INTRODUCTION :
It is the branch of engineering which deals with the
measurement, monitoring, display etc. of the various of energy exchanges which
take place during process operations. "In short Instrumentation is the
study of Instrument."
INSTRUMENT
:
Instrument is a devices which is used to measure,
monitor, display etc. of a process variable.
Que. : What
are the process Variable ?
Ans. : The
process Variable are :
1. Flow
2. Pressure
3. Temperature
4. Level
5. Quality i. e. % D2, CO2, pH etc.
Que. : Define
all the process Variable and state their unit of measurement. ?
Ans. : FLOW
: Kg / hr, Litter / min, Gallon / min. M3 / NM3 / HR. ( GASES )
PRESSURE
: Force acting per unit Area. P = F/A
Units : Bar / Pascal / Kg / CM /, Pounds
LEVEL : Different between two heights.
Units
: Meters, mm, cm, %.
TEMPERATURE : It is the degree of hotness or coldness of a
body.
Units : Degree Centigrade, Degree Fahrenheit,
Degree Kelvin, Degree Rankine.
QUALITY : It deals with analysis
pH, % CO2, % 02, Conductivity, Viscosity.
D E F I N A T I O N.
DEAD ZERO SIGNAL :
The advantage is that it does not have to be biased to
true zero. A "Live zero" gives the computer additional information,
so that it can takes appropriate alarm action in case of a measurement failure,
because it can discriminate between a transmitter operating, but transmitting a
zero measurement and a failure, in the signal system.
Que.
: What is force balance and motions
balance principle ?
Ans.
:
FORCE BALANCE PRINCIPLE :
"A
controller which generates and output signal by opposing torque’s".
The
input force is applied on the input bellows which novas the beam. This crackles
nozzle back pressure. The nozzle back pressure is sensed by the balancing
bellows which brings the beam to balance. The baffle movement is very less
about 0.002" for full scale output.
MOTION BALANCE PRINCIPLE :
"A
controller which generates an output signal by motion of its parts".
The
increase in input signal will cause the baffle to move towards the nozzle. The
nozzle back pressure will increase. This increase in back pressure acting on
the balancing bellows, will expands the bellows, there by moving the nozzle
upward. The nozzle will move untill motion (almost) equals the input (baffle)
motion.
Advantages of force Balance :
1.
Moving parts are fever.
2.
Baffle movement is negligible.
3. Frictional losses are less.
AUTOMATIC CONTROLLER :
It is a device
which measured the value of variable quantity or condition and operates to
correct or lie it deviation of this measured value from a selected reference.
AUTOMATIC CONTROL SYSTEM :
It is any operable arrangement of one or more automatic
controllers in closed loops with one or more processes.
SELF OPERATED CONTROLLER :
It
is one in which all the energy needed to operate the final control element is
derived from the controlled medium through the primary element.
RELAY OPERATED CONTROLLER :
It is one in which the energy transmitted through the primary
element is either supplemented or amplified for operating the final control
element by employing energy from another sources.
PROCESS :
A process comprises the collective function performed in
and by the equipment in which a variable is to be controlled.
SELF REGULATION :
It is an inherent characteristic of the process which aids
in limiting the deviation of the controlled variable.
CONTROLLED VARIABLE :
The controlled variable is that quantity and condition
which is measured and controlled.
CONTROLLED MIDIUM :
It
is that process energy or material in which a variable is controlled. The
controlled variable is a condition or characteristic of the controlled medium.
For e.g. where temperature of water in a tank is automatically controlled, the
controlled variable is temperature and controlled medium is water.
MANIPULATED VARIABLE :
It is that quantity or condition which is varied by the
automatic controller so as to affect the value of the controlled variable.
CONTROL AGENT :
It
is that process energy or material of which the manipulated variation is a
condition or characteristic. The manipulated variable is a condition or
characteristic of the control agent. For e.g. when a final control element
changes the fuel gas flow to burner the manipulated variable is flow the
control agent is fuel gas.
ACTUATING SIGNAL :
The
actuating signal is the difference at anytime between the reference input and a
signal related to the controlled variable. This basically known as error
signal.
DEVIATION :It is the difference
between the actual value of the controlled variable and the value of the
controlled variable corresponding with set point.
OFFSET :
It
is the steady state difference between the control point and the value of the
controlled variable corresponding with setpoint
CORRECTIVE ACTION :
It is the variation of the manipulated variable produced
by the controlling means. The controlling means operates the final control
element ( control value ) which in turn varies the manipulated variable.
REFERENCE INPUT :
It is the reference signal in an automatic controller.
SET POINT :
It
is the position to which the control point setting mechanism is set.
CONTROL POINT :
It is the value of the controlled variable which under any
fixed set of conditions the automatic controller operates to maintain.
D E F I N A T I O N.
ACCURACY :
A
number or quantity which defines the limit of error under reference conditions.
ATTENUATION :
A
decrease in signal magnitude between two points, or between two frequencies.
DEAD TIME :
The
interval of time between initiation of an impact change or stimulus and the
start of the resulting response.
DRIFT :
As
undesired change in output over a period of time, which change is unrelated to
input, operating conditions, or load.
ERROR :
The
difference between the indication and the true value of the measured signal.
SPAN ERROR :
It
is the difference between the actual span and the specified span and is
expressed as the percent of specified span.
ZERO ERROR :
It is the error of device operating under the specified
conditions of use when the input is at the lower range value.
STATIC GAIN :
It
is the ratio of the output change to an input been change after the steady
state has been reached.
HYSTERESIS :
The
maximum difference between the upscale and downscale indications of the
measured signal during a full range traverse for the same input.
INTERFERENCE :
Interference is any spurious voltage or current arising
from external sources and appearing in the circuits of a device.
COMMON MODE INTERFERENCE :
It
is the form of interference which appears between the measuring circuit
terminals and ground.
NORMAL MODE INTERFERENCE :
It is the form of interference which appears between
measuring circuit terminals.
LINEARITY :
The
closeness to which a curve approximate a straight line.
RANGE :
The
region between the limits within which a quantity is measured received or
transmitted, expressed by stating the lower and upper range values.
REPEATABILITY :
The closeness of agreement among a number of consecutive
measurements of the output for the same value of the measured signal under the
same operating conditions.
REPRODUCIBILITY :
The closeness of agreement among repeated measurements of
the output for the same value of the input made under the same operating
conditions.
RESPONSE :
It is the general behavior of the output of a device as a
function of input both with respect to time.
SIGNAL TO NOISE RATIO :
Ratio of signal amplitude to noise.
TIME CONSTANT :
The time required for the output to complete 63.2 % of the
total rise or decay.
SPAN :
The algebraic difference between upper and lower range
values.
ZERO SHIFT :
Any parallel shift of the input output curve.
