The Principle of Weighing Technology for the Fully Automated Filling of Liquids
In many industries, packaging processes are automated. There are numerous different methods for making sure that precisely the right quantity of a product is in its packaging at the end of the process: piece goods are sorted, bulk materials are metered, and liquids are bottled. The challenge for all these processes is the same: the packaging must contain the stated quantity of product. If the fill quantity in bottles is too low, for example, this contravenes EU Prepackaging Directive 76/211/EEC and German Pre-packaged Product Act (§22 FPackV). These stipulate the percentage by which a production batch may fall short of the specified minimum fill quantity. Here, "misconduct" can lead to penalties that include the shutdown of production.
Therefore, during production manufacturers often exceed the minimum quantity in packaging. However, such safety overfilling can be costly. A Chinese milk producer fills around 100,000 bottles of coconut milk per day. By reducing the overfill in individual bottles by a few grams using new, precise weighing technology, the company saved several million euros a year. Manufacturers of complete bottling plants should therefore offer systems that measure the fill quantities with high precision and keep overfilling to an absolute minimum.
Four methods for measuring fill quantities of liquids
Different methods of measuring fill quantities have been developed specially for liquids. Not every technique is suitable for all liquids – and there are also considerable differences where precision is concerned. However, all production processes have some things in common: liquids never flow through the pipes of the bottling machine at a uniform pressure. Density can fluctuate due to air bubbles in the liquid and changes in the temperature or recipe. Therefore, the correct fill quantity cannot be determined simply on the basis of time.
There are four common methods for measuring fill quantities. These methods differ in cost, accuracy, speed and hygiene. Not all methods are suitable for all liquids, for beverages need particular hygiene, expensive cosmetics require special precision, while engine oil has a different flow characteristic to, say, mineral water. The four methods introduced in this article are:
Gravimetric measurement using a load cell
Measurement using a level sensor
Mass flow measurement
First Method: Gravimetric filling using a load cell
In the gravimetric measurement method, or filling by weight, a digital or analog load cell measures the weight of the empty bottle and the fill content. The load cell features strain gauges for this purpose, which precisely detect the change in weight during the filling process. The way the strain gauge functions is explained in more detail in our HBM article "How does a load cell work?”. The advantage of digital load cells is especially simple communication with a PLC via digital I/Os. But analog load cells can also be digitized quickly and easily using a PAD amplifier.
To fill a bottle, the filling machine first positions it on a platform or suspends it by the neck from a fork. At the same time, the filling machine weighs the empty bottle. While the machine fills the bottle, the load cell measures the change in overall weight. Once the predefined overall weight has been reached, the filling process automatically stops and the bottle is conveyed to the capping station. In a rotary filler with 20 to 80 filling heads, this process takes around five to eight seconds for a typical 1-liter bottle. Not only is filling by weight highly precise, it comes with further enormous advantages.
Advantages of filling by weight
- The load cell weighs each bottle before filling commences. Broken bottles are lighter than the permitted minimum weight, so these bottles can be detected easily and removed immediately.
- If a bottle exceeds the permitted weight, this may indicate the presence of cleaning agent and disinfectant residues. The filling machine immediately removes these bottles, too, to prevent any risk to health.
- If the overall weight of a bottle does not increase as quickly as stipulated during filling, this indicates a leak from which liquid is escaping. The machine stops the filling process to prevent contamination by the product. Since cleaning is expensive and time-consuming, the load cell helps to maintain high system availability.
Simple load cell programming and calibration
Filling by weight is a process with high-precision results: the content stated on the packaging can be accurately filled down to the gram. Furthermore, load cells are very easy to calibrate with calibration weights – like a kitchen scale. This guarantees that the bottle does actually contain the 1,000 grams shown. As well as producers, machine construction firms also benefit from the advantages of digital weighing technology, because programming the filling algorithm in the machine control system is a real challenge. Obtaining the correct ratio of flow rate to filling process is extremely complex, and requires knowledge of filters, control technology, settling times and response times. A digital load cell from HBM considerably facilitates this setting process, because it comes with an integrated filling algorithm that reflects decades of experience in filling by weight. The algorithm can be adapted with ease using free PanelX configuration software. The load cells can run entirely as stand-alone units, communicate with the machine control system via I/Os or fieldbuses, or continuously supply the control system with filtered measured values, if the company wishes to use its own filling algorithms.
