Pressure Cells Types, Working Principle and Applications

1. Pressure Cells: Types, Working Principle, & Applications

2. 1 WHAT IS A PRESSURE CELL? 2 USES OF PRESSURE CELLS IN GEOTECHNICAL FIELD TABLE OF CONTENT 3 WORKING PRINCIPLE OF A PRESSURE CELL 4 TYPES OF PRESSURE CELLS

3. 1 WHAT IS A PRESSURE CELL? Pressure cells are one of those geotechnical instruments that are designed to moni- tor stress or pressure on concrete structures, retaining walls, abutments, embank- ments, buildings, tunnel linings, etc. A pressure cell is a transducer that converts pressure or stress into a readable elec- trical output. They are specially designed to measure the total stress in various con- struction fields. Pressure cells are available in different shapes, sizes, and types.

4. 2 USES OF PRESSURE CELL IN GEOTECHNICAL FIELD The rock/soil formation is completely dependent on the rock/soil type, stress history, shear, and normal stress levels, boundary and drainage conditions, and several environmental effects. The precise assessment of stress and changes that occur in foundation soil/rock formations is of great importance for safe engineering design. While constructing the large civil engineering projects like tall buildings, dams, un- derground tunnels, etc., measurement of stress plays a vital role. The types of stress can be categorized as: Measurement of total stress at a point within a soil mass/foundation rock/ concrete structure. Measurement of contact stress against the face of a structural element. • •

5. 2 USES OF PRESSURE CELL IN GEOTECHNICAL FIELD The vibrating wire pressure cells are much reliable and faster than all the other types as they are capable of taking readings electrically as the frequency output is almost immune to external noise. The cable from the pressure sensor is connected to the readout unit or data logger and is protected against any possible damage during construction to give all-around reliable data.

6. 2 USES OF PRESSURE CELL IN GEOTECHNICAL FIELD The applications of pressure cells (stress cells) are as below- • • • • • Pressure cells are designed to monitor radial and tangential stresses in shotcrete tunnel linings, underground excavations, unlined caverns, foundations. Measurement of contact stress against the face of a structural element. They are used to monitor mine backfill. To measure the orientation and magnitude of total pressure and stress distribution within dam embankments and the interface of soil and concrete. To determine contact pressure on retaining and diaphragm walls, piers, and abutments.

7. 3 WHAT IS THE WORKING PRINCIPLE OF A PRESSURE CELL? A pressure sensor converts the stress or applied pressure into measurable electrical output. Pressure can be defined as the applied force per unit area that a fluid exerts on its surroundings. The pressure is given by: Pressure (P) = Force (F)/Area (A) The pressure transducers consist of a sensor with a constant area and respond to the force applied by fluid pressure. The force applied tends to deflect the diaphragm inside the pressure cell. The de- flection of the diaphragm is calculated and converted into readable electrical output

8. 4 TYPES OF PRESSURE CELLS Model EPS-30V-Series Earth and Concrete Pressure Cells The Encardio-Rite earth and concrete pressure cell primarily consist of a flexible and circular flat capsule constructed from two stainless steel discs. These stainless steel discs are welded around the periphery and connected to a spe- cially designed Encardio-rite pressure transducer incorporating the latest vibrating wire technology. The vibrating wire technology allows for remote and digital readout. The pressure cell has inherently high sensitivity and, it is not necessary to produce a large strain on the area being monitored for the measurement of stress.

9. 4 TYPES OF PRESSURE CELLS Encardio-Rite offers three types of Model EPS-30V-Series pressure cells: • Model EPS-30V-S Earth Soil Pressure Cell – Suffix ‘S’: for use in soil, earth or rock fills The Model EPS-30V-S Earth Soil Pressure Cell is 200 mm Φ x 7 mm thick. It consists of two stainless steel diaphragm plates welded around the periphery. The pressure pad is connected to the vibrating wire pressure sensor through a 6 mm dia x 165 mm long stainless tube. The narrow cavity inside the pressure pad and the tube is filled with fluid.

10. 4 TYPES OF PRESSURE CELLS • Model EPS-30V-C Concrete Pressure Cell – Suffix ‘C’: for use in concrete The Model EPS-30V-C Concrete pressure capsule is similar to EPS-30V-S. For applications in which the concrete pressure cell is embedded in concrete, a 600 mm long pinch tube is welded to the transducer. Once the concrete is cured, due to cooling the shrinkage of capsule is created which creates a gap between the concrete and cell which prevents the transmission of pressure on the cell. Thus to fill the gap, a pinch tube is squeezed to push the fluid to inflate the cell and develop a positive pressure on it.

