Encyclopedia of Environmental Science and Engineering, Volume I and II

546 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

placed in four categories—modification of sensor output,

amplification, mathematical operation, and signal modifica-

tion for readout.

The electrical components used in these circuits are of

two types, active and passive elements. Active elements,

such as solid state devices add energy to a circuit; whereas

passive elements, such as resistors, capacitors, inductors,

diodes add no energy. Both elements are combined to form

active and passive circuits. Active circuits change signals in

a complex way. Passive elements are used in active circuits

to provide necessary conditions for the proper functioning

of active circuits. Some active devices are ionization cham-

bers, vacuum phototubes, operational amplifiers and gas dis-

charge tubes.

Readout Devices The sensor signal modified by the

conditioning circuits is ultimately converted into a visual

form by the readout device or output transducer. The read-

out signal may be analog or digital requiring a compatible

readout device. Analog readout devices comprise record-

ers, meters, oscilloscopes, photographic plates and integra-

tors; printers, computers and digital meters with optical

displays provide digital readouts. A digital computer may

be interfaced to an instrument, in order to compute values

from a digital output signal and produce a hard (printed)

copy of the data using a printer. Analog output signals may

be digitized in order to utilize a computer. The advantages

of digital outputs are the statistical benefit derived from

counting and analog outputs are advantageous in feedback

control systems.

Analog Devices The automatic recording potentiometer

or potentiometric recorder has been, over the years, the

most frequently used readout device providing a continu-

ous trace on a chart of an analog signal. Its operation is

based on a low power servomechanism utilizing a feedback

system. The instrumental signal to be measured is com-

pared to a standard reference signal. The amplified, differ-

ence or error signal activates the pen-drive motor moving

the pen on the chart to a position representing the magni-

tude of the analog signal. The control of the pen, based on

the error signal, denotes the feedback system and the total

system is referred to as a servomechanism. 9,10 Two types of

recorders, the Y -time or X – Y, allow the recording of a signal,

Y, as a function of time or of two signals representing the

ordered (data) pair, x, y, respectively. In the Y -time device,

a constant-speed motor moves the chart in the x direction

while the servomechanism deals with the y signal. The X – Y

recorder has two servo- systems, one for each signal, x and y.

However, recorders may be limited by the rate that the data

flows from the instrument. Some recorders can adequately

respond to signals during fast scans. For example fast scans

in cyclic voltammetry of about 1 volt/sec. can be transcribed

using a recorder, however, at faster rates an oscilloscope is

necessary.

Almost any instrument can utilize a potentiometric

recorder. A Y -time analog recorder is commonly used to

trace gas and liquid chromatograms; the abscissa, X axis,

is for retention volume or time and the ordinate is for the

detector response.

The oscilloscope is a measuring device with complicated

circuitry that allows accurate display and measurement of

non-sinusoidal or complex waveforms. The oscilloscope’s

basic part is the cathode ray tube, CRT. A CRT is a vacuum

tube containing an electron gun pointing to a fluorescent

screen at the tube’s end. The electron gun provides a beam

whose movement is controlled by two sets of deflector plates

perpendicular to each other. The plates receive the signals

representing the waveforms. These analog signals are dis-

played on a fluorescent screen as Y -time or X-Y curves. The

display is photographed to provide a hard copy of the analog

data. The oscilloscope can display data that is generated at

high rates, since there are no mechanical movements used

in manipulating the electron beam. Where very fast events

must be recorded, an oscilloscope is an effective readout

device.^11 (See the previous paragraph on the potentiometric

recorder.) Oscilloscopes have facilities to store, compare,

and manipulate signals.

Analog meters are based on the D’Arsonval meter move-

ment. The electrical current signal passing through a moving

coil, to which is fixed a pointer, induces a magnetic field in

the coil. A static magnetic field from a permanent horseshoe

magnet surrounds the coil. The interaction between the two

fields causes the movement of the coil: the degree of move-

ment is determined by the magnitude of the signal current.

Analog meters require the analyst to interpret or read the

output signal value by the position of the indicator needle

or pointer using a calibrated scale mounted on the meter.

A resistance placed in series with the meter movement allows

FIGURE 2 Chemical sensor.

CHEMICAL SENSOR

INTERFACE ZONE

TARGET

or

ANALYTE selective reaction

(chemical or physical

TRANSDUCER

electrical

signal

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