PRESSURE MEASUREMENT

1. PRESSURE MEASUREMENT

2. “In any given plant, the number of pressure gauges used is probably larger than all other instruments put together” Pressure Sensors

3. Pressure Definition • Static Pressure. Pressure, P, is defined as force, F, per unit area, A:     P = F/A

4. Pressure = Force / Area Pressure can be used to measure other variables such as Flow and Level Pressure plays a major role in determining the Boiling Point of Liquids Fluids exerts pressure on the containing vessel equally and in all directions Pressure Measurement

5. Mechanical Methods Electrical Methods Pressure Measurement Methods

6. Types of Pressure measurements • Absolute pressure is measured relative to a perfect vacuum (psia) • Gauge pressure is measured relative to ambient pressure (psig) • Differential pressure is the difference in pressure between two points of measurement. (psid). • Note that the same sensor may be used for all three types; only the reference is different.

7. Pressure Sensing Pressure • Pressure is sensed by mechanical elements such as plates, shells, and tubes that are designed and constructed to deflect when pressure is applied. • This is the basic mechanism converting pressure to physical movement. • Next, this movement must be transduced to obtain an electrical or other output. • Finally, signal conditioning may be needed, depending on the type of sensor and the application. Figure illustrates the three functional blocks. Sensing Element displacement Transduction element electric Signal Conditioner V or I output

8. Manometer method Elastic pressure transducers Pressure measurement by measuring vacuum Electric pressure transducers Pressure measurement by balancing forces produced on a known area by a measured force Pressure Measurement Methods

9. Manometers Manometers measure a pressure difference by balancing the weight of a fluid column between the two pressures of interest. • Where • p is unknown pressure • Po is Ambient pressure • is fluid density g is gravitational force h is fluid column

10. Pressure by Elastic Transducers • Bourdon • Bellows • Diaphragm Uses flexible element as sensor. As pressure changes, the flexible element moved, and this motion is used to rotate a pointer in front of dail

11. Sensing Elements • The main types of sensing elements are Bourdon tubes, diaphragms, capsules, and bellows • All except diaphragms provide a fairly large displacement that is useful in mechanical gauges and for electrical sensors that require a significant movement

12. Bourdon tubes are generally are of three types C-type Helical type Spiral type Bourdon Tube Pressure Gauge 12

13. Bourdon Tube Pressure Gauge 13

14. Output of LVDT

15. Uses B-tube as elastic transducer & LVDT as an inductive transducer

16. Pressure Measurement Bourdon gauge A Bourdon gauge uses a coiled tube, which, as it expands due to pressure increase causes a rotation of an arm connected to the tube.

17. Pressure Measurement A second type of aneroid gauge uses the deflectionof a flexible membrane that separates regions of different pressure. The amount of deflection is repeatable for known pressures so the pressure can be determined by using calibration. The deformation of a thin diaphragm is dependent on the difference in pressure between its two faces. Diaphragm The reference face can be open to atmosphere to measure gauge pressure, open to a second port to measure differential pressure, or can be sealed against a vacuum or other fixed reference pressure to measure absolute pressure. The deformation can be measured using mechanical, optical or capacitive techniques. Ceramic and metallic diaphragms are used.

18. Diaphragm are popular because they required less space and the motion they produce is sufficient for operating electronic transducers Diaphragm Pressure Gauge

19. They are used to measure gauge pressures over very low ranges. Two types of diaphragm pressure gauges are: 1. Metallic diaphragms gauge (brass or bronze) 2. Slack diaphragms gauge (Rubber) Diaphragm Pressure Gauge

21. Bellows The bellows pressure sensor is made of a sealed chamber that has multiple ridges like the pleats of an accordion that are compressed slightly when the sensor is manufactured. When pressure is applied to the chamber, the chamber will try to expand and open the pleats. which uses a spring to oppose the movement of the bellows and provides a means to adjust the amount of travel the chamber will have when pressure is applied

22. More sensitive than bourdon type gauge. Used to measure low pressures Brass, phosphor bronze, stainless steel, beryllium copper etc. Doesn’t work well with dynamic pressure due mass & large displacements Bellows

23. Mechanical Pressure Transducers to Pneumatic Pressure Transmitters to Electric Pressure Transmitters Electric Pressure Transducers PEC – UET Lahore Dr. Shahid Naveed 5. Januar 2020 / Dr. –Ing Naveed Ramzan 23

24. Why Electrical Pressure Transducers? Electric Pressure Transducers Transmission requirements for remote display as electric signal transmission can be through cable or cordless. Electric signals give quicker responses and high accuracy in digital measurements. The linearity property of the electric signal produced to pressure applied favors simplicity. They can be used for extreme pressure applications, i.e. high vacuum and pressure measurements. EPTs are immune to hysteresis, shock and mechanical vibrations.

25. Pressure sensing element such as a bellow , a diaphragm or a bourdon tube Primary conversion element e.g. resistance or voltage Secondary conversion element Electric Pressure Transducers

26. Piezoelectric pressure transducers Capacitive pressure transducers Inductive transducers Strain gauge pressure transducers Potentiometer pressure transducers Types of Electric Pressure Transducers

27. Piezoelectricity Phenomenon of generating an electric charge in a material when subjecting it to a mechanical stress (direct effect) and Generating a mechanical strain in response to an applied electric field (converse effect). Piezoelectric materials are Anisotropic – Electrical and mechanical properties differ along different directions

29. Pressure Measurement Piezoresistor For typical values of the stress voltage drop along the resistor Vr varies

30. Pressure Measurement Strain Gauge 900 1800 2700 00

31. Pressure measurement – Capacitive Tducer In a capacitance-type pressure sensor, a high-frequency, high-voltage oscillator is used to charge the sensing electrode elements. In a two-plate capacitor sensor design, the movement of the diaphragm between the plates is detected as an indication of the changes in process pressure.

32. Pressure measurement – Capacitive Tducer

33. Pressure measurement – Capacitive Tducer • The variable gap created by a moving diaphragm can be used as a capacitance sensor. • The capacitance of the sensor is related to pressure.

34. Pressure measurement – Capacitive Tducer • Advantages: • More sensitive than piezoresistive • Less temperature dependant • Great dynamic pressure sensor • Disadvantages • Requires special diaphragm • Diaphragm mechanical properties

35. Pressure Measurement Potentiometer The potentiometric pressure sensor provides a simple method for obtaining an electronic output from a mechanical pressure gauge. The device consists of a precision potentiometer, whose wiper arm is mechanically linked to a Bourdon or bellows element. The movement of the wiper arm across the potentiometer converts the mechanically detected sensor deflection into a resistance measurement, using a Wheatstone bridge circuit

36. Pressure Measurement Resonant of wire The resonant-wire pressure transducer was introduced in the late 1970s. In this design, a wire is gripped by a static member at one end, and by the sensing diaphragm at the other. An oscillator circuit causes the wire to oscillate at its resonant frequency. A change in process pressure changes the wire tension, which in turn changes the resonant frequency of the wire. A digital counter circuit detects the shift. Because this change in frequency can be detected quite precisely, this type of transducer can be used for low differential pressure applications as well as to detect absolute and gauge pressures.

37. Applications • AUTOMOTIVE • MEDICAL DEVICES • ERGONOMICS • ROBOTICS