Precisa’s Guide to Balance and Scale Calibration
Differences between Scale Calibration and Adjustment
“Calibration” and “adjustment” are often confused as being the same process. Although they both work to improve balance measurement accuracy and provide accurate balance readings, they differ in their procedures. Calibration is the understanding of how a scales behaves, while adjusting is the changing of this behaviour.
Calibration is a set of processes under controlled conditions that show the relationship between the values of quantities using measurement and the corresponding values according to standards set. The standards output and accuracy are known which means the measurement can be compared with a measurement from the instrument being calibrated. Calibration reports can be made which show whether or not the balance has passed or failed certain conditions.
Adjustment is the process of altering a measuring device so that it is in a state of performance that is suitable for its use. The amount of adjustment carried out can be determined by a “found calibration” which is a pre-adjustment calibrations which involved carrying out the first calibration, making the adjustment and then carrying out the second calibration.
Calibration is about establishing a relationship between a known value (the standard taken from test weights) and the measured value. The difference between these two values show how you understand the behaviour of the scales. If you place 1kg on the scales and the scales reads 2kg, then you can presume that by placing 5kg on the scales, it will read 10kg. This understanding of behaviour is calibration.
Calibration of a scales often does not require any adjustment if everything is running well. A “tolerance” is if the scales is measuring well enough to successfully complete the task, without being 100% precise. Tolerances are subject to change and can be set by customers and manufacturers
What is Measurement Uncertainty?
Measurement uncertainty is becoming increasingly emphasized in measurement communities. Measurement uncertainty is the “doubt” of measurements. Every measurement we make has some level of doubt so we need to assess how much “doubt” there is so we can establish how reliable the reading is. If the measurement uncertainty for a device is not know, then there is no point in taking a reading at all. Measurement uncertainty must be included in calibration certificates according to the ISO 17025 standard for assessing the competence of calibration and testing laboratories
Measurement uncertainties can stem from environmental factors, the measurement device, operator or procedure errors and many more. Error is not the same as uncertainty. Error is the difference between the device reading and the standard, but an error reading cannot be accurate until the measurement uncertainty is established.
Measurements must be repeatedly taken in order to establish what the typical deviation is. Calibrators must have valid traceability to national standards and attention must be paid to the calibration process’ total uncertainty before deciding to pass or fail performances.
Measurement uncertainty should be included in the calibration reading in order to enhance the understanding of the scales.
What is the difference between Internal Scale Calibration and External Scale Calibration?
External Calibration is a manual process. It involves using trade approved calibration weights. A wide variety of calibration weights are used to test the weight capacity and level of accuracy. The weights are placed on the scale and their reading is set as the standard. The purchasing and management of these weights is very important. Weights must be bought from a trusted source and preserved carefully to make sure they don’t gain or lose mass.
Internal Calibration is a more automatic process that does not involve calibration weight sets or manual user input. This is used for scales with built in calibration weights and some of these scales can even be programmed to calibrate at specific times.
In summary, external calibration is less expensive but takes up time and staff resources and both scales and weights need to be properly maintained. Scales with internal calibration functionality tend to be more expensive but they account for more environmental changes, such as movement, stability and temperature.
When deciding which option is best for you, you will need to assess your budget, your time resources, if weighing conditions frequently change and frequency of use.