Intelligent pigs may also use calipers to measure the inside geometry of the pipeline. In 1961, the first intelligent pig was run by Shell Development. It demonstrated that a self-contained electronic instrument could traverse a pipeline while measuring and recording wall thickness. The instrument used electromagnetic fields to sense wall integrity. In 1964 Tuboscope ran the first commercial instrument. It used MFL technology to inspect the bottom portion of the pipeline. The system used a black box similar to those used on aircraft to record the information; it was basically a highly customized analog tape recorder. Until recently, tape recording (although digital) was still the preferred recording medium. As the capacity and reliability of solid-state memory improved, most recording media moved away from tape to solid-state. Capacitive sensor probes are used to detect defects in polyethylene pipe gas pipeline. These probes are attached to the pig before it is sent through the polyethylene pipe to detect any defects in the outside of the pipe wall. This is done by using a triple plate capacitive sensor in which electrostatic waves are propagated outward through the pipe's wall. Any change in dielectric material results in a change in capacitance. Testing was conducted by NETL DOE research lab at the Battelle West Jefferson's Pipeline Simulation Facility (PSF) near Columbus, Ohio. Modern intelligent or "smart" pigs are highly sophisticated instruments that include electronics and sensors that collect various forms of data during their trip through the pipeline. They vary in technology and complexity depending on the intended use and the manufacturer. The electronics are sealed to prevent leakage of the pipeline product into the electronics since products can range from being highly basic to highly acidic and can be of extremely high pressure and temperature. Many pigs use specific materials according to the product in the pipeline. Power for the electronics is typically provided by onboard batteries which are also sealed. Data recording may be by various means ranging from analog tape, digital tape, or solid-state memory in more modern units. The technology used varies by the service required and the design of the pig; each pigging service provider may have unique and proprietary technologies to accomplish the service. Surface pitting and corrosion, as well as cracks and weld defects in steel/ferrous pipelines are often detected using magnetic flux leakage (MFL) pigs. Other "smart" pigs use electromagnetic acoustic transducers to detect pipe defects. Caliper