POTENTIOMETRIC RECORDER

Typical Number in Hospital: 20

Cost Bands: 3,4

References: 3

Slow-moving variables such as trends of pressure, heart rate or output from analytical instruments in the pathology laboratory may be recorded on wide paper using a potentiometric recorder, usually in the flat-bed format. The pen is mounted on a wire which moves it across the width of the recording paper with the aid of pulleys driven by a motor. One of the pulleys is mounted on the spindle of a potentiometer (although this can be a single length of resistance wire) which generates a voltage output proportional to the position of the pen. This voltage is applied to the input of the amplifier which drives the motor, as a negative feedback signal, so that the pen is always moved to a point corresponding to the amplitude of the input signal to be recorded. Preamplifiers are added to scale and condition the signal according to its amplitude, frequency content, and type (e.g. differential).

The writing speed is determined by the power of the motor, mass of the moving system (wires pulleys and pens) so that the pen has a maximum acceleration and velocity. The system is attractive where wide paper is desired and where high linearity is required (e.g. 0.2%). The system is unsuitable for fast moving signals or where multichannel operation is required. Multichannel systems exist but different colours are needed and there must be a small offset between each pen to avoid them colliding. X-Y recorders are usually potentiometric.