An apparatus and method is disclosed to automatically maintain a desired back pressure on a slurry to facilitate the proper operation of pumping equipment and to eliminate or diminish scaling or other buildup in piping and related equipment. The present invention also relates to a display device comprising a substrate, a first transparent conductive layer, a light modulating layer, and a second conductive layer, wherein the second conductive layer is UV curable. A door spring for urging the slide door toward the closing position and holding the same in the closing position is disposed on the door passage.