Olafur Jonsson (Tovalia)
When producing animal feed, controlling the moisture content is one of the most important control parameters. Moisture Measurement Sensors in Animal Feed are a very important part of how accurately we can sense and control process moisture.
Moisture content will affect the quality of the finished feed and its shelf life. Moisture content in the feed will also directly affect the profitability of the feed mill.
It is therefore evident that this parameter should be monitored and controlled to an optimum level at each stage in the process.
Moisture Measuring Techniques
Measuring moisture content can be done by the so-called moisture balance method; this method involves heating a certain amount of material and the weight loss is interpreted as moisture. This method takes from 25 – 25 minutes and is not very practical if the operator wants to react quickly to changes in the production.
Measuring moisture in animal feed instantly and continuously is however tricky. The three most common technologies for doing these measurements are:
- Measuring the response of Near Infrared (NIR) Light as it is reflected off the material
- Measuring the dielectric properties of the material with Electrical Capacitance
- Measuring the dielectric properties of the material with Microwaves
The advantage of NIR is that it can measure more than just the moisture content. It can measure protein and fat. However, the NIR requires a separate calibration for each new recipe which is not ideal for compound feed, where recipes constantly change. For pellets the NIR technology has the disadvantage that it is a surface measurement, it will not measure the moisture inside the pellet.
Electrical capacitance is measured at low frequency. This is the cheapest method. However at this frequency, the ionic conduction effects are considerable. The effects of density as well as temperature are also considerable.
The Microwave method for online moisture measurements of animal feed is becoming the recognised method in the industry. At microwave frequencies above 2-3GHz, the ionic conduction effects are negligible. The measurements of dielectric properties resulting from dipolar orientational polarisation of the water molecules inside the material are directly correlated with moisture content. However, microwaves will be affected by temperature as well as density of the material.
There are several sensor technologies or sensor types that can be used with microwave technology to measure online moisture content in animal feed, but the most common sensor type is the planar sensor, or cavity resonator. This is a cylindrical cup placed on the outside of a silo or chute.
The installation is fairly easy. This sensor can be mounted on a hopper or a chute.
The disadvantages of this design is that the measurement area is unknown. The radiation of microwaves into the material is not defined, but rather depends on the material and the density of it. Therefore this method had 3 unknowns :
- the moisture content,
- the density and
- the size of the area being measured.
It is often estimated that the measurement area is a few square centimetres. These sensors need to be cleaned regularly in case there is a dirt settlement on the sensor surface, as the dirt will influence the moisture measurement of the material, because the proportion of the dirt will be significant to the total measurement area.
A New Solution in Moisture Sensing Devices
There is a new design of microwave moisture sensors on the market now, that use planar antennas. There are two plates separated by a specific distance. A Transmitter is in one plate and a receiver in another. This device is mounted on a silo or a hopper and the product passes between these two specifically designed plates as it is sensed for moisture content. This means the measurement area is known.
The amount of material measured is a cross section of the volume of material between the two plates, which is considerably more than what current planar sensors do. Effects of dust, dirt or buildup on the sensor are therefore negligible.
As with the planar sensors, this new design is installed in a hopper or a chute. The requirement for this sensor to work well is that the two sensor plates need to be fully covered with material.
Tested and Proven
Graph 1. Comparison of moisture content by Tovalia sensor and the moisture balance method. In total 71 samples were analysed from 39 different recipes. The Tovalia sensor used a single calibration for all measurements.