Power Measurement Equipment 65
Chart-recording Instruments
The types of instruments discussed previously are used when no
permanent record of the measured quantity values is needed. Howev-
er, instruments can be employed that provide a permanent record of the
quantity values. Also, values measured over a specific time period can be
recorded. A chart-recording instrument is such an instrument. The types of
chart recorders include both pen and ink recorders and inkless recorders.
Pen and ink recorders have a pen attached to the instrument. This pen
is controlled by either electrical or mechanical means, which cause it to
touch a paper chart and leave a permanent record of the measured quanti-
ty on the chart. The charts utilized may be either roll charts (which revolve
on rollers under the pen mechanism) or circular charts (which revolve on
an axis under the pen). Chart recorders may use more than one pen to re-
cord several quantities simultaneously. In this case, each pen mechanism
is connected so as to measure a specific quantity. The pen of a chart re-
corder is a capillary tube device that is actually an extension of the basic
meter movement. The pen must be connected to a constant source of ink.
The pen is moved by the torque that is exerted by the meter movement,
just as the pointer of a hand-deflection type of meter is moved. The chart
used for recording the measured quantity usually contains lines that cor-
respond to the radius of the pen movement. Increments on the chart are
marked according to time intervals. The chart must be moved under the
pen at a constant speed. Either a spring-drive mechanism, a synchronous
AC motor, or a DC servomotor can be used to drive the chart. Recorders
are also available that use a single pen to make permanent records of mea-
sured quantities on a single chart. In this case, either coded lines or differ-
ent colored ink can be used to record the quantities.
Inkless recorders may use a voltage applied to the pen point to pro-
duce an impression on a sensitive paper chart. In another process, the pen
is heated to cause a trace to be melted along the chart paper. The obvious
advantage of inkless recorders is that ink is not required.
Chart-recording instruments are commercially available for measur-
ing almost any electrical or physical quantity. For many applications, the
recording system is located a great distance from the device being mea-
sured. For accurate system monitoring, a central instrumentation system
may be used. Power plants, for instance, ordinarily use chart recorders at
a centralized location to monitor the various electrical and physical quan-
tities involved in the power plant operation.
The operation of a typical roll-chart recording instrument involves