11. 4 TYPES OF PRESSURE CELLS • Model EPS-30V-I Soil and Rock-Concrete Interface Pressure Cell – Suffix ‘I’: for use at concrete and soil interface The Model EPS-30V-I Soil and Rock-Concrete Interface Pressure Cell is similar to the above except that it is 200 mm Φ x 10 mm thick and the diaphragm on one of the side is rigid.

12. 4 TYPES OF PRESSURE CELLS Model EPS-30V-J Jack Out Pressure Cell The total stress measurement in the soil can be categorized as follows: • • Measurement within a soil mass. Measurement at the face of a structural element. Encardio-Rite Jack-Out Pressure Cell falls in the latter category. The name of the cell is derived from the use of a hydraulic jack during installation by activating it to keep the cell in position while the concrete is poured in.

13. 4 TYPES OF PRESSURE CELLS Encardio-Rite Model EPS-30V-J-Ф Jack-Out Pressure Cell is used for monitoring changes in stress base slabs and diaphragm/slurry walls etc. It is extensively used: • • • Where concrete is cast directly against soil, i.e. cast in place structures. As a safety measure to monitor the earth pressure in excess of designed limits. For verification of design assumptions and to provide data and information for safer and more economical future designs. The Model EPS-30V-J Jack-Out Pressure Cell is designed to measure the total stress i.e. the effective stress due to the soil together with the pore water pressure in the voids between soil grains. It is suitable for measuring static or slowly varying stresses only.

14. 4 TYPES OF PRESSURE CELLS Construction of Model EPS-30V-J Jack Out Pressure Cell The Jackout Pressure cell is 14 mm thick and has a diameter of either 200 mm or 125 mm. It is constructed from two stainless steel plates welded together around the periphery so as to leave a narrow space between them.

15. 4 TYPES OF PRESSURE CELLS This space is completely filled with de-aired fluid and connected hydraulically to a pressure sensor. The sensor has an all welded construction such that space confining the hydraulic fluid is entirely metal, not requiring ‘O’ rings that tend to trap air and reduce cell stiffness. The hydraulic fluid is de-aired which materially improves fluid stiffness and performance of cell. The pressure transducer is made of stainless steel construction with size 42 mm Ф x 176 mm long. It incorporates the latest vibrating wire technology and has an inherently high sensitivity.

16. 4 TYPES OF PRESSURE CELLS The transducer converts the fluid pressure into an electrical signal in the form of fre- quency, which can be accurately measured by model EDI-51V vibrating wire readout unit or any other conventional vibrating wire readout unit. The signal can also be transmitted over long distances to a remote automatic data acquisition system like Encardio-Rite Model EDAS-10.

17. 4 TYPES OF PRESSURE CELLS Model ESC-30V Shotcrete-Concrete Stress Cell The Model ESC-30V is used to monitor the radial and tangential stresses in the shot- crete of tunnels, particularly where the New Austrian Tunnelling Method and under- ground works are being implemented. Measurement of radial and tangential stresses within shotcrete lining is a vital fea- ture of this type of stress cell.

18. 4 TYPES OF PRESSURE CELLS The stress cell has a rectangular pressure pad which is constructed with two stain- less steel plates, welded around the periphery. The pressure pad and the electron beam welded vibrating wire pressure sensor are connected together by a 6 mm Φ x 165 mm long stainless steel tube. The cavity inside the pressure sensor and the pressure pad is filled with de-aired fluid.

19. 4 TYPES OF PRESSURE CELLS Construction of Model ESC-30V Shotcrete-Concrete Stress Cell Increase in stress in the concrete causes a vibrating wire pressure cell and is con- verted to an electrical signal which is transmitted as a frequency signal to read out the unit. The pressure cell consists of a magnetic, high tensile strength stretched wire, one end of which is anchored and the other end fixed to a diaphragm which deflects in some proportion to the applied pressure. Any deflection of the diaphragm changes the tension in the wire, thus affecting the resonant frequency of the vibrating wire. The resonant frequency with which the wire vibrates can be accurately measured by the vibrating wire readout unit.

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