Reducing the weight of some products is often desirable for performance as well as economic reasons. Many manufacturers strive to develop materials that are partially porous yet still fulfill their function. Density measurements play a key role in these developments. In other products, preventing porosity is important. Closed air bubbles (cells) are desirable in home-insulating materials, but an open cellular structure is required for automobile and furnace filters. Producers thus keep a check on closed and open cells by density evaluation.