VOLUMETRIC OR GRAVIMETRIC DOSING?
Gravimetric dosing is the principle whereby the components are dosed in a balanced way instead of moving a certain volume. Within the group gravimetric dosing there are two variants, one weighing the decrease in weight (loss in weight principle) and the other the increase in weight (gain in weight principle).
LOSS IN WEIGHT
The loss in weight systems are mostly volumetric dosing machines with a weighing cell under the hopper with the material to be added or underneath the machine. In practice, these load cells have a very wide range, sometimes even up to 20 kg. As a result, the machine is often unable to detect a single dose. Only after a number of doses can a weight decrease be seen by the weighing cell and it can adjust the following doses to these values. This can take a long time to start, which often leads to a lot of rejection. In addition, it is still not possible to dose half a pitch or pocket. The advantage of such a system is that, theoretically, it is self-calibrating and in practice will therefore be a lot more accurate than volumetric dosing, but it is just as easy to use. However, by doing research we found out that software is key in accuracy here. When the software is continuously changing the values it can result in an even worse accuracy.
GAIN IN WEIGHT
With the gain in weight principle, all components are weighed one by one and combined in one batch, which give these machines the nickname batch blenders. For dosing the same example product as above, a batch of 500 grams will be made. The weight to be added now becomes 7.5 grams, which is approximately equal to 300 grains. Suppose this is not 300, but 330, a deviation of 10% arises. However, this can now be compensated in the second component, which is also weighed. At first 19,700 granules of natural were requested, but are now being adjusted to 21,670 (+ 10%). The last component can also have a deviation, suppose that 20 grains are dosed too much, then the deviation is only 0.09%. This makes a batch blender much more accurate than a volumetric dosing machine or loss in weight dosing machine. Finally, everything is weighed, resulting in almost perfect traceability and reproducibility.
Volumetric dosing means that dosing is based on “volume”. This can be the contents of a screw or the contents of a “pocket”. The advantage of volumetric dosing is that it is relatively easy to use. The machine is placed between the intake and the machine hopper and you can start dosing almost immediately. With volumetric dosing, the machine is calibrated by entering the weight of an x number of revolutions. Provided regular calibration is done, this can be fairly accurate. In practice, however, too little is calibrated, as a result of which too many additives are usually consumed. In addition, dosing is only carried out when “it” is needed.
As an example:
Suppose we have a shot weight of 100 grams where 1.5% has to be added, the weight to be dosed is 1.5 grams. We know from experience that for most components 1 gram is approximately equal to 40 granules.
In this case, therefore, 60 granules must be dosed. If it happens that not 60 but 65 grains are dosed, an overdose of 8% occurs. When 55 grains are dosed, the under dosage is also 8%. When too much is dosed, this often has little consequences for the end product, except that it contains more expensive materials than necessary. If too little is dosed, this results in rejection. What we see in practice is that standard dosing is already too much, not 1.5% but 2%, so that in any case there is no rejection. Finally, a volumetric dosing machine has either a pitch or a pocket. It is not possible to dose half a pitch or pocket.