling of chemical compounds.
What is repeatability?
Repeatability (also referred to as precision) is the ability to consistently produce identical results when the same weight is placed on the same weighing balance multiple times under identical conditions. Approved calibration weights are used when calculating repeatability in scales and balances to ensure they are weighing accurately and consistently.
Identical weighing conditions are essential for maintaining balance repeatability
Despite accurate balance calibration, factors such as vibration, temperature, humidity and location still affect the accuracy of a weighing balance. It is crucial that these variables are reduced where possible. Here are some recommended ways in which to keep your balance weighing optimally:
- Ensure the same user is performing the test using the same weighing method on the same balance.
- Ensure the same object is placed in the same place on the same pan support.
- Ensure the balance is located in the same location for each measurement, and those measurements are taken in immediate succession.
If you are getting results that are inconsistent or unexpected, you might want to check to see if any of those conditions are affecting your measurements. Some high-precision balances such as analytical balances come with protective draft shields around the weighing chamber that can be used to negate the effects of drafts and other temperature or weather concerns. In addition, anti-vibration tables can be used to increase the stability of a balance which can be affected by vibrations from nearby machinery, footfall and even exterior traffic.
How to find the repeatability of a balance’s measurement of a 10g item
When testing repeatability it’s a good idea to use calibration weights, which are specifically designed to ensure consistent weighing. If calibration weights are unavailable, you can use any solid, non-porous object that isn’t magnetic, as long as it is clean and doesn’t hold static.
- Weigh a 10g item 10 times and record each measurement.
- Figure out the average of those 10 measurements by adding the weights and dividing the total by 10.
- Determine the difference between the average and each individual measurement.
- Square those numbers to get the squared difference for each measurement.
- Add together the squared difference amounts.
- Divide that total by the number of measurements minus one (10-1=9), so in this case, you’re dividing by nine.
- Find the square root of that amount. That figure signifies the repeatability of the balance when weighing a 10g item.
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