MOISTURE MEASUREMENT in potting soil substrates and other more or less inhomogeneous bulk products

Different moisturesensors. Flat or Cylinder

For most professional substrate producers it is of great importance to know the moisture content of their produced substrates. Especially when their substrates are used by customers who process those substrates in a mechanic way. Also for many other processes, it is very important to know the moisture content of the processed product. Some examples of those are industrial dryers, burning or pelletising biomass like woodchips and sawdust and producing bricks in brickworks.

There are many ways of determining the moisture content of those substrates and other inhomogeneous products. However, all of those known methods have their own disadvantages.

* HEATING STOVE *
Nowadays, the most common way of determining the moisture contents, is to take a sample from the produced soil, put that in a stove, and determine the decrease in weight after having heated this sample for several hours, in some cases even 24 hours.
The main disadvantages of this method are, that you only get the result many hours after production, and that it is very labor-intensive. In most cases the soil has already left the premises or is allready processed by the time the results of the test becomes available. So there is less possibility to use the result for quality control, or for preventing wrong products being delivered to your customer. Because the volume of the sample used in this method is relatively small, this is not mostly not representative for the total batch. When producing substrates which contain different sorts of raw materials, and probably are wetted also, field tests have shown that such a small sample certainly does not represent the batch. This is also the case in for instance brickworks, because there a raw mixture of sand and clay has to be sampled

There exists many measuring methods which can be operated on-line. The best known are the following two:

* NEAR INFRA RED *
This is a rapid system. However, it measures only the outside of the mixed - and sometimes (pre-)wetted - product, and hence the measured value does not represent the inside. In case the color of the product changes, also the measured value will deviate. Also a change in environmental light or temperature can have a devastating effect on the measurement.

* MICROWAVE RADIATION or CAPACITANCE MEASUREMENTS *
Both of these are fast systems. Unfortunately, their results depend also heavily on the salinity of your soil. In most substrates fertilizers are added, so that prevents the use of such systems for measuring ready produced substrates. Also some base materials have a high salinity, and thus these also cannot be measured in that manner. Furthermore they both give their results as a volume percentage. So you have to know the exact Bulk Density of the measured product, otherwise the result is just a figure, but not an absolute one. After all, the volumetric moisture content increases, when the product is compressed. Furthermore, all the existing systems based on these principles depend on the product making a predictable and reproducible contact with the sensor-head. With product containing much void space, like peat and mixtures of sand and clay, this cannot be achieved. Hence the measuring result will deviate a lot also.

All the enumerated previous problems in measuring the moisture content of your soil, was for us the reason to develop a system that does not have any of those before described disadvantages.

We already had a broad experience the potting soil production, because of our Densimeter, our Belt Weighers and our Quantity Checker. After investigating all systems available on the market, it became clear that there was no commercial system available that could fit all of our requirements. Having ended this survey, we decided that we had to develop one ourselves!!.