function addNote(bytes32 _productID, bytes20 _serialNumber, string _text) onlyOwner public returns (uint) {
        Note storage note = notes[notesLength];
        
        note.productID = _productID;
        note.serialNumber = _serialNumber;
        note.text = _text;
        
        emit noteInfo(_productID, _serialNumber, _text);
        
        notesLength++;
        return notesLength;
    }