Checkweigher buying guide

Short answer: buy a checkweigher for the rules you must meet and the packs you actually run. First confirm which net-quantity rules apply in each market you sell into. Then ask each maker for accuracy on your pack at your line speed, proven in a test. Next, make sure the reject system removes every failed pack safely, and that the data it records will satisfy an auditor. Finally, check hygiene, changeover and integration with your filler and metal detector. Its purchase price is only part of the cost. Accurate data that lets you reduce overfilling often matters more.

What does a checkweigher do?

A checkweigher is an inline scale that weighs every pack while it moves. A typical machine has three conveyor sections:

• An infeed belt spaces the packs out and brings them to the right speed for weighing.
• A weigh belt is mounted on a load cell. This may be a strain-gauge cell or an electromagnetic force restoration (EMFR) cell. EMFR cells are used for high-speed, high-precision weighing.
• A reject section removes packs that fall outside the limits.

As each pack crosses the weigh belt, the machine's computer takes many readings and combines them into one weight. It then sorts the pack as underweight, accepted or overweight and removes it if needed. It also records the result.

Checkweighers usually sit straight after the packaging machine and before case packing. They are often combined with a metal detector or an X-ray system on one frame, which saves space and uses one set of controls.

Which weight rules does it help you meet?

A checkweigher does not make a pack legal on its own. It is a tool for meeting the net-quantity rules of the market you sell into, and those rules differ.

• European Union and the e-mark (℮). Under Directive 76/211/EEC, packs marked with the ℮ must follow three "packers' rules". The average contents of a batch must not be less than the declared quantity. Only a small proportion of packs may be short by more than the tolerable negative error. No pack may be short by more than twice that error. The tolerable negative error depends on the declared quantity. It ranges from 9% for packs of 50 g or 50 ml or less to 1.5% for packs of 1 kg or 1 litre or more. The ℮ is also used in the United Kingdom and some other countries. The average-quantity system also expects packers to keep records, which is why checkweigher data matters.
• United States. Officials check net contents using the test procedures in NIST Handbook 133, Checking the Net Contents of Packaged Goods, as adopted by the National Conference on Weights and Measures.
• Other markets have their own rules. Some large wholesale buyers also require accurate weight checks and charge fees for inaccurately filled packs.

Before you talk to makers, write down the declared weights you pack, the markets you sell into and the rules for each. A maker can then set up reject limits and reports to match.

How accurate does it need to be, at what speed?

Accuracy and speed trade off against each other. The faster a pack crosses the weigh belt, the less time the machine has to take readings. Vibration from nearby machines, draughts from air-conditioning or refrigeration, and the pack bouncing or tilting on the belt all add noise. The belt length, the load cell type and the way the pack is triggered and timed on the belt all affect the result.

So there is no single accuracy figure worth comparing across makers. Instead:

1. Give each maker your pack sizes, weights, materials and line speeds.
2. Ask for the accuracy they will guarantee on your pack at your speed. Ask how the figure is expressed, and whether it comes from a test on real product.
3. Ask to see the test, or run it at the maker's test centre or on your own line.

Heavier and less stable packs, such as tall bottles or loose bags, are harder to weigh at speed than flat, rigid cartons. Plan for your most difficult product, not your easiest one.

How does the reject system work, and is it safe?

The reject device must remove every failed pack at line speed without damaging good packs or jamming. Common types are:

• An air blast, for light packs.
• A pusher or diverting arm, for heavier packs.
• A drop-down or lifting belt, for fragile packs that must slide gently into a bin.

Because a checkweigher often protects consumers as well as margins, look for fail-safe features. These include confirmation that the reject actually happened, a full-bin sensor and a lockable reject bin, so that failed packs cannot be put back on the line by hand. A combined checkweigher and metal detector should also stop the line safely if it cannot confirm a reject.

What should the data do for you?

Modern checkweighers store every weight and can send them to a plant computer. This data serves three purposes:

• Compliance: records for average-quantity systems and for auditors.
• Process control: trends in average weight and spread show when the filler upstream is drifting. Many checkweighers can send a feedback signal to the filler, such as a multihead weigher or auger filler, to adjust the fill automatically.
• Giveaway: reducing the spread of fill weights lets the average run closer to the declared weight while staying within the rules. Most of the saving comes from a steadier filler. The checkweigher is what shows you how steady it is.

Ask what reports come as standard, which plant systems the machine can connect to, and how long data is stored.

How is it calibrated and checked?

Makers describe two kinds of calibration. Static calibration uses a reference ("master") weight or product on a stationary belt. Dynamic calibration passes a reference product across the belt at running speed to fine-tune the in-motion reading. Ask how often each is needed, how long it takes, and whether the machine guides operators through it. Build routine performance checks into your quality plan, just as you would test a metal detector with test pieces at set intervals.

What about hygiene and the environment?

Checkweighers work in very different places, from dry bakeries to wet, chilled meat and dairy plants. For washdown areas, look for a full-washdown design with stainless steel construction and load cells rated for the cleaning regime. Tool-free belt removal speeds up cleaning and changeover. If the machine will sit in a cold room or near a freezer, ask how temperature and condensation affect weighing.

What drives the cost?

Compare quotes on the same scope. The main cost drivers are:

• Accuracy and speed: high-precision load cells and faster control systems cost more.
• Pack range: heavy, large or unstable packs need longer, wider or specially designed conveyors.
• Reject system and safety features: fail-safe options and lockable bins add cost but are often required by customers.
• Combination systems: integrated metal detection or X-ray on one frame.
• Hygiene rating: washdown construction costs more than a dry-area frame.
• Software and connectivity: data export, filler feedback and links to plant systems.
• Service: installation, calibration, training and spare parts.

A cheaper machine that cannot hold accuracy at your line speed may make you run the filler with a larger safety margin. That means more product given away on every pack.

What should you ask makers before you buy?

1. Which weight rules does your standard setup support for my markets, and what reports come with it?
2. What accuracy will you guarantee on my heaviest, lightest and least stable pack at my line speed?
3. Which reject device do you recommend for my packs, and which fail-safe features are included?
4. Can the machine send feedback to my filler, and which fillers have you connected to before?
5. How are static and dynamic calibration done, and how often?
6. Is it rated for my cleaning regime, and how long does a product changeover take?

Then compare checkweighers by maker on this site, where each specification links to the maker's own document. If you pack into film bags, our guide on choosing a form-fill-seal machine covers the machine upstream.