This would most likely be easiest accomplished with a development board containing a RISC processor, A/D chip, and buffered I/0. A load cell or other weighing component connects to the A/D converter, and the buffered I/0 conntcted to the valve. Under program control, the processor stores the input from the user, then reads the weight from the A/D converter ( and performs any conversion/linerization, etc ) then sends the signal to open/close the valve.
In the future, try your questions in the technical forums for better responses.