Process automation and optimization with online moisture measurement

Moisture measurement systems for the determination and control of the moisture content in all types of bulk solids are already used in the following industrial sectors: concrete, glass, ceramics, gypsum, nutrition, brick and tile and chemical industries.

Some examples of the measured materials are: sand, silica sand, lime, gravel, ore, sewage sludge, cereals, concrete aggregates, sugar, rice, dairy products, chemicals, gypsum, washing powder, silicates, in short terms: just about everything in which moisture influences quality and further processing and incurs follow-on costs (dryer, water, energy, weight, etc.).

An in extensive material list, you can receive here.

In the field useful on-line moisture measurement.
During the years moisture measurements have drastically improved, for the necessity of industry and handwork , and they are required for the high technical standards.

The moisture measurement sensor, type (D)MMS can be connected to any existing or new control system, SPS or PC with an input of 0/4-20 mA or 0 - 10VDC.
The principle is based on the high-frequency dielectric moisture measurement method (HFC principle), for the determination and controlling of the product moisture in a wide range of materials.

In many companies cost savings had been achieved between 20 and 40% due to the intervention of moisture measurement systems. The earn back time of one week is very common in some industries. 

...of this moisture measurement system is to be expected:

• Due to permanent (on-line) moisture measuring, the quality will improve, the production line will be continuously monitored and there will be a time saving in the laboratory for regular sample taking and it will prevent bad production charges getting trough.
• Processes can be monitored and controlled by the moisture content, f.i. pelletizing, for reproducible switch-off points, end-points and it will be possible to influence the moisture content by the control program.
• Recipes in all possible ranges and mixings can be produced reproducibly, controlled and corrected by the moisture content of each separate component.
• Due to the control of the moisture content, money savings can be expected in dryer processes and by water uses.
• A moisture protocol can be set up and the data can be stored for process validation
• By the moisturizing to set-point, losses by dry-out during processing can be compensated in the process (sowing-seed production)

Every material has a dielectric constant or relative permittivity, which is important for the principle of measurement (high frequency, dielectric) applied by ACO. For better understanding of the principle of operation, a very simplified explanation follows:

Water has a relative permittivity εr of around 80, most other materials an εr of around 1 to 10. The εr of sand, for example, lies between 3 and 4. This means that there is a large measurable difference between the relative permittivity εr of water (80) and that of the material to be measured (1 to 10).

This difference can be measured and correlated to a direct moisture value. The moisture is then the output in the form of a standard signal (0-10VDC or 0-20 mA) as a value for the moisture content in mass percent.

In other words: the more water or moisture is contained in the material, the closer the value of its relative permittivity εr is to 80.

The online measurement is a real-time measurement. The material will pass the sensor, or vice versa, and the measurement signal is available instantly - even when fast-flowing products are measured. The measurement of solid bodies is also possible.

The analogue output measurement signal of the moisture measurement sensor of 0/2...10 VDC or 0/4...20 mA can be processed directly in a process sequence and connected to a control, PC or PLC system.

Also moisture measurement through other (non-conductive) materials e.g. ceramic, glass, Plexiglas or acryl glass or Teflon, is no problem for the ACO moisture sensor.

Depending on the material and its properties, the measuring probe reaches a measurement depth of around 100 mm till 150 mm into the material. The total product moisture, i.e. the core moisture as well as the surface moisture of the material, will be analysed. On account of this high penetration depth, soiling and minor deposits on the measurement surface are insignificant.

The very rugged sensors are made of high-graded stainless steel. They are designed for the toughest applications and can be retrofitted in existing plants with limited mechanical effort.
The measurement is independent of the colour of the material, the minerals it contains, the pH value and salt additions.
A solid casting of the probes guarantees impact strength and water resistance as well as exceptional operating reliability even when exposed to strong vibrations.

Many options and additional products are available: like f.i. temperature measurement; measurement of the wear-out of the ceramic; highly wear-out resistant ceramic, Teflon or rubber measuring surface as well as special mounting conditions for the different applications.

The use of moisture sensors in most bulk goods is simple and without problems.
A one-time configuration of the product , while empty, is obliged.

The out-put signal of the moisture sensor will be configured on the wished start and end value (f.i. 2% moisture = 0 VDC and 18% moisture is 10 VDC)

Due to the high reproducibility, the sensor is commonly used for controlling production differences. There will be, like with a difference monitoring, a value configured at the right output value and at the wrong output value (f.i. at the right output value 7 VDC or 15 mA). If the measured value differs from the configured output signal, the SPS or PC will take over the control, f.i. with control tasks or  corresponding alarm settings etc.

The modular design of the entire moisture measurement system makes it a complete, flexible and user-oriented modular system. From this, a selection is made to meet the specific requirements of the application; in a straightforward case, the solution may be a single sensor for direct connection to a

PLC, or alternatively a ready-to-use pre-wired PID control system with integrated averaging function (plug and play) may be necessary.

Extensive accessories make the product range complete, e.g.: laboratory devices, evaluation instruments, self-optimizing process controllers with RS 485 interface, calibration curve memory

for 100 different materials, analogue and digital displays, microprocessor-controlled averages, high-temperature sensors for up to 200° C and mechanical accessories such as sleds and different flanges.

This item earns a special signification. The best sensor produces nothing when installed on the wrong application. In practice , misplacement of sensors accounts for 85% of measurement problems!

Some examples of typical applications for moisture sensors are in the bin discharge slope, on transfer plates in belt conveyor systems, mounted on a sled on belt conveyors, in screw conveyors, in bypass and glide plates.

Discharges and grids of extruders and in the collecting chamber of eddy current dryers are other options for installation. These are just a few examples for the application of the sensor. For installation, only a hole for the sensor (approx. 76 mm in diameter) and 3 bore holes for the 6-mm screw fastening are require.