GALVANOMETRIC RECORDER

This is a type of chart recorder using pens which are deflected by a moving coil mechanism (galvanometer) similar to those employed in most electrical meters. A small coil of wire is mounted on a spindle between the poles of a powerful magnet. When an electric current passes through the wires of the coil it forces the spindle to rotate. If this rotation is restrained by a spring the angle through which it turns is proportional to the current through the coil. In galvanometric recorders the input signal is amplified and delivered as a current to the coil of the galvanometer. A pointer attached to the end of the spindle marks the recording paper to indicate the magnitude of the input signal. The marks may be made by ink (the pointer is a narrow tube through which ink is forced), or it may write by pressure or heat on special paper. Some types do not have a pointer but spray the ink from a hole in the spindle or have a small mirror mounted on the spindle or coil which reflects light on to a photographic paper. These latter types can operate at higher frequencies (e.g. 1 kHz) because of their very low inertial mass.

Galvanometric pen recorders are widely used for recording ECG, EEG, blood pressure and other signals with frequency components up to 100 Hz, and are thus found in many hospital departments as single or multichannel instruments. The mirror and ink jet recorders may be used for electromyography (EMG) or other applications where the wider bandwidth is important.

A small defect with the pen types is that the pen draws an arc on the paper and not a straight line. Straight line recording is usually achieved by feeding the paper over a 'knife edge' writing bar, but this introduces a linearity error at the extremes of deflection. There would be approximately 10% error at 30 degrees deflection.

Content and Design Copyright 2000 Dr. Malcolm C Brown.  See Title Page for more details