Feature

Monitoring the moisture in the raw mate-rials, controlling the water addition dur-ing the mixing/blending process and final-

ly checking the moisture of the materials in pel-letised form, not only ensure an end product with

consistent quality, but will enable substantial sav-ings to be achieved in the production process.

Hydronix, who originally pioneered the use of the microwave measurement technique for the concrete industry, have developed a range of digital microwave sensors that are particularly well suited to measuring in animal feed materials.

In selecting a moisture sensor or system then there are five important criteria to be considered:

• The physical aspects of the measurement technique are inherently accurate and suitable for process and materials concerned

• The design of the sensors should enable them to be installed at the appropriate positions where it is necessary to measure the moisture. The ease of installation is clearly another factor to be taken into account

• The sensors need to interface easily with the control system

• The sensors or the moisture control system must be easy to operate and in particular must have an easy yet accurate calibration procedure

• As may be anticipated the sensor should be rugged, reliable and cost effective

Accuracy – why use a microwave? Compared with other measurement techniques, microwave

offers the best solution. The resistive techniques that attempt to relate the moisture to the electrical resistance or conductance of

Digital microwave sensors for the feed industry

by Rolf Laffan, Chairman, Hydronix Limited

Figure 2: The Hydro-Probe II

Figure 1: The water molecule

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Feature

the material are cheap and cheer-ful but fail to provide a reliable measurement as, for instance, rain water/distilled water does not conduct electricity whereas sea water certainly does.

So this technique is severe-ly affected by pollutants in the water.

Capacitive techniques, which are more widely used, are a

definite improvement, but nevertheless are still similarly affected by contaminants in the water.

Infrared systems again are quite widely used but being a reflective technique only provide a surface measurement which will not be representative of the total moisture in the product.

Nuclear techniques offer an accurate measurement technique, but are generally not practical as they need a relatively large volume of material in which to measure, they are not very user friendly and are relatively expen-sive.

The microwave technique, as pioneered by Hydronix, provides a sound basis for measurement. The principal depends on the dipolar structure of the water molecule (Figure 1) that will try to align itself with the magnetic field to which it is subjected, in a similar way to a compass needle. When subjected to a rapidly changing electro-magnetic field, such as in a microwave oven, the rapidly oscillating water molecule will expend energy which then heats the food.

Thus, the interaction of the water molecule with a rap-idly varying electromagnetic field provides a reliable basis for the measurement of water present in a material.

Selecting a suitable frequency is important compro-mise. The higher the frequency of the EM field the less the measurement is affected by extraneous impurities, but

so also is the penetration of measure-ment. Hydronix has selected a frequency that will generally (depending on the properties of the material) penetrate by approximately 100mm.

Hydronix are the only manufactur-ers to have adopted digital techniques that not only provide a greater accuracy and range of measurement compared to analogue techniques, but also greatly assist remote communication with the sensors.

The sensors radiate an extremely low-powered electromagnetic micro-wave field into the material (significantly less than a mobile phone) which detects the resonant frequency of the system. Due to the dipolar effect of the water molecule, the resonant frequency chang-es with the changing water content. These changes are measured in ‘unscaled units’ that are then scaled by a process of calibration to provide a precise readout of the moisture present. For standard

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Figure 4: The Hydro-Mix V

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Featureapplications default calibrations are written into the sensor.

The sensors are internally compensated for changes in temperature, and also have a fast response ‘external’ temperature sensor to provide a readout of the temperature of the material if required.

How and where may the sensors be fitted?

The sensors enable online measurements to be taken at various situations in the production process.

The Hydro-Probe II (Figure 2) has been designed to measure in flowing materials in the neck of bins, material in free fall and material on conveyor belts. Figure 3 shows a Hydro-Probe II adapted to measure in the pellets on a conveyor belt. The method of mounting is simple and basic.

The Hydro-Mix V sensor (Figure 4) has been designed for applications were it is necessary to monitor the moisture during the water addition process and is flush mounted to the side of the mixer. This sensor may also be used to measure material flowing in a chute. Again the mounting of the sensor is simple but will entail cutting a hole in the mixer or chute.

Interfacing sensors with control systems

The sensors are wired back to the control system with appropriate screened cable. Analogue, digital or both outputs may be selected.

The sensors, using a flash eprom, may be remotely interro-gated from a laptop or the host com-puter. The internal parameter settings may be changed and software upgraded using the Hydro-Com communication and diagnostic software package available for free download from: www.hydronix.com.

Ease of operation and calibrationOnce integrated with the control system

there should be no further requirement for attention, apart from periodically checking the calibration is satisfactory.

Ensuring a sensor is correctly calibrated is probably the most import aspect for ensur-ing consistent performance from the system. Hydro-Cal is a utility within the Hydro-Com software package that greatly facilitates the calibration process, that is particularly appro-priate to the Hydro-Probe II sensors. This enables an accumulation of calibration data to be entered over a period of time, or all at once, that will provide the best fit calibration line for these cali-

bration points. Also the system is designed to assist calibration with only limited test results

(Figure 5).

Di f fer ing mater ia l s wi l l genera l l y have d i f ferent ca l ibrat ion parameters . I f these a re a l ready known then they may be r emote -l y c h an ged i n the sen -sor f rom the computer or t he con t ro l sys tem.

For certain process appli-

cations, were absolute meas-

urement is not so important as in relation to just maintaining a consistent moisture content, then it may not be necessary to change the calibration settings when changing materials. Instead it may be sim-pler to just change the target value for the moisture content to be achieved.

Cost effective sensors & systemThe total investment clearly needs to be

justified in relation to the benefits that accrue as a result of installing a moisture control sys-tem. These are not always easy to quantify.

The increased consistency of product will allow the producer to work more closely to the permitted tolerances for the final moisture content of the product, as well as

a saving in energy by controlling the production processes more

closely, par-ticularly with regard to drying proc-esses.

Feedback from cus-tomers indi-cate a pay-back period of months.

Another factor is the life and cost of mainte-nance of the equip-ment. The sensors are extremely rugged and

robust, constructed from stainless steel with extremely high wearing ceramic ‘windows’ or face plates through which the electromagnetic field is radiated into the material. The sensors would normally be expected to last 10 years in the type of environment associated with animal feedmills.

The cost of maintenance under nor-mal circumstances is nil, and the power requirements absolutely minimal. With approaching 40,000 sensors operat-ing in over 50 countries in a variety of industries, Hydronix has an established reputation for reliability of product and after sales service second to none.

For more inFormation:Hydronix Limited7 Riverside Business CentreWalnut Tree CloseGuildfordSurrey GU1 4UGUnited Kingdom

Tel: +44 1483 468900Fax: +44 1483 468919Website: www.hydronix.comEmail: info@hydronix.com

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Figure 3: The Hydro-Probe II in pelletised material

Figure 5: Calibration curve

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