Defying gravity
A new weight-measurement technology addresses some of the shortcomings associated with traditional systems.
By Arek Druzdzel and Arne Wieneke
The production of specialty filters and next-generation tobacco products requires precise measurements to ensure the quantities of raw materials applied are consistent with the brand formulations. Product quality parameters must mean the same to everyone in the production process. They must also comply with industry quality standards and local government regulations. Traditional scales with load sensors have limitations, however. This article will discuss the benefits of novel, contactless measurement systems.
Traditional methods of weight measurement are based on comparisons with accepted standards. Over the past 200 years, the kilogram has become such a standard. In the International System of Units, it is defined as a mass standard and is used as a base for weight measurements worldwide. Unlike other base units in the system, the kilogram is defined as an artifact: a chunk of metal, for comparison and reference, which is stored in Sevres, France.
For single measurements under laboratory conditions, using a standard mass in calibration procedures on load cells is sufficiently precise and repeatable. However, load cells require frequent maintenance and calibration, which is a disadvantage when fast and accurate measures of single milligrams and micrograms are required during the production process. Under such conditions, correct adherence to regulations and production targets might not be ensured at the same time.
For starters, scales with load cells require adjustment to the geographical location; otherwise the weight is measured incorrectly. This is because scales measure weight, which is then translated into mass, taking into account the local gravity force.
The mass-reading error is caused by variation in the gravitational acceleration and the resulting gravity force (weight), which varies around the world. For an object of a given constant mass, its weight depends on both latitude and altitude of the location of the measurement device. Diagram 1 shows the variation in gravitational acceleration around the world, at a constant altitude of 100 meters above sea level.
The gravitational acceleration at the equator amounts to approximately 9.78 meters per second squared (m/s2 ), while at the poles it is approximately 9.832 m/s2, resulting in a discrepancy of 0.052 m/s2, or 0.53 percent.
Additionally, gravitational acceleration is affected by altitude, the tilt of earth’s rotational axis, precession, equatorial bulge, etc. Gravity-related effects apply when, for example, calibrating weight measurement devices to a mass standard. The greater accuracy required, the more time and effort are needed for scale calibration and measurement.
Furthermore, measurement precision and accuracy of scales and load cells change with time from the last calibration, as they depend on elastic properties of materials in load cells, environmental conditions and other components of a weighing system.
This discrepancy causes variation in weight readings when an object of a given mass is measured at different latitudes and altitudes. Scales compensate for this error by providing reference masses for pre-calibration. Evidentially, this calibration becomes critical when quickly measuring small masses—e.g., in the milligrams range—requiring more frequent calibration to ensure reliable measurement in line with regulations.
Moving away from the laboratory environment to the production environment, more factors start influencing the weight measurement of small masses. Machines vibrate, causing slow measurements and/or potentially incorrect readings; potent products require contained handling inside closed chambers, which in turn require controlled ventilation and frequent cleaning; products may vary in water content during processing where the dry weight is in focus. Accumulation of these factors limits the useable range of accuracy of load cells. Sometimes they even make it impossible to measure small masses accurately and quickly.
Furthermore, in the next-generation products primary department, substances are added and/or mixed in a way that does not permit the on-line monitoring of those processes. It happens mainly because the process yields a variety of quality-related issues, depending on the product’s dry mass and moisture content, its morphology, and whether the process is closed or continuous.
Such applications may prevent the use of load cells, leaving manufacturers only the options of off-line sampling and indirect weight estimation. These issues become even more challenging in the secondary department, where cartomizers, atomizers and other small containers are filled with novelty products, frequently in a pre-defined sequence. The off-line monitoring of small weights is not desirable, because it represents only products sampling, leaving thousands of product units uncontrolled.
Removing gravity and ambiance from the equation
Overcoming the described challenges requires a time-stable and gravity-independent measurement system, capable of measuring the mass of objects on-line—i.e., the amount of substance instead of weight of fast-moving objects, such as vials, capsules, liquids or powder. Such measurements would be identical around the world and independent of the external factors, allowing not only for tight and on-line monitoring of substances but also for direct data comparison.
Over the past years, Aiger Group has developed such a novel system and successfully installed it in a number of factories around the world. The system emits a local energy field, which is alternated as substances pass through it. Once measured, the signal can be instantly converted to mass or the local weight units. Knowing exactly the quantity of the substance dosed—that is, the mass of an object—the dosing-measuring combination is automatically calibrated to the conditions of the measuring device’s location, without the risk of overdosing or underdosing due to the earlier described factors.
As only the substance changes the output signal, the signal is ambience-independent and allows for positive, on-line process control. Also, the sensor signal data processing is fast enough for closed-loop control.
Aiger has built an industrial version of the described system and verified it with a variety of small objects, including pharmaceutical capsules in the 150 mg to 700 mg range, as well as with the micro-dosing of powders from 1 mg to 100 mg into various types of containers. The system has a proven stable precision and accuracy within one sigma ranging from 0.25 percent to 3 percent, the dispersion depending mainly on materials structure and morphology.
Most important for production environments, the proposed system ensures quick, precise and accurate mass measurement of a wide range of objects, with no need for major system adjustment, special environmental conditions, leveling, isolation from vibration and ventilation, or prolonged measuring time. The system has no moving or flexing elements, hence it is free from the disadvantages associated with load cells.
Aiger’s new technology is developed for the production of next-generation products that require compliance with pharmaceutical regulations. Not only does it ensure 100 percent product control by a gravity-independent weight measurement system, but it also documents all values for in-detail analysis. Equally important, the technology also helps maintain brand consistency.
Arek Druzdzel is business development director at Aiger Engineering. Arne Wieneke is business development manager at Aiger.
