MEASUREMENT, TRANSMITTERS AND FILTERING

Transcription

1 MEASUREMENT DEVICE CHE302 LECTURE II MEASUREMENT, TRANSMITTERS AND ILTERING Professor Dae Ryook Yang all 2001 Dept. of Chemical and Biological Engineering Korea University CHE302ProcessDynamicsandControl Korea University 2-1 INTRODUCTION TO SENSOR What is Sensor? Sensor converts the physical quantity to signal that can be recognized by other components such as display, transmitter and etc. PhysicalVariable (T, P,, L, x, ) Sensor Measurement (V, ma, psig, ) Sensor types Temperature: thermocouple, RTD, thermister Pressure: bels, bourdon tube, diaphragm rate: orifice, venturi, magnetic, ultrasonic, Coliolis effect Liquid level: float, differential pressure ph: ph electrode Viscosity: pressure drop across venturi or vane deflection Composition: density, conductivity, GC, IR, NIR, UV CHE302ProcessDynamicsandControl Korea University 2-2 Transducer: Sensor+Transmitter Transmitter generates an industrial standard signal from the sensor output. Standard instrumentation signal levels Voltage: 1~5VDC, 0~5VDC, -10~+10VDC, etc. Current: 4~20mA (long range transmission with driver) Pneumatic: 3-15psig Signal conversion I/P or P/I transducer: current-to-pressure or vice versa I/V (I/E) or V/I: current-to-voltage or vice versa P/E or E/P: pressure-to-voltage or vice versa Analog-to-Digital (A/D) converter Continuous signal converted to digital signal after sampling Specification: sample rate, resolution (8bit, 12bit, 16bit) CHE302ProcessDynamicsandControl Korea University 2-3 TRANSMITTERS Transmitter Gain (K m ): adjustable Amplification ratio: (output span)/(input span) Span and Zero: adjustable Span: magnitude of range of transmitter signal Zero: er limit of transmitter signal Ex) Temp. Transmitter (20 ma 4mA) K m = = 0.16 [ ma / C] (150 C 50 C) span=100 C zero=50 C T m (ma) Other functions: square-root extractor, T(ºC) CHE302ProcessDynamicsandControl Korea University zero Actual span Ideal

2 Principle Thermal Expansion Resistance EM Radiation TEMPERATURE SENSORS Gas expansion Liquid Bimetal Type Pt-100 Thermistor Thermocouple IC temp. sensor Pyrometer Usable range (ºC) -230~ ~350-50~ ~500 < ~ ~150 Very wide N2 Oil Accurate, linear, self heating Cheap, inaccurate, nonlinear Low sensitivity Remarks High voltage, accurate, linear Noncontacting, need accurate calibration TemperatureSensorAdvantages/Disadvantages Sensor Advantages Disadvantages Thermocouple RTD Thermistor Infrared Self -powered Simple Rugged Inexpensive Wide variety Wide range Most stable Mostaccurate More linear than thermocouple High output ast Two-wire ohms measurement No contact required Very fast response time Good stability over time Highrepeatability No oxidation/corrosion to affect sensor Non-linear Low voltage Reference required Least stable Least sensitive Expensive Currentsourcerequired Small Low absolute resistance Self heating Non-linear Limited range ragile Currentsourcerequired Self heating High initial cost More complex/support electronics Spot size restricts application Emissivity variations affect readings Accuracy affected by dust, smoke and background radiation CHE302ProcessDynamicsandControl Korea University 2-5 Temperature Sensor Attributes Criteria Thermocouple RTD Thermistor Cost-OEM Quality Low High Low Temperature Range Very wide -450º +4200º Wide -400º +1200º Shot to -100º +500º Interchangeability Good Excellent Poor to fair Long-term Stability Poor to fair Good Poor Accuracy Medium High Medium Repeatability Poor to fair Excellent air to good Sensitivity (output) Low Medium Very Response Medium to fast Medium Medium to fast Linearity air Good Poor Self Heating No Very to High Point (end) Sensitive Excellent air Good Lead Effect High Medium Low Size/Packaging Small to large Medium to small Small to CHE302ProcessDynamicsandControl Korea University 2-6 CHE302ProcessDynamicsandControl Korea University 2-7 Metal 1 Metal 2 THERMOCOUPLE Ice Point Thermocouple Types Chromel- alumel (K- type): most popularly used Iron- constantan (J- type): er electromotive force ( emf) Chromel- constantan (E- Type): cryogenic temperature 13% Rh. Pt Pt (R- type): temperature (> 900 ºC) Typical emf is about 0.041mV/ºC for K type Needs signal amplification Ice point can be a ice bath or an electronic device to compensate the ambient temperature. CHE302ProcessDynamicsandControl Korea University 2-8 V

