The majority of SPEC dispensing systems are gravimetric and operate by controlling pneumatic pumps and valves to very accurately weigh each component in a formula. This method of automated dispensing has several benefits. A single measuring device (the scale) means only one component to calibrate.
In addition, the scale accuracy can be easily confirmed by placing a certified weight on the scale and reading the indicated weight. Volumetric dispensing systems have a measuring device (metering pump) for each component being dispensed. Twelve components on the system means twelve metering pumps. There is no direct feedback to confirm or determine if each metering pump is metering accurately. As the SPEC system dispenses each component, the set point weight is displayed on the PC monitor along with the actual weight as it is being dispensed. At the end of each batch, a label is printed that gives all the batch information (color #, job #, weight, etc.) along with the ingredients in the batch and the set point and actual weights of each. It is very easy to confirm exactly what went into the batch. Since there is no direct feedback from metering pumps on a volumetric system, no actual batch dispense information can be given.
For example, what happens if the supply container runs out of ink while dispensing on a volumetric system? Does all or part of that component simply get left out of the formula? In this case, the SPEC gravimetric system would sense no increase in weight on the scale and display a “no flow” alarm and wait for the operator to refill or replace the supply container. The operator is prompted to press R to restart the dispense right where it left off to complete the dispense correctly. This closed loop control method provides real dispense data which is very helpful whenever a problem occurs to help determine what caused the problem (off shade, wrong viscosity, etc.)
In addition, since gravimetric systems weigh each component, they are not susceptible to changes in temperature. The weight does not change with temperature variations. Volumetric systems measure volume, which changes with temperature.