Chart recorder

Circular chart recorders have a rotating disc of paper that must be replaced more often, but are more compact and amenable to being enclosed behind glass.

[1] Here is how he described it: "A roll of paper a thousand feet in length was slowly unwinding itself upon the long table ... About a dozen pens connected with a bridge crossing the middle of the table were each marking its own independent curve gradually or by jumps ..." The paper advance was geared to the wheels of the railroad carriage, while pens recorded time, the drawbar pull of the locomotive, and numerous other variables.

[5] Chart recorders are still used in applications where instant visual feedback is required or where users do not have the need, opportunity or technical ability to download and view data on a computer or where no electrical power is available (such as in hazardous zones on an oil rig or in remote ecological studies).

One common method is to use a miniature synchronous motor which turns at a constant speed related to the power frequency; a gear-train is used to propel the paper.

In the telegraphic siphon recorder of 1858 a fine capillary tube is connected to an ink reservoir and is deflected by the process signal.

One form of sensitive and high-speed recorder used beams of ultraviolet light reflected off mirror galvanometers, directed at light-sensitive paper.

[6] The earliest instruments derived power to move the pen directly from the sensed process signal, which limited their sensitivity and speed of response.

To lessen the strain on the galvanometer the pen might instead only intermittently be pressed against the writing medium, to make an impression, and then move while pressure is released.

[citation needed] Where greater sensitivity and speed of response is required a mirror galvanometer, might be used instead, to deflected a beam of light which can be recorded photographically.

The original models used a small mirror attached to a galvanometer to aim a high-intensity beam of light at photosensitive paper.

One of the first digital units was designed by William (Bill) C. McElroy Jr. working for Dohrman Instrument Company in Santa Clara, California.

McElroy's design was an instant loading paper roll 'table-top' unit using an Integrated Chopper Circuit for signal conversion.

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.