Posted on: 6 January 2015
Your laboratory depends on the accuracy and reliability of your weighing equipment. No matter how sophisticated your precision laboratory balances may be, they are still prone to error. They need meticulous care and re-calibration on a regular basis to maintain consistent and accurate readings.
The following guidelines will assist you in developing suitable care and maintenance procedures and getting the most out of your lab balances. Of course, your main objective is to ensure that errors are prevented.
Recognising Error Types
Weighing errors in the lab may be either systematic or random.
Systematic errors occur as a result of a fault in the balance itself or by the actions of the operator. This type of error always produces readings which deviate in one direction. For example, a balance that has been incorrectly calibrated will always produce a reading that deviates a little above true value. Systematic errors are often rather challenging to detect, the main difficulty being that they do not average out. The best procedures for finding these errors are to test by measuring a known quantity or compare readings made by using a different apparatus which is known to be accurate.
Random errors, in contrast, produce deviations in both directions and over a large number of readings, they average out. For example, by weighing the same sample load repeatedly, each weighing will produce slightly different results, either above or below true value.
Errors are best avoided by a thorough care and maintenance routine in the lab.
Confirm And Test Repeatability
When placing the same sample load on the weighing pan for a subsequent weighing, it should produce an identical reading, but usually there is a minute variability. Standard deviation is the difference between the highest and lowest result for a specified number of measurements. To test for repeatability of the balance:
- Choose a test weight
- Begin with an empty weighing pan
- Reset the reading to zero
- Place and remove the weight ten times
When the variation between the higher and lower readings is under the allowed tolerance level outlined in the balance manufacturer's manual, your instrument has passed the test.
Confirm And Test Corner Load
For this test you can make use of the same weight used in the repeatability check.
- Place your weight in the centre of the weighing pan and reset the reading to zero.
- Move the weight from the centre to the edge at the right, left, front and rear of the pan and obtain readings at each position to check accuracy.
Confirm And Test Linearity
Use four weights and measure their combined mass to record what is called the full scale value.
- Subsequently weigh two at a time and record the weight for each set.
- Sum up these readings to obtain what is called the weight sum value.
- Calculate the difference between the full scale and weight sum values and divide the result by two.
This figure produces the linearity error which should fall within the accepted range for the instrument.
Don't Overlook The Minor Details
Normal practices and basic precautions should not be ignored in the necessary vigilance required to avoid errors. For example, remember to recalibrate balances each time they are moved to a different position, subjected to a sizable ambient temperature change and disconnected from the main power for an extended period as they will need to warm up to operating temperature.
All details of routine maintenance and repairs or malfunctions should be recorded in an equipment log for each instrument.
Many laboratory managers, being aware of their absolute dependence on the accuracy and performance of their balances, seek the assistance of professionally qualified personnel. These experienced technicians perform all the necessary checks on the instruments at regular intervals. This ensures that the laboratory operates at maximum efficiency and eliminates waste of costly materials and wasted time in error detection.Share