Batch production: weighing it up

As process control instruments proliferate in the plant, operators are turning to new technologies to provide a chain of traceability against national standards.

Whether you’re dealing with food, chemicals or pharmaceuticals, the ability to ensure that each batch is a precise match for your chosen recipe is critical.

And, while different industries present their own challenges in terms of hazards and cleaning regimes, there are some useful underlying principles that can guide the set-up of an ideal batching station.

One of the most basic decisions involves opting for a gain-in-weight (GIW) or loss-in-weight (LIW) approach, or some combination of the two.

Whether you’re dealing with food, chemicals or pharmaceuticals, the ability to ensure that each batch is a precise match for your chosen recipe is critical

For very small-scale production, GIW systems may be as simple as placing a batching vessel onto a scale platform and adding the ingredients one by one. Bulk ingredients would normally go in first, with ingredients that require more precision often being weighed separately on a finer scale and then added.

Even if ingredients are being added manually, complex recipes may be stored in associated software to keep operators on track and promote effective record keeping.

“A lot of recipe systems are bespoke systems for the user,” says Paul Lloyd, product manager for software and weight indicators with Avery Weigh-Tronix. “All ingredients must be lot controlled and recorded for traceability.”

According to Sharon Nowak, global business development manager of food and pharmaceuticals at Coperion K-Tron, GIW batching stations designed for larger batches and a more automated approach would typically comprise a hopper mounted on load cells (aka a scale hopper), receiving bulk solid components automatically via volumetric feeders such as screw feeders or wafer valves in a pneumatic delivery system.

The feed systems usually supply the ingredients rapidly until they hit around 90% of the target weight and then slow down to improve accuracy.

Scale model

This automated GIW set up requires ingredients to be added sequentially, which can prolong batching times, and achieves overall scale accuracies of around +/-0.5% of the full scale capacity.

This level of accuracy makes GIW more suitable for bulk ingredients than for minor ingredients, notes Nowak.

Alternatively a batching station might also include gravimetric feeding devices, such as screw, weigh belt or vibratory feeders, each mounted on their own load cells. These feeders deliver the product to the batching vessel by means of LIW feeding.

“Since all of the ingredients are being delivered at the same time [with LIW feeding], overall batch times are greatly reduced,” says Nowak.

“In addition, due to highly accurate load cells specifically sized for the individual ingredient batch weights, the accuracy of this type of system is much higher. This method is often used for more expensive micro ingredients due to the resultant batch accuracy.

“However, it should be noted that this system can also be more costly, due to the individual weighing devices required for each feeder.”

With these pros and cons in mind, she says that it’s often advisable for manufacturers to opt for a combination of GIW and LIW batching: “Where small amounts of micro ingredients are required for a total overall batch, both methods can be combined: LIW feeders for the micros and minors, and GIW batchers for the major ingredients.”

While establishing the right weighing arrangement up front is one thing, maintaining accuracy in the longer term requires careful monitoring to spot any deterioration

This is a frequently used set-up in extruded snacks and pharmaceutical manufacture, for example.

While establishing the right weighing arrangement up front is one thing, maintaining accuracy in the longer term requires careful monitoring to spot any deterioration in weighing performance over time.

The most common type of weighing technology in industry is the strain gauge load cell.

“A [strain gauge] load cell is a piece of metal that bends,” explains Lloyd.

“Over time it actually bends out of shape so you have to recalibrate at least once a year to keep the accuracy correct.”

He also cautions that weighing machinery performance can deteriorate much more rapidly if the equipment is abused by overloading or shocking it.

Suppliers have therefore been developing systems that are as robust as possible. Avery Weigh-Tronix's latest weighing platforms have massive overload capacity built in – 800% in the case of the company’s ZQ 375 checkweigher, for example – plus shock absorption. “Older weighing equipment just wouldn’t take the abuse,” says Lloyd.

Spot the culprit

But even equipment that isn’t subject to harsh treatment can deteriorate over time.

Stephen Brown, product technical support specialist with Mettler Toledo UK’s industrial weighing division, says: “Load cells are built for a given number of weighing cycles. You’d expect to replace them after a few years of use.”

Many industrial systems rely on three or four load cells, combining their signals in a junction box and sending that to the weighing terminal, where it is converted into a digital version.

But this set-up can make it difficult to spot the culprit if one of the load cells starts to produce unreliable readings.

“If one of three or four load cells starts to fail it’s not always obvious where the problem lies. You’ll be getting the incorrect weight but you won’t know why,” explains Brown.

Mettler Toledo’s IND780 weighing terminal includes TraxEMT (Embedded Maintenance Technician), which is a predictive maintenance feature equipped with algorithms that can identify when the response of a load cell starts to deteriorate.

It’s used in combination with a smart junction box that converts the signals into a digital format before combining them, allowing them to be more easily monitored.

What’s more, since even worn load cells tend to behave in predictable ways, the TraxEMT system’s ‘Run Flat’ algorithm (named after the ability of a car to run a short distance on a flat tyre) can correct for the resulting drift.

Some applications are too safety- or quality-critical to take advantage of Run Flat, but in many applications it makes it possible to defer repairing or replacing the worn cell until it’s convenient to do so, minimising downtime, and enabling users to complete a batch once a worn load cell is detected, rather than having to throw it away.

“These algorithms can predict a load cell’s declining performance,” says Brown.

“The system can then correct for it to keep the weight within tolerance because it knows how to compensate.”

“Vibrating wire weight sensors exploit the well understood relationship between a wire’s tension and its resonant frequency

Sharon Nowak, business development manager, Coperion K-Tron

Meanwhile, Nowak advises that alternative load cell technologies can also help to minimise the problem of long-term drift in the weighing signal.

She highlights vibrating wire technology such as the Coperion K-Tron Smart Force Transducer range as suitable for gravimetric feeding and batching applications, as its digital design needs no calibration.

“Vibrating wire weight sensors exploit the well understood relationship between a wire’s tension and its resonant frequency. Today, the technology is used in high-performance all-digital weighing applications where the load to be measured directly governs the tension of the wire, and the resulting frequency of vibration is detected and evaluated to obtain the weighment,” she says.

This technology can maintain extremely accurate results over time since it is very resistant to drift.

The smart circuitry associated with the transducer can also distinguish effectively between the weigh signal itself and any interference from ambient vibration and shock, says Nowak.