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Digital Re-print - January | February 2012 The Benefits of using Digital Microwave Moisture Measurement

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Feed and grain producers will be familiar with the benefits of being able to control moisture during the various

stages of the production process.

Monitoring the moisture in the raw mate-rials, controlling the water addition during the mixing/blending process and finally checking the moisture in the final product will enable substantial savings to be made. Product yields may be maximised, ingredients efficiently utilised, energy consumption reduced and a quality finished product ensured.

For these reasons it is becoming more common for producers to start considering installing moisture control systems and they therefore need to look for solutions that offer the best accuracy and flexibility at the most competitive price. There is an increas-ingly wide choice of moisture measurement systems available that all claim to have the best and latest technology, but how does the producer, whose area of expertise lies in other technologies, know which system to choose?

There are five main technologies available on the market, although their degree of accuracy and their ability to solve the prob-lem varies considerably. There is the option of resistive, capacitive, infrared, nuclear or microwave techniques.

The first four all have their limitations: resistive and capacitive are susceptible to impurities that may lead to erroneous results; infrared tends to be relatively expensive and

can be affected by dust and vapour and therefore typically lends itself to dry, clean, controlled environ-ments; nuclear systems are

very expensive, and although very accurate in certain applications can be difficult to maintain.

However, microwave sensors are not affected by dust, vapour or colour and have proved overall to be the best option in terms of ease of use, reliability, accuracy and cost effectiveness, usually covering the capital expenditure in a matter of months.

Having settled on the microwave measure-ment method, choosing the right sensor tech-nology now starts to become more involved. There are microwave sensors that are purely analogue, those that are analogue but output their results in a digital format, and those that actually use a digital measuring technique.

Measurement techniqueThere are various methods of using

microwaves to measure moisture. One of the most common methods used

is the resonant cavity technique. In the past, an analogue microwave sensor would measure moisture through a combination of frequency shift (the left to right movement of the response) and amplitude attenuation (the change in the height of the response) of the resonator, see Figure 1. This combination was measured as a single analogue response and therefore frequency shift and attenuation could not be separated.

In the 1980s, Hydronix, a leading manufac-turer of microwave moisture measurement systems introduced the world’s first digital microwave sensor. This broke new ground, for the first time enabling the frequency shift component to be accurately measured using digital techniques. This development resulted in two significant improvements in the sen-sors, an improvement in accuracy and also a very significant extension of the moisture range for which the sensor would give a true linear response as moisture levels increased.

Every material will have a different effect on the microwave field generated by the sensor. As moisture increases the response will shift and reduce in amplitude.

Less advanced sensors simply measure a change in amplitude at a fixed frequency. Measuring across a custom frequency range gives more accurate results than those sen-sors that work within the confines of a single frequency using an open band such as those used by Burglar Alarm Systems or Wi-Fi (433MHz or 2.4GHz). The compo-nents required to manufacture a sensor that measures a frequency shift as opposed to a change in amplitude at a fixed frequency are more complex but provide a superior

by Jason Laffan, Managing Director, Hydronix Limited, UK

Figure 2: The Measurement

Figure 1: Moisture causes a change in the dielectric property of a material

resulting in frequency shift and amplitude attenuation

The benefits of using

Digital microwave moisture measurement

Grain&feed millinG technoloGy18 | January - february 2012

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Grain&feed millinG technoloGy January - february 2012 | 19

www.hydronix.comenquiries@hydronix.com

Hydronix sensors include:

• Digital technology with precise linear output• Wide moisture measurement range• Suitable for chutes, silos, mixers or conveyors• Choice of measurement modes• Not affected by dust or colour• Different installation options• Temperature stable

Hydro-Mix VII

The Hydro-Probe III is the latest release in the Hydronix range of sensors specifically designed to measure in organic materials. Using an existing and proven design, the Hydro-Probe III now includes the measurement modes which are already being used successfully in the grain industry in the Hydro-Mix VII.

Hydro-Probe IIINEW

Hydronix digital, microwave moisture sensors provide accurate and cost effective moisture control in feed meals and pellets, grain, cereal and pulses

Hydronix Moisture Sensors

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nique requires the sen-sor reading to undergo significant mathemati-cal manipulation in order to output what appears to be a linear response to changes in mois-ture.

A digital multi-frequency measure-ment has the ability to measure from 0 percent moisture through to mate-rial saturation while maintaining sen-sitivity and preci-sion across the full

working range. This also highlights a key difference between an analogue sensor that claims to be digital because it can process the signal and output a seemingly linear measurement and a sensor that uses a digital microwave moisture measurement technique that is inherently linear.

The latter will overlay the linear response with subsequent processing to fine tune the curve and incorporate features such as on-board signal smoothing, alarms and averaging functions.

Accuracy requires calibrationCalibration is the process of defining the

relationship between the changes in the sen-sor response with the change in the moisture content of the material. This is required because each material will have its own electrical properties (dielectric properties), which affect the microwave field differently.

A good sensor should only require cali-brating once for a given material. A poor quality sensor will require continual recali-brating due to errors. When well calibrated,

an accurate and temperature stable moisture sensor should glide up and down the linear moisture line all year round.

