What is a combination scale?
Combination weighing scales are most often round or radial scales that use the method of weight combinations to arrive to the overall target weight. Of course to use the combination of weights, the scale needs to have multiple load cells with a portion of the overall target weight. That is why this technology is referred to as a combination scale.
The smallest scale money can buy is a 10 head combination scale. Not limited to, but usually the scale would try to find the combination of 3 weights to make the target weight. Meaning, after the scale has made 1 combination, it has only 7 weighments left to make another combination. That is true when running high speeds or large amounts of product. There is time that goes by from replenishing the product into the weigh bucket, but at lower speeds that should not be a factor. Often, the scale needs to wait for the material handeling device below (poucher or indexer). This is plenty of time for the scale to complete its combination, meaning a majority of the time 10 weigh buckets will be fast enough.
What scale is right for you?
Of course, you can upgrade to 14, 20, or even 28 heads. Increasing the number of heads will certainly offer better accuracy as the scale now has that many more weighments to choose from. Theoretically, it also should increase speed and that is certainly true when running heavy product like beans, rice or similar. When the scales job is to dispense a light and fluffy product, a larger scale does not necessarily do any favors as the product needs to travel further on a less aggressive slope. In most cases there is no reason to step above 14 heads as the accuracy increase does not offer much more benefit over and above the price increase and space required.
Most important to keep in mind, not every combination scale is like the other. Disregarding most of them looking simular to the laymans eye, there are huge differences in quality and design. From the actuator module material (Cast aluminum versus Stainless Steel), to the mechanics of bucket design, electronics and operator control. Just looking at one system from one manufacturer will ultimately leave you uneducated about the differences other systems offer. So like with everything, it is the buyers duty to educate themselves on the purchase.