Weighing onions and potatoes.

Weighing the harvest.

The four most important ways of weighing onions or potatoes prior to sale are:

Don't weigh , hand weigh , Semi-automated and Automated.

1) Weigh several bags at once as opposed to one by one..

Method: Fill the sacks by hand with onions or potatoes. Fill them with roughly the same quantity, for example 25 Kg. When the product is sold and the buyer wants for example 5 bags, place the 5 bags on a weigher and charge the exact weight indicated. This Method has the advantage that there is no product giveaway due to errors when weighing and filling bags, but the buyer must accept that the weight per bag may vary.

Another alternative (see photo): Weigh the lorry when empty, and weigh it again when loaded. The difference gives a fairly accurate weight.

2) Weighing bag by bag.

Weighing bag by bag

Weighing bag by bag

Adjusting weight bag by bag

A well-earned rest

Bag by bag

Bag by bag

Bags can be weighed one by one when filled by hand. The result is an exact weight in each bag, but the process is inconvenient and laborious. Furthermore the final result is not as accurate as the previous Method, because someone filling and weighing bags will always add a little more than the amount indicated (5, 10, 15 or 25 kg), in order to avoid disagreements or mix-ups at the time of sale.

However, the inefficiency and labour implications are huge, not to mention back breaking.

Now semi-automated crop weighing. Suitable for 5 Kg and above.

3) Using a double or single belt weigher/filler (conveyor).

Oldehuis Agro weigher rear view.

SKALS weigher front view. Both companies have single and double belt weighers.

The semi-automatic double belt weigher has:

1. A hopper where the product to be fed into the machine is collected.
2. Two belts, one wide and one narrow, to feed the hopper that weighs the batches.
3. A discharge hopper where the batches are weighed with an outlet valve and an attachment for the bags to be filled.
4. This equipment is controlled electronically.

To better understand how these weighers/fillers work let's look at a complete cycle.

When the weighing hopper is empty, the two lifting belts start to move at top speed filling the hopper until the target weight is almost reached. At this point, the wide belt stops and the narrow belt slows down, adding the last remaining potatoes or onions bit by bit until the target weight is reached.

A single dosing belt.

A double dosing belt

The principles of this system are made clear in the video on the right.

The accuracy of these one or two belt-weighing systems.

This system works well whilst the target weight is not too low in relation to the individual weight of the potatoes or onions. For example, if the bags are 10 Kg and the onions on average weigh 100 grams, one onion per bag will be given away to ensure that all of the bags reach the nominal weight, i.e. 100 grams per 10 Kg, only 1%. However if we use this system to fill one kilogram bags, we would be giving away 100g per 1kg, or 10%.

The table shows the percentages of giveaway produce in relation to the individual size of the onions or potatoes, and the required weight of the bag.

As you can clearly see, the bigger the product and smaller the bag, more of the product is given away, and therefore NOT CHARGED.

For this reason, this system of weighing and packaging of onions and potatoes is only used if the packaging unit is more than 5 Kg.

The capacity of this system in semi-automatic mode is about 3.000 Kg per hour in 5 Kg bags or 10,000 Kg per hour in 25 Kg bags.

The capacity of this system in fully automated mode is about 16,000 Kg per hour in 5 Kg bags or 18,000 Kg per hour in 25 Kg bags.

If you are interested in calculating how much money these percentages represent for your particular business, or how these percentages relate to the different sizes of your potatoes or onions, please go to: Double belt weigher.

4) Use of a fully automated multi-head weigher/filler. Suitable for weights between 0.3 Kg to 50 Kg

These weigher/filler machines fill up to 30 tonnes/hour in 25 Kg bags and 18,000 Kg/hour filling 1 Kg bags, with a maximum deviation from the target weight of 0.25%.

The 16 head weigher manufactured by Eqraft BV is the latest in crop weighing

How do these machines work?

These machines have between 10 and 18 aligned or circular weighers, hence the name multi-head or multi-channel. How these weighers are filled can be seen here.

In this example, we have used a machine with 10 channels and we want to fill 500 gram bags with 7 to 9 cm diameter onions. The channels, tracks or heads are filled to around 50% of the target weight. A computer then looks for the combination out of the 45 two track combinations that is closest to the nominal weight.

The results of a multi-head weigher with 10 heads.

The table on the left shows the weights of the 10 channels of onions. These 10 channels or heads give 45 useful combinations (head 1 and head 2 are one combination, but head 2 plus head 1 is the same).

On this occasion, we want to package units of 500 grams.

In the upper part of the table in black we can see that head 4 has 258 grams and head 9, 242 grams. Therefore 258 + 242 - 500 (the target weight) = 0 grams. This is the combination that the weigher's computer will use to fill the bag: head 4 + head 9. In this case, the bag will be filled to the exact target weight.

Of course, the computer does not always find such an exact combination, but this example gives a good idea of the accuracy of the multi-head weighers/fillers.

To see how accurately these machines work with large 7 to 9 cm diameter onions and with 500g bags, play with our Multi-head weigher/filler Simulation.