However, a good sensor that is poorly

calibrated or an unreliable sensor that is well calibrated may, for a short period perform well when operating at or around the mois-ture range to which it was calibrated. As the sensor moves away from this moisture range the reading will exhibit an ever-increasing error. The same error will also exist for a sensor with an underlying non-linear meas-urement technique that is well calibrated.

For the demanding end-user, even a small calibration error can lead to large errors as the moisture varies even with an accurate sensor as shown in Figure 4.

As appealing as it may be, leading manu-facturers would agree that any claim made by a sensor manufacturer that their equip-ment does not require calibrating will lead to sub-optimal results in the field regardless of technical claims concerning the measure-ment technique of the sensor.

A well-calibrated sensor that operates with a different calibration slope with for example wheat and barley is likely to be more accurate than a sensor that claims one calibration fits all.

Figure 4 shows the increasing inaccuracy arising from either a very small calibration error or the assumption of the gradient of the calibration slope. At the point of calibration, between 12.5 percent and 13.5 percent moisture the sensor is reasonably accurate but as the working moisture range moves away from this range, perhaps with a change of material from a different supplier, the error will progressively increase shown by the grey area which is the difference between the theoretically correct calibration and the one in use.

It is usual for moisture sensors measur-ing in the silos to be calibrated to an abso-lute moisture percentage. Hydronix always recommends that the best and most failsafe method for calibrating a sensor is to use a multi-point technique such as that used in its free software, Hydro-Com, see Figure 5.

result. The reason for this is demonstrated in Figures 2 and 3.

As the moisture increases the frequency shifts from f1 to f2, f3 and then f4. The shift between each is similar in magnitude. A sen-sor with a digital measurement technique will continually scan the frequency response and will track the equal changes in frequency as the material becomes wetter.

For the same moisture changes, a sin-gle frequency sensor simply measuring the change in the amplitude at frequency f1 will measure the changes from A1 to A2, A3 and then A4. It can be seen that the sensor progressively loses the ability to register a change in reading as the material becomes wetter.

Typically, a good analogue sensor will lose the ability to register additional changes in moisture from about 12 percent onwards.

This not only effects the sensors ability to register changes in moisture above this level but it also means that the entire moisture curve lacks linearity as shown in Figure 3.

Utilising a non-linear measurement tech-

Figure 5: A multi-point calibration technique is recommended

“There are a variety of moisture measurement sensors available

on the market, all of which claim to provide the best and

most accurate results. It is well known that the microwave

measurement technique provides the best results in

the feed and grain industry”Figure 3: The effects of a non-linear measurement technique

Grain&feed millinG technoloGy20 | January - february 2012

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nique that is capable of maintaining linearity from the absorption point (also known as S.S.D) to complete saturation by definition will give excellent accuracy limited only by the accuracy of the calibration.

The Hydronix range of sensors is sup-ported in over 65 countries worldwide and our team of engineers is backed-up by our free global exchange program.

If you would like to know more about measuring moisture at your feed plant, or about using the latest microwave moisture measurement technology, contact Hydronix:

More inforMation:Jason LaffanManaging DirectorHydronix Limited

Tel: +44 1483 468900Email: enquiries@hydronix.comWebsite: www.hydronix.com

added. For a relatively small capital outlay, both labour and energy costs as well as the amount of material spoilt are reduced, potentially saving the producer many 10’s of thousands of pounds over the year.

To concludeThere are a variety of moisture

measurement sensors available on the market, all of which claim to provide the best and most accurate results. It is well known that the microwave measurement technique provides the best results in the feed and grain industry. When choosing a sensor, you should ensure that it is fully temperature stable and

completely linear over the working range at which you wish to operate. A digital sensor utilising an underlying measurement tech-

Advances in technologyMore advanced microwave moisture

sensors, such as those manufactured by Hydronix now provide the user with the ability to choose between different measure-ment modes, which is of par-ticular interest to feed and grain producers.

Different organic materials have their own distinct dielec-tric property behaviour and will therefore respond differ-ently to each of the underly-ing measurement modes. The best mode may now be select-ed for the material and the application to best account for the temperature range, preci-sion required and bulk density changes over time.

The new measurement modes were introduced into the Hydronix Hydro-Mix VII sensor for mixers and convey-ors during 2010, and proved to be extremely successful in the feed and grain indus-try providing producers with more accurate results than ever before.

This new technology was then introduced in 2011 into the new generation of Hydro-Probe sensor. Typically installed underneath silos or on conveyors, the new Hydro-Probe III has proved to be even more successful than its predecessor. Eliminating the need for continual and time-consuming manual moisture tests, the Hydro-Probe pro-vides an instant and continual moisture level directly to the control system.

This allows driers to be adjusted immediately, or the correct amount of additives, such as mould inhibitors to be

Figure 4: The effects of assuming a calibration slope

Grain&feed millinG technoloGy January - february 2012 | 21

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In this issue:

• Increasing storage capacity

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January - February 2012

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