Getting to the Core

Moisture analysis of raw materials, in-process or semi-finished materials, and final products is among the most common analytical methods used in manufacturing processes. However, until now, this analysis has been usually employed for spot checks only.

Whether its food products, building materials, chemical or pharmaceutical products, knowing the moisture content in raw materials, semi-finished products and final products is a decisive economic factor for production and commerce. The moisture content in a semi-finished product or a final product for example, determines its processing capability as well as shelf-life, and often enough has a decisive effect on quality. If certain raw materials are too moist, they can stick and form lumps or may deteriorate prematurely during storage. If they are too dry, they may not mix easily or be further processed mechanically.

Also, the price of products or raw materials is frequently determined by weight, which in turn depends on the moisture content of the materials. For example, the maximum permissible water content in roasted coffee is limited by legal regulations to 5 g per 100 g in some countries. This is to protect the consumers from paying for moisture instead of the actual product.

Staying within the limit values as precisely as possible has an economic advantage for the manufacturer. Many products, such as cereals, can be stored only if their moisture content is below a specific limit. Otherwise, they have to be dried to remove excessive moisture, a process which entails high energy costs and other expenses.

The intrinsic water content also has an influence on the quality and storage life of medicinal products, as well as the release of their active components. The list of applications for moisture analysis equipment is as varied as is the range of industrial products offered. Since the American Food and Drug Administration (FDA) issued its PAT Guidelines, on-line moisture analysis methods have become more important than ever.



Direct methods
Moisture analysis can be sub-classified into direct and indirect methods. In the case of the direct method, the value measured is directly linked to moisture and does not require calibration. The classic representative of the direct method is the oven drying method; this, however, involves analysis times of up to 24 hours. In this procedure, a sample is dried and then weighed to determine its loss in mass.

Thermo-gravimetric moisture analyzers operate faster than the drying oven. A heater that emits infrared rays or microwaves warms a sample directly on the weighing system or balance. These analyzers can detect the sample’s decline in weight in the drying process. Other direct methods are analytical methods such as the Karl-Fischer titration, the calcium carbide and the phosphorous pentoxide methods, which are more complicated than the thermo-gravimetric method.

All direct methods mentioned use relatively small quantities of sample, destroy or alter a sample as it undergoes moisture analysis and require analysis times of several minutes or more. For this reason, these methods are suitable only for spot-check measurements, while rendering continuous process monitoring or even process control impossible.

On-line analysis
Indirect methods overcome the shortcomings described above, although these methods require calibration. Spectroscopic methods have proven to be highly promising in moisture analysis. Electromagnetic waves interact with the sample during analysis. A well-known possibility is near-infrared (NIR) absorption spectroscopy, in which a sensor head directs NIR beams on to the product at hand. The reflected radiation is then analyzed. Moisture in the product is determined on line by evaluating a specific range of the spectrum detected, the so-called “water band.”

There are, however, several factors which may affect the spectrum near the water band and interfere with the result. Another disadvantage of this method is the low penetration depth at which NIR rays analyze the sample. Hence, bulk material with a small grain size can be examined for moisture only on its visible surface.

However, if a product is to be analyzed completely by “penetrating” down to its core moisture so to speak, long-wave electromagnetic radiation is more suitable than NIR, because microwaves can penetrate non-metallic materials. In the frequency range of about 2 to 3 GHz relevant for moisture analysis, this is easily accomplished to a depth of several centimeters.



In-depth analysis with microwaves
On account of their size and their dipole properties, water molecules can follow the alternating field of these frequencies by dipolar alignment, absorbing energy and changing the electromagnetic field with their dielectric properties.

The measuring principle of the microwave resonance method is based on this attenuation and alteration of the electromagnetic field. Beyond the surface of a planar sensor, a weak microwave field is generated. The resonance frequency of the sensor system is analyzed by continuous scanning of the microwave frequency.

If a product containing water is now passed over the sensor, the resonance frequency shifts and the amplitude is attenuated. Both the attenuation and the resonance frequency shift increase as the quantity of water rises. This is also obviously the case when the bulk density of the product to be measured increases.

