mirror with a stationary fiber-optic cathode ray tube that was in direct contact with the paper. These recorders had several flaws. The photo-sensitive paper was very expensive, and would quickly fade when exposed to ambient light. High chart speeds meant that test durations were extremely short. These instruments were intended to capture short-duration events such as NASA rocket launches in the 1960s and a broad range of ballistic events. Potentiometric (servo) instruments Analog chart recorders using a galvanometer movement to directly drive the pen have limited sensitivity. In a potentiometric type of recorder, the direct drive of the marking pen is replaced with a servomechanism where energy to move the pen is supplied by an amplifier. The motor-operated pen is arranged to move the sliding contact of a potentiometer to feed back the pen position to an error amplifier. The amplifier drives the motor in such a direct as to reduce the error between desired and actual pen position to zero. With a suitable signal processing amplifier, such instruments can record a wide range of process signals. However, the inertia of the servo system limits the speed of response, making these instruments most useful for signals changing over the span of a second or more. Digital chart recorders A modern chart recorder is an embedded computer system with an analog-to-digital converter, a microcontroller, and a hard-copy printing device; such instruments allow great flexibility in signal processing, variable chart speed on process upsets, and can also communicate their measurements to remote points. One of the first digital units was designed by William (Bill) C. McElroy jr working for Dohrman Instrument Company in Santa Clara, California. Up until this unit, most chart recorders were rack mounted and had one speed and one sensitivity range. Mr. McElroy's design was an instant loading paper roll 'table-top' unit using an Integrated Chopper Circuit for signal conversion. The unit had plug in circuit boards, plug in single or multi-range modules and plug in single or multi-speed modules. The recorder's sensitivity was 1 microvolt to 100 volts full-scale, which at the time was an industry first. Mr. McElroy also aided in the design and build of the Gas Chromatograph used for analysing dirt and rock samples from the 1969 Apollo 11 moon landing. See also Barograph Electrocardiograph Plotter (operates in two dimensions) Polygraph Seismograph Thermo-hygrograph References Recording devices