Permanent Magnet Moving Coil Instruments

The permanent magnet moving coil instruments are most accurate type for direct current measurements. The action of these instruments is based on the motoring principle. When a current carrying coil is placed in the magnetic field produced by permanent magnet, the coil experiences a force and moves. As the coil is moving and the magnet is permanent, the instrument is called permanent magnet moving coil instrument. This basic principle is called D’Arsonval principle. The amount of force experienced by the coil is proportional to the current passing through the coil.

The PMMC instrument is shown in the below images.

The moving coil is either rectangular or circular in shape. It has number of turns of fine wire. The coil is suspended so that it is free to turn about its vertical axis. The coil is placed in uniform, horizontal and radial magnetic field of a permanent magnet in the shape of a horse-shoe. The iron core is spherical if coil is circular and is cylindrical if the coil is rectangular. Due to iron core, the deflecting torque increase, increasing the sensitivity of the instrument.

The controlling torque is provided by two phosphor bronze hair springs.

The damping torque is provided by eddy current damping. It is obtained by movement of aluminum former, moving in the magnetic field of the permanent magnet.

The pointer is carried by the spindle and it moves over a graduated scale. The pointer has light weight so that it deflects rapidly. The mirror is placed below the pointer to get the accurate reading by removing the parallax. The weight of the instrument is normally counter balanced by the weights situated diametrically opposite and rapidly connected to it. The scale markings of the basic d.c PMMC instruments are usually linearly spaced as the deflecting torque and hence the pointer deflections are directly proportional to the current passing through the coil.

The top view of PMMC instrument is shown in the below image.

Advantages of PMMC

The various advantages of PMMC instruments are,

  • It has uniform scale.
  • With a powerful magnet, its torque to weight ratio is very high. So operating current of PMMC is small.
  • The sensitivity is high.
  • The eddy currents induced in the metallic former over which coil is wound, provide effective damping.
  • It consumes low power, of the order of 25 W to 200 mW.
  • It has high accuracy.
  • Instrument is free from hysteresis error.
  • Extension of instrument range is possible.
  • Not affected by external magnetic fields called stray magnetic fields.

Disadvantages of PMMC

The various disadvantages of PMMC instruments are ,

  • PMMC is Suitable for direct current measurement only.
  • Ageing of permanent magnet and the control springs introduces the errors.
  • The cost is high due to delicate construction and accurate machining.
  • The friction is due to jewel-pivot suspension.

Bourdon’s Tube | Pressure Gauge: Principle, Diagram, Explanation

Basic Principle of Bourdon tube pressure gauge:

when an elastic transducer ( bourdon tube in this case ) is subjected to a pressure, it defects. This deflection is proportional to the applied pressure when calibrated.

Description of Bourdon tube Pressure Gauge:
The main parts of this instruments are as follows:

An elastic transducer, that is bourdon tube which is fixed and open at one end to receive the pressure which is to be measured. The other end of the bourdon tube is free and closed.
The cross-section of the bourdon tube is eliptical. The bourdon tube is in a bent form to look like a circular arc. To the free end of the bourdon tube is attached an adjustable link, which is inturn connected to a sector and pinion as shown in diagram. To the shaft of the pinion is connected a pointer which sweeps over a pressure calibrated scale.

Operation of Bourdon tube:

The pressure to be measured is connected to the fixed open end of the bourdon tube. The appilied pressure acts on the inner walls of the bourdon tube. Due to the appilied pressure, the bourdon tube tends to change in cross – section from ellipitcal to circular. This tends to straighten the bourdon tube causing a displacement of the free end of the bourdon tube.

This displacement of the free closed end of the bourdon tube is proportional to the applied pressure. As the free end of the bourdon tube is connected to a link – section – pinion arrangement, the displacement is amplified and converted to a rotary motion of the pinion.

As the pinion rotates, it makes the pointer to assume a new posistion on a pressure calibrated scale to indicate the appilied pressure directly. As the pressure in the case containing the bourdon tube is usually atmospheric, the pointer indicates gauge pressure.

Applications of Bourdon Tube pressure gauge:

They are used to measure medium to very high pressures.

Advantages of Bourdon tube pressure gauge:

  • These Bourdon tube pressure gauges give accurate results.
  • Bourdon tube cost low.
  • Bourdon tube are simple in construction.
  • They can be modified to give electrical outputs.
  • They are safe even for high pressure measurement.
  • Accuracy is high especially at high pressures.

Limitations of bourdon tube pressure gauge:

  • They respond slowly to changs in pressure
  • They are subjected to hysterisis.
  • They are sensitive to shocks and vibrations.
  • Ampilification is a must as the displacement of the free end of the bourdon tube is low.
  • It cannot be used for precision measurement.

U- Tube Manomerter | Theory, Explanation, Diagram, & Working Equations

Construction of U-tube Manometer:
This manometer consists of a U shaped tube in which the manometeric liquid is filled. The manometer is used to measure the pressure which is unknown by the balancing gravity force and acceleration due to gravity, g = 9.81 m/sec2
The manometer consists of a steel, brass and aluminum material. It has a glass tube made up of pyralex glass. The graduations are made on the tube in terms of mm or in some condition it is graduated in kilo Pascal.
Working of U-tube Manometer:
The unknown pressure is applied in the one arm of the tube and the mercury in the tube or manometeric liquid filled in the tube moves in the tube or rises to the constant region and then the movement is stopped. The height of the liquid is measured and noted. The pressure is calculated by using the formula,
                                                                    P1-P2 = Pmhg
The above equation is arrived by
P1 = Pthg = P2+Pmhg
P1-P2 = hg(Pt – Pm)
P1 = applied pressure
P2 = 0
Pt = specific gravity of the liquid or water
g = acceleration due to gravity.
P1 – P2 is approximately equal to Pmhg.
Advantages of U-tube Manometer:
Simple in construction
Low cost
Very accurate and sensitive
It can be used to measure other process variables.
Disadvantages of U-tube Manometer:
Fragile in construction.
Very sensitive to temperature changes.
Error can happen while measuring the h.
Characteristics of liquid used in U-tube Manometer:
Viscosity should be low.
Low surface tension is required.
The liquid should stick on the walls.
Should not get vaporized.

