Pressure Transmitter calibration

Dead Weight Tester

Bourden type Pressure Gauge

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.

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.

What is Instrumentation ?

Instrumentation is a branch of physics which deals with the measuring and controlling variables in a process industry, the instruments related to the process variables and its calibration. Some of  the industrial process variables are Level, Pressure, Temperature, Humidity, Flow, pH, Force, Speed etc,. The technology which used to design and develop an instrument for the measurement and control is called instrumentation technology.

Definition: Instrumentation is defined as "the art and science of measurement and control".

An instrument is a device that measures and regulates the process variables such as flow, pressure, temperature, level, or pH.Instruments are classified in to mechanical instruments and electronic instruments. In the early years the process industry field uses mechanical devices for the measurement and control. At that time process indicators and control elements such as valves were monitored by an operator that walked around the unit, by adjusting the valves to obtain the desired process variables. After the boom of electronics all mechanical instruments give the way to electronic equipments.The transformation of instrumentation from mechanical pneumatic transmitters, controllers, and valves to electronic instruments reduced the maintenance costs the systems used for measurements & control in the process industry field. This also increased the efficiency, and production due to their increase in accuracy.

The progress of the field in industry took place largely in the 1930's after the invention of the reliable instrument to record the temperature. With the growth of the continuous manufacturing and 24 hrs working in the process industries, the need for continues measurements and automatic controls became necessity. There are continuous demands for the improvements in the quality of measurement, control and for the development of new methods based on newly found laws and its effects in the electrical, mechanical fields.

Aim of Instrumentation:- To obtain maximum production (quality products) from the industry with the minimum usage of raw materials.