Page 814 - Basic Electrical Engineering

The output of analog instruments has to be converted into digital form
using analog to digital converters (A to D converters) before interfacing with
computers. However, this conversion costs money and some time is lost

which may be critical in a fast-changing control-system operation. That is
why digital instruments are preferred where fast measurement and control

operations are involved.

11.7.3 Indicating, Recording, and Integrating Instruments

Indicating instruments give the output as a function of time through the

movement of a pointer over a graduated scale. These are, therefore, called
analog instruments. The deflection-type ammeters, voltmeters and wattmeters

in your laboratory are indicating instruments.
Recording instruments create a written record usually on paper of the time-

varying quantity. The measurement system carries a pen which is used to
record the value of the time-varying quantity on a paper which is driven by a

slow-moving motor drive. The curve traced on the paper indicates the actual
variation in the value of the quantity being measured. For example,
temperature can be measured and recorded continuously using a recording-

type instrument. In an ECG machine the status of health of your heart is
recorded on a slow-moving paper and can be classified as a recording-type

instrument
Integrating instruments record the total value of the variable quantity over
a period of time. For example, the electric meter (kilowatt hour meter)

installed in our residences records the total amount of electricity consumed
over a period of time. It is a summing or integrating instrument.

11.7.4 Deflection- and Null-type Instruments

In deflection-type instruments deflection of a movable pointer provides a
basis for measurement of the quantity which has created the deflection. This

deflection is controlled by an opposing force created by some spring action.
At a steady-state deflected position, deflecting torque is equal and opposite to

the controlling torque. The deflection is measured on a calibrated scale.