Check weigher

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Example checkweigher. Product passes on the conveyor belt where it is weighed

A checkweigher is an automatic machine for checking the weight of packaged commodities. It is normally found at the offgoing end of a production process and is used to ensure that the weight of a pack of the commodity is within specified limits. Any packs that are outside the tolerance are taken out of line automatically.

A checkweigher can weigh in excess of 500 items per minute.

Checkweighers often incorporate additional checking devices such as metal detectors and X-ray machines to enable other attributes of the pack to be checked and acted upon accordingly.

1 A typical machine
2 Tolerance methods
3 Data Collection
4 Application considerations
5 External links

[edit] A typical machine

A typical checkweigher incorporates a series of conveyor belts. Typically there are three:

An accelerating belt to stabilise the speed of the pack and to bring it up to that required for weighing
A weighbelt. This is typically mounted on a high-speed weight transducer which can typically be a strain-gauge load cell or a servo-balance (also known as a force-balance). It is usual for a built-in computer to take many weight readings from the transducer while the pack is on the weighbelt to ensure an accurate weight reading
A reject conveyor to enable the out-of-tolerance packs to be removed from the normal flow while still moving at a know velocity. The reject mechanism can be one of several types. Among these are a simple pneumatic pusher to push the reject pack sideways from the belt, a diverting arm to sweep the pack sideways and a reject belt that lowers or lifts to divert the pack vertically. A typical checkweigher with a bin to collect the out-of-telerance packs.

[edit] Tolerance methods

There are normally two tolerance methods:

The traditional "minimum weight" system where weights below a specified weight are rejected. Normally the minimum weight is the weight that is printed on the pack or a weight level that exceeds that to allow for weight losses after production such as evaporation of commodities that have a moisture content.
The European Average Weight System which follows three specified rules known as the "Packers Rules".

[edit] Data Collection

There is also a requirement under the European Average Weight System that data collected by checkweighers is archived and is available for inspection. Most modern checkweighers are therefore equipped with communications ports to enable the actual pack weights and derived data to be uploaded to a host computer. This data can also be used for management information enabling processes to fine-tuned and production performance monitored.

Checkweighers that are equipped with high speed communications such as Ethernet ports are capable of integrating themselves in to groups such that a group of production lines that are producing identical products can be considered as one production line for the purposes of weight control. For example, a line that is running with a low average weight can be complimented by another that is running with a high average weight such that the aggregate of the two lines will still comply with rules.