Filling by weight is especially suitable for the food industry, as the sensor does not come into contact with the product. All HBM load cells have the maximum possible degree of protection IP68/69K, and can therefore withstand continuous submersion and pressure washing. Some load cells, such as the PW27, are certified to EHEDG, for particularly stringent hygiene requirements. Bacteria from protein or sugar, for instance, are unable to breed on the electropolished, completely rounded contact surfaces of these load cells. They are therefore also suitable for the latest packaging trend: aseptic cold filling without pasteurization and without the addition of preservatives.
Second Method: Measurement with a level sensor
The level sensor method focuses on the level of liquid in the bottle. Here, a level sensor is inserted into the bottle opening and the bottle is filled until the liquid reaches the sensor. Therefore, only electrically conductive products – i.e. liquids with a minimum salt content – are suitable for this process. Plant and mineral oils, for example, contain too little salt. The level sensor method is the least accurate measurement technique, with the most scattered results for determining the fill quantity, because the container volume fluctuates dramatically, particularly in the case of glass bottles. Consequently, level sensors are used almost exclusively for cheap, conductive products. Another major disadvantage is that the sensor comes into contact with the product. It therefore carries a little of the product from one bottle to another, and as a result is unsuitable for hygienic bottling.
Fill level measurement with a level sensor is ideal for isobaric filling, when identical pressure conditions – e.g. three bars for bottling beer – have to be maintained in both the pipe and the bottle. Carbonated liquids require this kind of environment, as otherwise they will lose carbon dioxide. Moreover, this method enables a visually identical fill level to be achieved when bottling crates of beverages – an important criterion to customers, particularly for beer or mineral water.
Third Method: Volume measurement
Volume measurement detects the quantity of liquid flowing through a filling valve. Here, the volume of liquid in the bottle is measured magneto-inductively. A magnetic field splits the ions of the liquid flowing through, producing a voltage at the measurement electrodes. This voltage can be measured, enabling the volumetric flow rate to be calculated. This procedure is only suitable for conductive liquids. In addition, the filling machine has to be individually calibrated for each liquid, because each product contains different numbers of ions.
Fourth Method: Mass flow measurement
Mass flow measurement uses the principle of the Coriolis force. Here, the liquid flows into the bottle through two vibrating pipes, and the Coriolis force acting on the pair of pipes generates a phase shift in these vibrations. This enables the mass of liquid that has flowed through the pipes to be calculated. This method is also suitable for non-conductive liquids. However, it is extremely expensive to purchase, because it requires time-consuming calibration during sensor production.
Comparing the different filling methods
In the end, the chosen method depends on the value and properties of the product being filled. What's more, the various methods differ considerably in precision and filling speed.
Gravimetric filling using weighing technology is much more precise than all the other measuring and filling methods. The level sensor method has a standard deviation of around two to five percent of the filling weight in glass bottles, while it is 0.5 to one percent for the volumetric method. 0.2 percent is the best that can be achieved with the mass flow method. Filling with weighing technology enables a standard deviation of around 0.1 of the filling weight to be achieved.
The various measurement methods achieve different filling speeds. When filling with a level sensor, a bottle can be filled in two to four seconds. With weighing technology or mass flow measurement, it takes around five seconds to fill a bottle. Flow rate measurement suffers the largest fluctuation in filling speed, with the time varying between two and five seconds.
Numerous product properties make filling by weight the ideal filling method
Filling by weight is a procedure that is suitable for virtually any liquid, regardless of conductivity, solids content and flow velocity. Exceptionally precise content quantities can be achieved with filling by weight. Costly safety overfilling can therefore be reduced to a minimum, enabling considerable savings, especially with high-quality products such as cosmetics and oils. For products with properties that make bottling more difficult, such as those with high viscosity or a high solid content, gravimetric measurement with a load cell ensures much greater certainty of results than the other measurement methods.
The only real choice for manufacturers wishing to respond to rising demand for aseptic fillers is between mass flow measurement and gravimetric filling by weight. In a direct comparison, the gravimetric measurement principle offers decisive advantages: due to its greater hygiene, it is the simplest way to achieve aseptic cold filling without pasteurization. What's more, filling by weight is much cheaper and more precise than mass flow measurement. Therefore, for manufacturers of bottling plants, gravimetric filling by weight is the ideal method for developing competitive filler systems.