Venturi Meter: Theory, Operations & Working

Basic principle:

When a venture meter is placed in apipe carrying the fluid whose flow rate is to be measured, a pressure drop occurs between the entrance and throat of the venturimeter. This pressure drop is measured using a differential pressure sensor and when calibrated this pressure drop becomes a measure of flow rate.

Construction of Venturi meter

The following are the main parts and areas of venture meter:

The entry of the venture is cylindrical in shape to match the size of the pipe through which fluid flows. This enables the venture to be fitted to the pipe.
After the entry, there is a converging conical section with an included angle of 19’ to 23’.
Following the converging section, there is a cylindrical section with minimum area called as the throat.
After the throat, there is a diverging conical section with an included angle of 5’ to 15’.
Openings are provided at the entry and throat (at sections 1 and 2 in the diagram) of the venture meter for attaching a differential pressure sensor (u-tube manometer, differential pressure gauge, etc) as shown in diagram.

Operation of venturi meter:

The fluid whose flow rate is to be measured enters the entry section of the venturi meter with a pressure P1.
As the fluid from the entry section of venturi meter flows into the converging section, its pressure keeps on reducing and attains a minimum value P2 when it enters the throat. That is, in the throat, the fluid pressure P2 will be minimum.
The differential pressure sensor attached between the entry and throat section of the venturi meter records the pressure difference(P1-P2) which becomes an indication of the flow rate of the fluid through the pipe when calibrated.
The diverging section has been provided to enable the fluid to regain its pressure and hence its kinetic energy. Lesser the angle of the diverging section, greater is the recovery.


  • It is used where high pressure recovery is required.
  • Can be used for measuring flow rates of water,gases,suspended solids, slurries and dirty liquids.
  • Can be used to measure high flow rates in pipes having diameters in a few meters.

Advantages of venturi meters

  • Less changes of getting clogged with sediments
  • Coefficient of discharge is high.
  • Its behaviour can be predicted perfectly.
  • Can be installed vertically, horizontally or inclinded.


  • They are large in size and hence where space is limited, they cannot be used.
  • Expensive initial cost, installation and maintenance.
  • Require long laying length. That is, the veturimeter has ti be proceeded by a straight pipe which is free from fittings and misalignments to avoid turbulence in flow, for satisfactory operation. Therefore, straightening vanes are a must.
  • Cannot be used in pipes below 7.5cm diameter.

LVDT: Basic Principle, Theory, Working, Explanation & Diagram | Linear Variable Differential Transformer

A very basic transducer which is always useful in the field of instrumentation, I have studied about this in my college days. Now let me explain about the LVDT with its Principle of Operation and I will explain how it is constructed for its well known operation and you can understand the working of LVDT.
Principle of LVDT:
LVDT works under the principle of mutual induction, and the displacement which is a non-electrical energy is converted into an electrical energy. And the way how the energy is getting converted is described in working of LVDT in a detailed manner.
LVDT consists of a cylindrical former where it is surrounded by one primary winding in the centre of the former and the two secondary windings at the sides. The number of turns in both the secondary windings are equal, but they are opposite to each other, i.e., if the left secondary windings is in the clockwise direction, the right secondary windings will be in the anti-clockwise direction, hence the net output voltages will be the difference in voltages between the two secondary coil. The two secondary coil is represented as S1 and S2. Esteem iron core is placed in the centre of the cylindrical former which can move in to and fro motion as shown in the figure. The AC excitation voltage is 5 to 12V and the operating frequency is given by 50 to 400 HZ.
Working of LVDT:
Let’s study the working of LVDT by splitting the cases into 3 based on the iron core position inside the insulated former.
Case 1:On applying an external force which is the displacement, if the core reminds in the null position itself without providing any movement then the voltage induced in both the secondary windings are equal which results in net output is equal to zero
i.e., Esec1-Esec2=0
Case 2:When an external force is appilied and if the steel iron core tends to move in the left hand side direction then the emf voltage induced in the secondary coil is greater when compared to the emf induced in the secondary coil 2.
Therefore the net output will be Esec1-Esec2
Case 3:When an external force is applied and if the steel iron core moves in the right hand side direction then the emf induced in the secondary coil 2 is greater when compared to the emf voltage induced in the secondary coil 1. therefore the net output voltage will be Esec2-Esec1
Advantages of LVDT:
* Infinite resolution is present in LVDT* High output* LVDT gives High sensitivity* Very good linearity* Ruggedness* LVDT Provides Less friction* Low hysteresis* LVDT gives Low power consumption.
Disadvantages of LVDT:
* Very high displacement is required for generating high voltages.* Shielding is required since it is sensitive to magnetic field.* The performance of the transducer gets affected by vibrations* Its is greatly affected by temperature changes.
Applications of LVDT:
=> LVDT is used to measure displacement ranging from fraction millimeter to centimeter.=> Acting as a secondary transducer, LVDT can be used as a device to measure force, weight and pressure, etc..