3 Thermocouple type B (Platinum / Rhodium) E (Chromel / Constantan) J (Iron / Constantan) K (Chromel / Alumel) N (Nicrosil / Nisil) R (Platinum / Rhodium) S (Platinum / Rhodium) T (Copper / Constantan) Overall Range ( C) 100~ ~ ~ ~ ~ ~ ~ ~400 EM (mv/ C) B,R,S: temp. sensitivity, cost S: very stable, use as the standard of calibration for the melting point of gold ( C). N: improved type K, getting more popular T: cryogenic use Ref: CHE302ProcessDynamicsandControl Korea University 2-9 LOW MEASUREMENT (1) Differential Pressure Cell Q = 1 C A P: Delta P across the orifice A 1 : area of f pipe A 2 : area of orifice 2 ( A / A ) ρ 2 1 C d : orifice coefficient d 2 2gc P Maximum pressure drop should be < 4% of the total line pressure Selection of orifice size and delta P range is very important for the reading precision CHE302ProcessDynamicsandControl Korea University 2-11 LOW MEASUREMENT(2) T E J N K R S B Vortex Meter The vortices create and pressure zones behind the bluff body. The vortex meter uses a piezoelectric crystal sensor to detect the pressure exerted by the vortices on the sensing wing. The piezoelectric crystal converts this vortex shedding frequency into electrical signals. Electromagnetic Meter Electrically conducting fluid passing through a magnetic field created by the device. CHE302ProcessDynamicsandControl Korea University 2-10 CHE302ProcessDynamicsandControl Korea University 2-12

4 CORIOLIS LOWMETER (3) SELECTION O LOWMETERS rate is measured by Coriolis effect (1835) Mass frate, vol. frate, temp. and density are simultaneously measured. Requirement accuracy Orifice ±2~4% of full span Positive displace ment ±0.2~0.5% Vortex ±1.0% Electro- Magnetic ±0.5% Acoustic ±1~5% of full span Coriolis ±0.5% Press. loss Initial Cost very Maintenance cost Application liq.; some Clean viscous liq. liq.; viscous conductive liq. and Dirty, viscous liq. and viscous liq. and some Upstreampipe size 10~30 None 10 to to 30 Viscosityeffect CHE302ProcessDynamicsandControl Korea University 2-13 Rangeability 4 to 1 10 to 1 10 to 1 40 to 1 20 to 1 10 to 1 CHE302ProcessDynamicsandControl Korea University 2-15 LOW MEASUREMENT (4) Ultrasonic Meter High accuracy No contact with f loat level sensor LEVEL MEASUREMENT Ultrasonic level sensor Use of DP cell Measure fluid head as Delta P Various implementation Positive Displacement Meter Turbine, gear, wheels Thermal Dispersion Meter over heating coil will change temperature CHE302ProcessDynamicsandControl Korea University 2-14 CHE302ProcessDynamicsandControl Korea University 2-16

5 OTHER MEASUREMENTS Composition measurements Expensive Long time delay High to maintenance cost Gas Chromatography IR, NIR, Raman, UV spectrophotometer ph sensor electrode: concentration of [H+] Secondary Measurements Density or temp. for binary composition Soft Sensors Estimated by a model based on other measurements The filter behaves as an interpolation between the measured output and previous filtered output. If =1, the measured output is ignored. (constant) If =0, the filtered output is same as the measured output (no filtering) If τ =0, =0 and no filtering is achieved. If =, =1 and the measured output is ignored. τ As τ Lighter filtering increases, heavier filter is applied. Heavier filtering CHE302ProcessDynamicsandControl Korea University 2-17 CHE302ProcessDynamicsandControl Korea University 2-19 ILTERING Noise Source Process nature (turbulence, vibration, oscillation ) Various noise source from environment Power line, electromagnetic force, etc. Removing noise dy Analog filter τ + y = y dt ilter time constant iltered output Measured output irst-order filter analogy 0 y y Previous filtered output τ + y = y t 0 τ / t y = y + (1 ) y where = (0< 1+ τ / t < 1) CHE302ProcessDynamicsandControl Korea University 2-18