Ounces of savings, pounds of profit

Sophisticated software may enable instant adjustments when a line changes over to run another product. Once all critical control measures for a product have been set and registered, they can be called up instantly, even if the product has not run for two months.

“With the programming capability of our equipment, changeover is usually done seamlessly,” says Loma’s Wilson. “With Ethernet compatibility, you can also tie directly into the data management system of the company. You can change from product A to product B from the manager’s office automatically.”

Hostile environment

Getting accurate weight measure for a package traveling on a high-speed packaging line is, indeed, challenging. Packages moving on and off a scale at rates of hundreds per minute can’t be measured with the consistent accuracy of a single package on a static scale. The speed of a moving package offers little time for a weigh cell to settle before the next package arrives.

The food environment poses natural difficulties to accurate measures with dramatic temperature fluctuations, product mess and dust. High-pressure water in washdown applications poses another serious threat to sensitive equipment.

The strain gauge on the load cell must be protected against moisture and contaminants. The checkweigh area must be clean. Foreign matter and loose product can offset the tare settings. Buildup on platforms and conveyors will compel the scale to re-zero repeatedly. With the sensitivity of the current generation of checkweighers, even drafty air can affect accuracy.

Vibration from a conveyor, hopper or other equipment brings background noise that can interfere with weighing accuracy. The latest generation of equipment attempts to filter out or isolate the scale from such interference.

Loaded for accuracy

The heart of the checkweigher is its load cell, which converts weight or force into electrical signals that actuate measuring. Pharmaceutical applications have demanded greater accuracy than the food industry historically, but profit pressures have turned food processors to reduce waste and giveaway, putting a higher premium on accurate measure.

“We went to a digital load cell over two years ago,” says Dinh of All-Fill. “It has fewer electronic components. Compared to the analog cell, it reduces electrical noise. You are using the same strain gauge load cell, but now everything is done inside the load cell to isolate interference.”

“Most checkweighers are equipped with analog load cells,” says Loma’s Wilson. “Our new AS checkweigher line’s state-of-the-art load cell has nine digital filters that remove background static.”

Promising greater accuracy at faster pack rates, Loma’s FIT (Fast Intelligent Transducer) load cell, produced by sister company HBM for the AS line, features “high torsion and bending stiffness” with outputs of up to 600 measurements per second.

The AS line is also designed to minimize interference from vibrations from nearby machines or the conveyor belt or from air pressure surges. The FIT load cell, unlike analog cells, does not need to compensate for temperature.
Manufacturers of highly accurate checkweighers argue the long-term advantages of accuracy outweigh the cost (or diminished flexibility) of higher-end equipment. Savings from reduced giveaway, waste and rerun accelerate return on investment.

“It’s just like plasma TV,” says Wilson. “People looking to buy checkweighers in the future will want digital load cell technology simply because of the range of features it offers.”

Sidebar: Uses of Checkweighers
Check packages for underweight or overweight filling
Guarantee net contents compliance
Validate the presence of components or package
Check mixes against weight limits
Reduce waste or giveaway by making accurate filler adjustments
Classify product by weight or contents
Helping to fulfill reporting requirements
Measuring and recording the efficiency of a production line
Assuring compliance with customer, agency, regulatory, or other specifications