The ratio of frequency shift and attenuation is a density independent measure of the water content and therefore key to moisture analysis. This ratio constitutes the microwave moisture value, which represents the total moisture i.e., surface and core moisture, allowing the moisture content of whole coffee beans for instance to be measured. Unlike the microwave oven, the heat increase for the product to be analyzed is not relevant as the output in the measuring field at less than 10 mW is far below the transmitting power of modern cell phones (1 to 2 W).



The construction of the planar resonators with their closed flux lines prevents an emission of microwaves in this measuring method, which results in good electromagnetic compatibility (EMC). Because the field changes instantaneously when the product to be analyzed enters the measuring field, the analysis speed depends only on the frequency of the scan. The Sartorius PMD300PA series enables frequencies of up to 40 measurements per second, even providing averaged results in less than one second.

Calibration accuracy
As the microwave resonance method is an indirect moisture analysis method, calibration is required. The microwave moisture value has to be uniquely correlated to material moisture. In this calibration process, the sensor system is used to measuring samples of different moisture content. This is followed by measuring the moisture content using a direct method, such as the oven drying method, thermo-gravimetric moisture analysis or Karl-Fischer titration. This calibration process—sampling and subsequent reference analysis—can be carried out directly in the process.

Correlating the microwave moisture values obtained with the corresponding absolute material moisture measured constitutes calibration of the measuring system. In this way, accuracies down to 0.1 percent moisture can be achieved. Experience has shown that these calibrations remain stable for a long time.
In the new Sartorius PMD300PA series, the individual values measured are averaged over a freely definable time, and subsequently a corresponding interface, such as Profibus, Ethernet or analog output is used to transmit the values to a PC for evaluation, a control panel or to a process logic controller (PLC). The advantages of the microwave resonance method lie in its extremely short analysis time and non-destructive mode of measurement; also the raw material analyzed can be processed directly.

Moisture analysis measures both core and surface moisture. A number of different sensors are available for this task. As the selection of the suitable sensors depends on the moisture content, the characteristics of the product and the process conditions, the moisture analysis system can be individually and ideally adapted to the process.



Depending on the type of sensor, the measuring range is between 0.1 and 60 percent of moisture. The sensors operate in a temperature range of up to 80 deg C. For this reason, they are also suitable for cleaning-in-place procedures (CIP). The uses of the PMD300PA on-line moisture analyzer are nearly unlimited. Based on its compact design and high international protection rating (IP65), all available sensors are suitable for use in the food processing, chemical and pharmaceutical industries. The PMD300PA’s highly sensitive planar sensors with their special ceramic surface show convincing advantages, for example, when used on conveyor belts, plate vibrators or in hoppers.

Bypass sensors are employed for free-flowing products that are conveyed through piping. In addition to measuring moisture, these sensors optionally determine the bulk density of a product.

Delays down
Especially in drying processes, it is indispensable to obtain the accurate moisture content instantaneously for use as a control parameter. Very frequently, a defined moisture content has to be achieved at a certain production stage before the process can be continued (batch processing).

Until now, production employees have had to wait for results from the laboratory. Particularly for production capacities of several tons per hour, time is of essence in providing analytical results. A delay of a quarter of an hour or more until the results arrive from the laboratory may result in off-spec production of several tonnes, adding up to damages of several thousand euros.

Based on continuous on-line analysis and control using a microwave moisture analyzer in the baking and drying processes, ideal drying and baking conditions can be adjusted and maintained without any delays. Moisture analysis results allow the temperature inside the oven, the air feed and the conveyor speed to be adapted according to the respective moisture content of a product. This not only saves valuable energy resources, but also increases process reliability and efficiency. At any time during the process, the user has control over the moisture content and can dispense entirely with complicated and time-intensive laboratory analyses.

Another application for the microwave resonance method is complete analysis and documentation of the incoming inspection of raw materials. Instead of individual spot checks, the complete batch is continuously monitored. This permanent control is part of the HACCP concept and also meets the requirements of the International Food Standard IFS V.5.

Reduce process costs
Thanks to continuous and precise measurement of the moisture content and the resulting increase in process reliability as well as the decrease in process costs, on-line moisture analyzers pay for themselves within a very short time. Irrespective of the industry in which they are used, they are the basis for an efficient and a transparent manufacturing process.