week 5-6 Pressure measurement devices lecture

measurement and instrumentation

1. EE218-Instrumentation & Measurement Dr. Mona Ibrahim

2. Lecture 5-Pressure Measurements

3. Pressure Measurement and Display Types: ❑Bourdon Tube ❑ Diaphragms ❑ Bellows Types: Manometers : U-type manometer Well-type manometer Inclined-type In order to display pressure values, two approaches are used:

4. Elastic Sensing Elements Diaphragm Bourdon Tube Spiral Helical Capsules C-Type Bellows Most pressure sensors used in process control, converts the pressure into a mechanical displacement Displacement is converted to a pointer movement via a link, quadrant and a gear arrangement The elastic element can be Bourdon tube, diaphragm, bellows or capsules.

5. Pressure Measurement: 2.Elastic Sensing Elements

6. Pressure Measurement: 2.Elastic Sensing Elements

7. Pressure Measurement: 2.Elastic Sensing Elements

8. Pressure Measurement: 2.Elastic Sensing Elements

9. Pressure Measurement: 2.Elastic Sensing Elements Any type of Bourdon tube can measure pressure proportional to the arc it subtends, i.e., the more curved the arc, the greater is the pressure sensitivity of the device. Thus, spiral and helical tubes are more sensitive to pressure changes than a C-type tube. This, however, increases the complexity, and consequently, the cost of manufacturing.

10. Pressure Measurement: 2.Elastic Sensing Elements

11. Pressure Measurement: 2.Elastic Sensing Elements

12. Pressure Measurement: 2.Elastic Sensing Elements Used when Bourdon tube is inappropriate to be used A diaphragm is nothing more than a thin disk of material which bows outward under the influence of a fluid pressure. Many diaphragms are constructed from metal (Metallic diaphragms gauge) (brass or bronze) , which gives them spring-like qualities and can either be flat or corrugated. Some diaphragms are intentionally constructed out of materials with little strength, such that there is negligible spring effect. These are called slack diaphragms (Rubber) , and they are used in conjunction with external mechanisms (e.g. springs) producing the necessary restraining force to prevent damage from applied pressure. The Diaphragm Pressure Gauge uses the elastic deformation of a diaphragm (i.e. membrane) instead of a liquid level to measure the difference between an unknown pressure and a reference pressure. The displacement of the diaphragm causes the pointer deflection. Diaphragms are used to measure relativity low pressures, viscous, impure and vibrating environment when its difficult to use Bourdon tubes. Used in many Pharmaceutical applications

13. Pressure Measurement: 2.Elastic Sensing Elements A typical Diaphragm pressure gauge contains a capsule divided by a diaphragm, as shown. One side of the diaphragm is open to the external targeted pressure, P Ext , and the other side is connected to a known pressure, P Ref ,. The pressure difference, P Ext – P Ref , mechanically deflects the diaphragm.

14. Pressure Measurement: 2.Elastic Sensing Elements

15. Pressure Measurement: 2.Elastic Sensing Elements

16. Pressure Measurement: 2.Elastic Sensing Elements More sensitive than Bourdon and Bellows, thus used with low pressure when high accuracy is needed (otherwise require an impractically large and thin diaphragm). Capsule is used in measurement of the aircraft altitude , and for the cabin pressure . The sensing element of a capsule pressure gauge consists of two corrugated diaphragms welded together at their periphery to form a capsule. The pressure to be measured is introduced into the capsule via an opening in the center of the first diaphragm. The center of the second diaphragm is connected to the transmission mechanism so that the deflection of the measuring element can be transmitted to the pointer . When the pressure rises inside the capsule, both diaphragms will slightly deform. By making use of two diaphragms, the total deflection of the measuring element is twice as large . For the measurement of very small pressure differences, the deflection of a single capsule may be too small. Therefore, multiple capsules can be stacked on top of each other until sufficient displacement is obtained to move the pointer across the full scale. In the pressure gauge animation the pressure is going in and out the capsule, turning the pointer to the right and back to the left .

17. Pressure Measurement: 2.Elastic Sensing Elements Capsule pressure gauges are suitable for the measurement of very low negative and positive pressures ranging from 0 - 0.5 mbar to 0-1000 mbar Absolute pressure sensors In absolute pressure sensors, the pressure chamber of the capsule is evacuated and hermetically sealed. The capsule is mounted in a sealed housing. The process pressure is then introduced into the sealed housing through a process connection, and applied to the exterior of the capsule. Since the process pressure is compared to a nearly perfect vacuum inside the capsule, we will be measuring absolute pressure. Capsule can measure both relative and absolute as well as differential pressure. Relative pressure sensors For the measurement of relative pressures, the process pressure is applied to the inside of the capsule. The capsule thus has a process connection in the middle of a diaphragm through which the process pressure is introduced. The reference pressure, being the atmospheric pressure, is on the exterior of the capsule. The measurement of relative pressure can be done with one or multiple capsules.. Differential pressure sensors If the relative pressure measurement is provided with a second connection, two different pressures can be compared with each other. The resulting pressure, which is measured by the capsules, is the differential pressure of the two process pressures.

18. Pressure Measurement: 2.Elastic Sensing Elements In certain applications, especially when measuring vacuum pressure Bellows are also used in low-intermediate pressure measurement as well as differential pressure measurement (dual bellows exist ) Bellows resemble an accordion constructed from metal instead of fabric. Increasing pressure inside a bellows unit causes it to elongate . Bellows are thin-walled metallic cylinders, with deep convolutions, of which one end is sealed and the other end remains open. The closed end can move freely while the open end is fixed. It is made of phosphoric bronze & stainless steel. When pressure is applied to the closed end, such as in the animation, the bellows will be compressed. The closed end will move upwards and the link, which is the rod in between the closed end of the bellows and the transmission mechanism, will go up and rotate the pointer .

19. Pressure Measurement: 2.Elastic Sensing Elements Absolute pressure sensors To measure absolute pressure we need two bellows. The first one is the reference bellows, which is provided with a perfect vacuum on the inside. The second one is the measuring bellows, which is subjected to the process pressure. The deflection of the bellows is transferred via the transmission mechanism to the pointer. Relative pressure sensors For relative pressure measurement the inside of the bellows is at atmospheric pressure whereas the outer side is subject to the process pressure. the spring can be located either on the inside of the bellows or on the outside.. Differential pressure sensors Just as differential pressure can be measured with a single bellows, as described in relative pressure measurement, we can also use dual bellows. The low process pressure is connected to the first bellows while the high process pressure is connected to the second bellows. Both of these process pressures will exert a force on the effective area of the bellows upon which they act. The resultant force rotates the pointer.

20. Pressure Measurement: 2.Elastic Sensing Elements