1. Why plc require in analog input 4-20amp & 0-10 volt?

Because these are the signals which can change with respect to signal.4-20 mA signal is use because if it can start from 0-20 then if there is an signal of instrument is not equal to or less then original signal and if there instrument is show 10% open valve at 4 mA then we can test it with giving it less then 4 mA.

2. Difference between mmwc and mmwg?

I hope that your question is the difference between mmwc (millimeter of water column) and mmHg (millimeter of Mercury column)

The force exerted by a water column of say 100 mm (ie. the force exerted by a water column of height 100 mm at the bottom point of unit

area ) is 100 mmwc. The pressure is defined as Force per unit area and the unit of pressure is mmwc. When the liquid column is replaced by Mercury then the pressure is measured as mmHg. The density of Hg(Mercury) is approximately 13.6 times greater than water and hence the pressure exerted by Hg column is 13.6 times greater than water column of same height.

3. What happen if i install my pressure transmitter below the elevation of pipe in natural gas service?

Pressure will be added up a bit more, see its detail:
1. Suppose,you want to install a pressure transmitter 10
meter below of pipe line.
2. Density of methane (Examlpe) 0.656 kg/m3 at 25C.
3. your pressure transmitter will show (10 x 1000 x0.656 =
6560mm) 6569 mm of water pressure more.
4. Suppose gas pressure is 20Kg/cm2, then due to this
elevation, transmitter will show 20.66Kg/cm2

4. What is Cryogenic?

Cryogenic means to work in lower temperature range (-50 and
below than that).

5. What are the automatic controller MODES?

Following are the mode of control. Ratio Control, PID
Control, PI Control, ON/OFF Control, Cascade Control,
Override Control, Feedback Control, Feed Froward Control.
These control are can be set in to MAN and AUTO Mode, If
this in MANUAL the mode of control is disable, If this is in
AUTO the mode of control is enable.

6. What is coriolis principle for mass flowmeter?

This meter uses the Coriolis effect to measure the amount of mass moving through the element. The substance to be measured runs through a U-shaped tube that is caused to vibrate in a perpendicular direction to the flow. Fluid forces running through the tube interact with the vibration, causing it to twist. The greater the angle of the twist, the greater the flow

7. How to use level tranasmitter in closed tank application?

In closed systems, the transmitter location is
restricted by the maximum allowable distance
above the lower tap. In pressurized systems, this is
the same as the 1 atmosphere equivalent seen
previously. In sub-atmospheric systems (vacuum
systems), the transmitter should be mounted at or
below the lower tap. This ensures the transmitter
always sees a positive pressure on both the
measurement and the reference sides.
In two seal systems, the distance between the taps
becomes the reference offset from zero. The
calculations are the same regardless of where the
transmitter is mounted.

8. if we have 450 mm heigth boiler drum level ,so what is span & zero value for a dpt tranmitter?

zero will be -400mm and span will be LRV will be -400mm and
URV will be 0 mm.

9. What is Difference between PLC and CNC?

CNC (Computer Numerical Control) is a particular application
usually to control a multi axis machine tool such as a
milling machine or a lathe but also could be a tube bending
machine punch press or similar. The CNC controller is
usually a purpose built controller using a micro computer
but could be a general purpose computer or a PLC. The
forerunner was NC (Numerical Control) before computers were
readily available, it used solid state logic and relays,
timers etc to provide the application.

A PLC (Programable Logic Controller) is a general purpose
logic controller specifically designed to allow the user to
create their own application. They select appropriate I/O
(Inputs Outputs) and perhaps function modules for purposes
such as communications and multi axis servo control and
program the PLC to perform the required functions.

10. why use MMWC unit?

milli meter water column used to measure pressure

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11. Explain types of valves?

various types of valves are used in industrial process
controls.Various types of valves are ball valve,gate
valve,globe valve,needle valve,mixing valve,diverting
valve,butterfly valve etc.Actually this is just an overview
of the types.To be elaborate we have several categories and
types.

12. How to trim the pressure transmitter?

Ensure that you have the necessary document on your hand
such as PTW.
Connect a HART Communicator, then close the Tx isolation valve
and open the VENT isolation to atmosphere.
Apply LRV and check w/ HART Comm, if the LRV is out, perform
TRIM function using HART 375/475

13. what is difference between differial pressure & delta pressure?

Differancial pressure means differanc between high pressure & low pressure
delta pressure means it is the pressure drop..

14. Explain working principle of lvdt?

When an AC excitation signal is applied to the Primary Coil
(P), voltages are induced in the two Secondary Coils (S).
The MAGNETIC CORE inside the COIL WINDING ASSEMBLY provides
the magnetic flux path linking the Primary and secondary
Coils.Since the two voltages are of opposite polarity, the
Secondary Coils are connected series opposing in the
center, or Null Position. The output voltages are equal
and opposite in polarity and, therefore, the output voltage
is zero. The Null Position of an LVDT is extremely stable
and repeatable.
When the MAGNETIC CORE is displaced rom the Null
Position, an electromagnetic imbalance occurs. This
imbalance generates a differential AC output voltage
across the Secondary Coils which is linearly proportional
to the direction and magnitude of the displacement.
As shown in the figure, when the MAGNETIC CORE is moved
from the Null
Position, the induced voltage in the Secondary Coil, toward
which the Core is
moved, increases while the induced voltage in the opposite
Secondary Coil
decreases.
LVDTs possess the inherent ruggedness and durability of a
transformer and
truly provide infinite resolution in all types of
environments. As a result of the
superior reliability and accuracy of LVDTs, they are the
ideal choice for linear
motion control.

15. What are the key components of control loop?

A closed control loop in a very basic manner consists of
sensor, transmitter, controller,signal converters and final
control elements.Actually in a practical loop there will be
many other instruments apart from this to support the
working of above mentioned instruments.

16. What is the meaning of wet leg & where is it used?

A process used to determine the differential pressure
present within a liquid-filled space.

The formula for determining the differential pressure
within a wet leg design is: d/p = h (SG)

Where: d/p = differential pressure, h = height of liquid
present, and SG = specific gravity

When the process vapors condense at normal ambient
temperatures or are corrosive, this reference leg can be
filled to form a wet leg. If the process condensate is
corrosive, unstable, or undesirable to use to fill the wet
leg, this reference leg can be filled with an inert liquid.

17. Square root calculation for dp flow transmitter?

So, the way you would go about doing square root extraction is by using an analog multipler. An analog multiplier has three inputs Vx, Vy & Vz. The output is given by Vo = (Vx*Vy)/Vz; If we set Vy = 1V and Vo = Vz through a feedback loop, we get Vo.^2 = Vx => Vo = sqrt(Vx).

If the interviewer further asks about an analog multiplier, you could mention that it can be designed by a combination of logarithmic/anti-logarithmic amplifier and an adder.