WEIGHING SYSTEMS AND WEIGHT ACCURACY EXPECTATIONS
Weight accuracy is a function of the weighing system and the uniformity of flow of the product into the weighing system. While the weighing system may be able to measure weight increments that are very fine, the final weight of the filled bag is normally far less accurate due to the process of stopping the product flow into the bag. It is of the utmost importance to remember that product testing in a manufacture’s factory will not yield the same results as actual weight accuracy in the buyer’s factory. This is due to many things such as:
- Temperature of the product
- Consistent head-pressure
- Ambient temperature
- Vibration
- Product density changes during shipment and over time
There are two methods of determining the weight of the product in the bag. The original method is the use of a mechanical scale to trigger a stoppage in product flow. A mechanical weighing system uses a lever similar to a child’s teeter-totter. When the filled bag weighs more than the counterweight, the scale pivots and trips a cutoff mechanism that stops the product delivery to the bag. The mechanical weighing system is simple, low cost, and can provide relatively high speed as compared to many electronic scale systems. It normally provides bulk-only flow and therefore is not as accurate as a controllable bulk-dribble infeed.
Electronic weighing systems provide higher weight accuracy than mechanical weighing. These are physically arranged to apply a force to a load cell proportional to the weight of a bag. Different linkages or direct suspension of the load are used in various versions. As more weight is sensed the electrical signal sent by the load cell increases. An electronic controller uses the electrical signal to determine actual weight and to control product flow cutoff at the desired weight. The controller provides bulk and dribble cutoff points to produce the most accurate weights. Once the dribble is actuated a smaller amount of product is dispensed until the desired weight is reach. Bulk/dribble control of product infeed provides the greatest weight accuracy.
Weight accuracy is generally expressed in terms of standard deviation. Typically, standard deviation may be reported as “± 120 grams # 2?.” For a system with a standard deviation (?) of 60 grams, we could expect:
- 68.2% of weights within ± 60 grams (1?)
- 95.4% of weights within ± 120 grams (2?)
- 99.7% of weights within ± 180 grams (3?)
