Electric Pressure Transducers

1. Electric Pressure Transducers Presentator…… Muhammad ShoaibMajeed 2007-CHEM-18-B

2. Introduction • A transducer is defined as a device which converts a physical quantity, a physical condition, or mechanical out put into an electrical signal. • Pressure sensing element such as bellow, a diaphragm or a bourdon tube. • Primary conversion element, e.g. resistance or a voltage. • Secondary conversion element.

3. Contd… • Concluding that a mechanical motion is first converted into a change in electrical resistance and then the change in resistance is converted into a change in electrical current or voltage.

4. Types • Strain Gauge Pressure Transducer • Basic principle “ The strain gauge is a fine wire which changes its resistance when mechanically strained , due to physical effects .A strain gauge may be attached to a diaphragm so that when the diaphragm flexes due to process pressure applied on it, the strain gauge stretches or compresses. This deformation causes the variation in length and cross-sectional area due to which its resistance also changes, as shown.”

5. Contd… • Basic Principle

6. Contd… • Construction and Working

7. Contd… • Advantages & Disadvantages • Small and easy to install • Wide range (from vacuum to 200,000 psig) • Good accuracy, good stability and high output signal strength • Good stock and vibrating characteristics with fast speed of response • Moderate to high cost • Electrical readout is necessary • Temperature compensation required due to problems presented by temperature variations

8. Reluctive Pressure Transducer • Basic Principle “Whenever the spacing between two magnetic devices changes, the reluctance between them also changes, which in turn changes voltage induced by one coil into another and is interrupted as pressure.

9. Contd… • There are 2 main types • Linear Variable Differential Transformer (LVDT • Working Principle & Construction

10. Contd… • Advantages & Disadvantages • Possesses a sensitivity & and infinite resolution • Very rugged in construction and can tolerate a high shocks and vibrations • Stable and easy to align & maintain due to simplicity and small size • Show a low hysteresis, hence repeatability is excellent under all conditions • Sensitive to stray magnetic field but shielding is possible • Temperature affects the performance of transducer

11. Contd… • Servo Pressure Transducer It is also called a “force balance pressure transducer”. It produces an electrical signal proportional t o Pressure. • Working Principle and Construction

12. Piezoelectric Pressure Transducer • Basic Principle “When pressure is applied on a piezoelectric crystals (such as quartz) an electric charge is generated. Piezoelectric materials are quartz, tourmaline and Rochelle salts.”

13. Contd… • Construction & Working

14. Contd… • Advantages & Disadvantages • No external power and is therefore self generating • Good high frequency response • Cannot measure static pressures • Output is affected by changes in temperature.

15. Potentiometric Pressure Transducer • In this type, there is a potentiometer (basically a variable resistance) which is made by winding resistance wire around an insulated cylinder. A movable electrical contact, called a wiper, slides along cylinder, touching the wire at one point on each turn. The position of the wiper determines how much wire, and therefore, how much resistance, is between the end of the wire and the wiper. • Some potentiometers are made covered so that the wiper can pivot in a circular motion rather than moving along a straight line

16. Contd… • Construction and Working

17. Advantages & Disadvantages • Resistance can easily be converted into a standard voltage or current signals • Simpler and less expensive than other types • Easy to design • Finite resolution • Wear occurs as the wiper moves back and forth • Each time the wiper makes and breaks contact with a turn of wire, it causes an extra-electrical signal, which is called noise.

18. Capacitive Pressure Transducer • Basic Principle The principle of operation of capacitive pressure transducer is based upon the familiar capacitance equation of the parallel plate capacitor, i.e. C=€0 €r A /d Where C=the capacitance of a capacitor in farad A=area of each plate in m2 d=distance between the two plates in m €0=8.854×10-12 farad /m 2 €r=dielectric constant (relative permittivity)

19. Contd… • Basic Principle

20. Contd… • Construction & Working

21. Contd… • Advantages • It gives rapid response to changes in pressure. Response time as short as ten milli seconds are possible. • It can withstand a lot of vibration and shoc. • It is extremely sensitive. • It has good frequency response. It can measure both static and dynamic changes.

22. Contd… • Disadvantages • The metallic parts of the capacitor must be insulated from each other. In general, the frames must be carthed. • The performance of a capacitive transducer is severely affected by dirt and other contaminants, because they change the dielectric constant. • The sensitivity of a capacitive transducer is adversely affected by changes in temperature. • Errors may be caused by erratic and distorted signals.