The two basic weighing principles to weigh onions and potatoes
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During the mechanised packing of harvested potatoes, onions, carrots and more, two weighing principles are applied:

The principles of how to weigh a harvest using automated format.

From now on in this section we will only discuss harvest weighing using automated machinery.

There are two different principles for doing this:

  1. Directly dosing each bag.
  2. Or dosing several hoppers at the same time, with for example 50% of the weight of each bag and subsequently choosing a combination of the two hoppers whose weight is closest to the weight of the final bag.

1. Directly dosing each bag, for packs of 5 Kg and more !

A weigher that weighs using the dosing principle.

A weigher that weighs using the dosing principle.

Machines with only one conveyor or elevator belt, work in the following way:

The conveyor or elevator belt begins to rapidly fill a hopper/weigher. When the weight approaches, for example, 90% of the target weight the conveyor or elevator belt reduces its speed and slowly doses the last of the potatoes, onions or other similar product, until the weight in the weigher/hopper has reached the target weight (in practice it will have gone over this weight, because no one wants to sell underweight bags as this would upset the customer). At this point the conveyor or elevator belt stops and waits until the weigher/hopper is empty. The cycle then begins again.

These weighers have limited accuracy, see: Weighing accuracy using dosing.

Machines with two conveyor or elevator belts, work in the following way:

These machines have two conveyor or elevator belts. One wide and one narrow.
The latter is the dosing conveyor or elevator belt.

The elevator or conveyor belts begin to fill the hopper/weigher. When the weight approaches, for example, 90% of the target weight the wide conveyor or elevator belt stops completely and the narrow conveyor or elevator belt reduces its speed and slowly doses the last of the potatoes, onions or other similar product, until the weight in the weigher/hopper has reached the target weight (in practice it will have gone over this weight, because no one wants to sell underweight bags, as this would upset the customer). At this point the narrow conveyor or elevator belt stops and waits until the hopper/weigher is empty. The cycle then begins again.

These weighers have limited accuracy, although slightly more than weighers with one conveyor or elevator belt. Weighing accuracy with dosing

The capacity of these machines is also fairly limited, especially when weighing bags of 10kg or less.

An indication of the capacity of these machines in relation to the weight of the bag:

  • For 2kg bags, the attainable capacity is 1 tonne per hour.
  • For 5kg bags, the attainable capacity is 2 tonnes per hour.
  • For 25kg bags, the attainable capacity is 7-9 tonnes per hour.
  • For 50kg bags, the capacity is 10-12 tonnes per hour.

Why do we use the word attainable?

For the simple fact that all of these capacities depend on the operator who places the bags under the weigher's discharge chute, and whether there is a semi-automatic or completely automatic bagging and sewing machine.

2. Combining different batches already dosed for packs of 0.2 Kg and more !

A weigher that uses the principle of combination dosing.

A weigher that uses the principle of combination dosing.

We have now reached the latest in weighing a harvest on a continuous basis, using large quantities and where accuracy is not affected by the size of the product or packaging.

In the photo on the right you can see the top of the ERCmachinery Multi-Head weigher. The weigher is continuously fed from above in the centre. Which is very important for the line feeding the machine. If the flow that feeds the weigher is correctly regulated, the line continues to move non-stop, as happens in weighers using the dosing principle.

The burgundy valves dose the weighing hoppers (the grey boxes) with (for example) 50% of the target weight for each bag. Now it is not necessary to fill the hoppers/weighers with a little over 50% of the weight, the machine is adjusted to exactly 50% of the weight. The result is that some hoppers/weighers receive a little more and others receive a little less than 50% of the target weight.

When all the hoppers/weighers are full, the weigher computer chooses two hoppers/weighers whose combined weight is as close as possible to the bag's target weight, and simultaneously drops their contents in the centre under the weigher. The hoppers/weighers are then immediately filled again and the cycle repeats itself.

The accuracy of an Eqraft BV Multi-Head weigher

These weighers are almost 100% accurate, see: Accuracy of weigher that combines two batches already dosed.

An indication of the capacity of these machines in relation to the weight of the bag:

  • 4.000 0.5 Kg bags or a capacity of 2 tonnes per hour.
  • 1,500 25 Kg bags or a capacity of 37.5 tonnes per hour.

The capacity increases linearly according to the weight of the container.

The following link provides all the features and applications of this machine. We can also see it in action. Eqraft BV Multi-Head comb weigher.

1. The dosing principle.

2. The dosing principle, combining two or more weighing hoppers.

The first principle/system uses dosing, this means: The weighing hopper is rapidly filled until the target weight is approached, once this target weight is almost reached, the machine slows down and begins to dose potato by potato, onion by onion until the weight is just over the target. This principle works perfectly well for relatively large sacks and relatively low hourly capacities. This system does not work for bags of less than 2 kilos with a relatively large product.

The second principle uses the dosing principle whilst combining different weighing hoppers. In this system several weighing hoppers (for example 16) are filled up to for example 50% of the target weight. The computer then chooses the two weighing hoppers whose combined weight is closest to the target weight.

This last combination dosing principle achieves impressive accuracy. In our page titled "Multi-Head Weighers" are very accurate, you will find a simulation programme where you can see for yourself exactly how accurate these machines are.

Furthermore, these machines have a huge capacity, as can be seen in the following table.


To mechanise your post harvest processes:
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Address: Farm San Isidro, Volcán, Panama.