An apparatus for identifying microscopic particles in a fluid, includes a laser beam that passes though a narrow detect zone, and photodetectors that detect light scattered by microscopic particles that pass through the detect zone. The touch screen is protected with a membrane adapted to the specific contour and profile of the electronic device and allows the user to use the touch screen interface with no shortcomings. A petroleum residue of atmospheric distillation or of vacuum distillation or a deasphalted oil are representative feedstocks of the feedstocks treated within the scope of this invention. This arrangement makes it possible to reduce the load because the timings causing the high load in removing the sheet members are shifted.