A scale is a simple measuring instrument. Consider a ruler or tape measure. One is not going to measure the length of a rugby field with a 3m tape measure or try and use a ruler graduated in cm to measure a tiny item accurately! So please consider the maximum weight the scale would have to measure up to and what the finest graduation is required to give you accuracy.
When selecting a scale, there are five things you should keep in mind to find the weighing device that best suits your requirements. Be aware of certain terms used in the weighing industry.
- What is the largest weight I need to weigh? (This is known as the CAPACITY {Max}of the scale)
- What is the size if the smallest graduation the scale indication must read in? (INCREMENT is what the scale is graduated in e.g. e = 0.1g; 5g; 0.1kg 0.5kg etc.)
- What is the minimum weight the scale is rated for? (MIN weight is usually calculated at 20 x the increment {e})
- Is this scale going to be used in general weighing or used to determine a monetary value like buying of scrap metal, prepacking goods for a store or selling fruit or meats (Legal Metrology)
- What is the physical size of the item I need to weigh? (The overall weight of an item might be within the scales max capacity, but it might be physically too large to fit on the scale pan!)
Example: A Micro A12E electronic platform scale. Specifications Max: 150kg e = 50g Min: 1kg SA1465 AA5 platform size:400×500
- The maximum weight the scale measures up to is 150kg
- The increment e=50g which is 0.05kg (so expect readings of 0.05; 0.10; 0.15; 0.20; 0.25; 0.30 etc.)
- The minimum weight is 1kg or 1000 grams. For accurate measurement to allow for rounding errors of the course increment size, the scale must not be used to determine results below this value.
- The NRCS approval number SA1465 AA5 denotes that this scale can be used for prepacking goods, determining weight at a courier service or buying/selling over.
- The platform size is 400mm x 500mm . This gives you a good indication if your parcels or goods will fit onto the platter.
Legal For Trade – Note that it is an offence to prepack, buy/sell over or use a scale to determine weight using a scale that has not been approved for such use by the NRCS – National Regulator for Compulsory Specification – in accordance to the Legal Metrology Act, Act 9 of 2014. All such scales must have a valid Verification Certificate and must be recalibrated within a 24 month period.
Calibration – Calibration is the process of adjusting a scales precision using known weights (calibration weights). Calibration should only be performed if your scale is not weighing properly. After calibration, the scale should display the exact weight that is placed on the tray (within a certain tolerance).
Calibration Certificate – A Calibration Certificate, not to be confused with a verification certificate (Legal Metrology) is provided upon request only by us as proof of calibration along with information about when the calibration was performed and documenting the accuracy and readings of the scale. Issue is not a legal requirement.
Calibration or Validation Weights – Calibration weights are highly accurate certified weights which come in a variety of nominal masses such as 100g; 500g; 5kg or 10kg etc. It is important to use the correct calibration weight when calibrating a scale. Calibration weights do not come with a scale, but are used by us for the initial setup or repair/service of scales in our workshop. Clients can also purchase their own validation weights to check their equipment on a regular basis as required by their quality systems.
Linearity – Linearity is a scales ability to produce accurate readings throughout the entire range of the scale from minimum to max capacity. Linearity is mostly listed on specification sheets for the high-end precision/industrial scales. The scale would be loaded in small increments with calibration weights and run up to maximum capacity with the readings noted.
Eccentric Test – Corner load testing is done to confirm that the scale is accurate with the weight placed on different corners of the weigh pan. Normally a third of the capacity weight is chosen, which is placed centrally on the pan and then the weight is moved to the four corners of the pan in sequence. The reading should be the same all-round the platter.
Repeatability – Repeatability, like Linearity, is a specification most seen on precision/industrial scale specification sheets. It refers to the scales ability to display the same weight reading consistently when a weight is placed on and removed from the scale platter. This cycle is repeated 5 or 6 times in a row noting the weight reading and if the reading returns to zero point on each weighing cycle.
Load Cell – Digital scales use a load cell to convert weight to a digital signal. Load cells typically consist of a block of metal with a strain gauge attached that can detect the slight variations in stress on the metal. The strain gauge detects the change in electrical resistance and converts this an analogue signal. The scales microprocessor converts this to a digital signal which is shown as weight on LCD displays.