Other ways to measure

Perhaps most important in a measurement system is the ability to detect moisture independent of other material changes. In a single energy system (such as microwave, or nuclear isotope systems) it is impossible to know if changes in the beam attenuation are due to changes in bulk density, wood density, volume, or moisture variations. Even after regular re-calibrations for seasonal changes. These instruments typically measure over a very small cone covering a fraction of the material. And frozen material renders them useless for many customers over long periods.

The natural variations in the raw material make measuring wood chips a challenge to any system. Infra-red systems can give no more than an indication of some change on the surface of the wood chips, with regular re-calibrations due to colour, temperature, dust and dirt necessary. Very few pulp mills can rely on this technology and none we know of use it to control the digester process.

There are many types of belt scales available with impressive accuracy quoted. The reality is often very different with greater uncertainty in the data a short period after recalibration. Time and resource goes to calibrations at very regular intervals so its good to know that the Flow Scanner requires just an initial calibration to maintain extremely high precision. 

Data from oven samples is used, together with belt scales, to estimate dry content. Whilst the oven method gives the only true accepted moisture content it is based on a very small unrepresentative sample size, is also subject to errors, and takes many hours and labour resources. The final analysis result is delivered long after the chips have been consumed and so can only be used as historical process analysis. However the data can be used to calibrate the Flow Scanner allowing it to measure all the material in the process.

Metal detectors are common at wood processing factories but do not detect rocks, stones, sand, gravel, or other non-ferrous metal items. False alarms are also common and can result in lost production searching for a phantom object. The Flow Scanner can instantly produce a highlighted image of any item with a different density hidden in the material. Operators can then make a quick decision to override and continue or stop and remove.

NMR spectroscopy has emerged as an interesting alternative recently. Perhaps its biggest limitation is the inability to scale up and measure all material in the process. The small volume of samples give rise to uncertainty of representing the whole material flow. Complex materials and multi-parameter measurements would also appear restrictive.