Module 3: Pressure Monitoring

1. Module 3: Pressure Monitoring Module Objectives Upon successful completion of this module, students will be able to: • explain the importance of pressure monitoring with the help of simple examples. • define the term “pressure” and its unit of measurement; • differentiate between atmospheric pressure and absolute pressure; • give examples of pressure measurement instruments and pressure gauges; • explain the operating principle of pressure sensors; • perform experiments related to pressure measurement and data logging/processing

2. 3.1 Introduction to Pressure Monitoring Pressure is the primary variable for a wide range of process measurements Food processing industries use high-pressure pasteurization systems to destroy many pathogens and produce safer and healthy food Pharmaceutical industries monitor and maintain the pressure inside drug manufacturing chambers to protect them from germs. Pasteurization process

3. Many types of industrial measurements are also derived from pressure, such as flow and liquid level as illustrated in figures 3.2 and 3.3. Figure 3.2: Measuring liquid level Figure 3.3: Flow measurement by Measuring pressure drop Doctors monitor the blood pressure of patients to check their health conditions as shown in here

4. More pressure monitoring applications include: 1. Monitoring gas pressure in petroleum industry 2. Monitoring tire pressure in vehicles 3. Pressure recording in the combustion chamber of an engine. 3.2 What is Pressure? Pressure is derived from force. It is defined as the force per unit area when the applied force is perpendicular to the surface of an object the force acting on the piston causes an increase in the gas pressure in the cylinder.

5. The Following formula is used to calculate the PRESSURE P= F/A P: Pressure in Pascal (Pa) F: Force applied to the surface in Newton A: Area of the surface in m2 Atmospheric pressure: Gases, vapors and liquids exert pressure. The atmospheric air exerts a normal pressure upon all surfaces with which it is in contact, and is known as atmospheric pressure The atmospheric pressure at sea level is called standard atmospheric pressure.

6. Absolute pressure: A pressure of absolute zero can exist only in complete vacuum. Any pressure measured above the absolute zero of pressure is termed as an absolute pressure. The SI unit of pressure is Pascal or N/m2. Other pressure units are atm (atmospheres), bar and psi (pound per square inch). Low pressures are often expressed in terms of mm of water or mm of mercury. Standard atmospheric pressure has the following equivalent values: 101.3kPa; 10.3 m of water; 760 mm of mercury. Calculating pressure A force of 5000 N acts on an area of 400 mm2. Calculate the pressure in Pascals

7. 3.3 Pressure Measurement Any physical parameter can be measured either directly with the use of instruments, or indirectly with the use of a measurement system that includes a sensor/transducer, signal conditioner and processor, and a recorder. 3.3.1 Pressure measurement instruments Manometers Manometers are simple devices used to measure low pressures. Manometers are classified as Simple Manometers and Differential Manometers.

8. 1-A simple manometer is one which consists of a fluid-filled glass tube whose one end is connected to a point where pressure is to be measured and the other end remains open to atmosphere. An applied gas pressure causes the fluid height to shift proportionately. Figure 3.6: Simple U-tube manometer 2-A differential manometer is used to measure the difference in pressure between two points. It consists of a U-tube containing heavy liquid, and the two ends of the tube are connected to the two points to determine differential pressure. Figure 3.7: Differential manometer

9. Mechanical gauges Manometers are suitable for comparatively low pressure. For high pressures they become unnecessarily bulky. Pressure gauges are used for measuring medium and high pressures They employ different types of mechanical elastic elements such as bellows, diaphragms, and bourdon tubes (shown in figure 3.9) to measure the pressure. The elastic deformation of these elements is used to show the effect of pressure. Some of the commonly used pressure gauges are: • Bourdon tube pressure gauge • Diaphragm gauge • Vacuum gauge

10. 3.3.2 Pressure Sensors There are many types of pressure sensors; each of them operates differently based on the type of the sensor. In case of electrical transducers/sensors for measuring pressure, examples of transformations needed to convert pressure into electrical signal can be: • The change of resistance of an elastic material such as a wire; • The change in capacitance when the distance between the capacitor plates is varied; • Creation of a voltage by the movement of a coil within a magnetic field 1. Resistance-type Pressure Sensor: Such sensors/transducers include a strain gauge as a pressure sensing element. They contain a diaphragm with integrated strain gauges, which is deformed by the pressure. This deformation stretches/compresses the strain gauge causing a change in resistance value. Figure 3.10 shows an example of its application. The resistance increases when stretchedand decreases when compressed.

11. Strain gauge pressure sensor application 2. Capacitive-type Pressure Sensor: They use a thin diaphragm as one plate of a capacitor. The diaphragm is exposed to the process pressure on one side and to a reference pressure on the other. Changes in pressure cause the diaphragm to deflect which in turn causes the distance between capacitor plates to change. Therefore, the capacitance changes. Figure 3.11 shows a capacitive pressure sensor construction.

12. 3. Gas Pressure Sensor The gas pressure sensor has a membrane that flexes as pressure changes. This sensor is arranged to measure absolute pressure. One side of the membrane is a vacuum and the other side is open to the atmosphere. The sensor produces an output voltage that is linear to the absolute pressure. An amplifier circuit is used to amplify/condition the signal from the pressure sensor. Gas pressure sensor