INSTRUMENTATION

INSTRUMENTATION Types Of Controls On/Off Control Continuous Control Control Elements: Pneumatic Control Valves Electrical Control Valves Hydraulic Cylinders Physical Quantities For Measurement Temperature Pressure Flow Level Speed Weight Displacement Gas Analysis Why Instrumentation? For measurement of physical parameters in the process and control of these parameters. Instrumentation Measurement Process Control Control System MEASUREMENTS Temperature Measurement Measuring Elements Thermometers Expansion Thermometers Filled System Thermometers Electrical Instruments Thermocouples Resistance Thermometers Thermistor Pyrometers Radiation Pyrometers Optical Pyrometers MEASUREMENTS Temperature Measurement The temperature of a substance or medium is a phenomenon expressing its degree of hotness or coldness. Various primary effects with respect to change in Temperature can be utilised for measurement: Change in physical or chemical state Variation in electrical parameters Generation of EMF Change in intensity of radiation Types Of Control Systems Open Loop Control System Closed Loop Control System MEASUREMENTS Temperature Measurement RTDs 3-Wire System 4-Wire System RTD RTD MEASUREMENTS Temperature Measurement Most Commonly Used Measuring Elements: THERMOCOUPLES Typical Arrangement: Transmitter E T Compensating Cable DCS Thermocouple PROCESS MEASUREMENTS Temperature Measurement Most Commonly Used Measuring Elements: THERMOCOUPLES PRINCIPLE: A current flows in a closed circuit made up of two dissimilar metals if the junctions of the two metals are kept at different temperatures. Thus the EMF generated is proportional to the temperature difference in a predictable manner. This phenomenon is known as Seebeck effect. Thermocouple Transmitter MEASUREMENTS Pressure Measurement Pressure is defined as the amount of force applied to a surface or distributed over it. It is measured as force per unit area. Different types of pressure: Gauge Pressure: Atmosphere pressure is considered as zero point. Absolute Pressure: Atmosphere pressure is included. Vacuum Pressure: Pressure below zero. Differential Pressure: Difference in two pressure heads. MEASUREMENTS Temperature Measurement RTDs (Resistance Temperature Detectors) PRINCIPLE : The resistivity of metals increases or decreases with increase in temperature. Such variations can be measured precisely with suitable electrical circuits Used for low temperature, high accuracy measurements. RTD Transmitter MEASUREMENTS Pressure Measurement Most Commonly Used Measuring Elements: MANOMETERS Principle: Orifice Flow High Pressure Tap P1 h1 Scale Low Pressure Tap P2 h2 P = (p- p₁) g h Where p= Density of fluid in U tube p₁= Density of fluid in pipe h = h2-h1 g = acceleration due to gravity MEASUREMENTS Pressure Measurement Measuring Elements Manometers Elastic Transducers Electrical Transducers MEASUREMENTS Temperature Measurement Selection Criteria Temp Range Type of transducer Type 900 degC RTD Platinum 400 degC RTD Nickel 700 degC Thermocouple T (Cu-Constantan) 900 degC Thermocouple Y (Fe-Constantan) 1370 degC Thermocouple K (NiCr-Ni) 1700 degC Thermocouple R (Pt87,Rh13-Pt) 3700 degC Radiation Pyrometer S (Pt90,Rh10-Pt) MEASUREMENTS Pressure Measurement Most Commonly Used Measuring Elements: DIAPHRAGM TRANSDUCERS Principle: DIFFERENTIAL PRESSURE 1 Input pressure P+ 2 Process flange 3 O-ring 4 Body of the measuring cell 5 Silicon pressure sensor 6 Overload diaphragm 7 Seal diaphragm 8 Filling liquid 9 Input pressure P- MEASUREMENTS Pressure Measurement Most Commonly Used Measuring Elements: Typical Arrangement: DCS PROCESS MEASUREMENTS Pressure Measurement Most Commonly Used Measuring Elements: DIAPHRAGM TRANSDUCERS Principle: PRESSURE 1 Reference pressure 2 Measuring cell 3 Process connection 4 Seal diaphragm 5 Filling liquid 6 Silicon pressure sensor Pg Input variable pressure MEASUREMENTS Flow Measurement Most Commonly Used Measuring Elements: MAGNETIC FLOWMETERS Principle: Uses Faraday's Law Of Electromagnetic Induction- 'Whenever a conductor moves through a magnetic field of given strength, a voltage is induced in the conductor which is proportional to velocity between the conductor and the magnetic field' In case of magnetic flow meter, electrically conductive flowing liquid acts as a conductor.