I. MEASUREMENT OF TEMPERATURE

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Emerald Poole
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1 I. MEASUREMENT OF TEMPERATURE Most frequent measurement and control Direct contact: thermometer, Indirect contact: pyrometer (detect generated heat or sensing optical properties) 1. Definition of temperature Temperature relates to average translational kinetic energy of molecule due to heat (no motion at absolute temp., 0K). Different from heat measurement (joules, calories). 2. Temperature scales Fahrenheit, Celsius, Kelvin 9 F = C C = (F 32) 9 K = C EE323 Measurements of temp, strain, pressure, motion Page 1 of 40

2 3. The thermocouple (wider measuring range) Current flow when one junction is at different temperature from the other. Current is proportional to the temperature difference. EE323 Measurements of temp, strain, pressure, motion Page 2 of 40

3 2 more junctions EE323 Measurements of temp, strain, pressure, motion Page 3 of 40

4 Voltmeter leads should be in an isothermal block placed in a known reference temperature. V (T1 T2 ) V= thermoelctric (Seebeck) voltage, V α = Seebeck coefficient, V/ O C EE323 Measurements of temp, strain, pressure, motion Page 4 of 40

5 EE323 Measurements of temp, strain, pressure, motion Page 5 of 40

6 Ex: A general-purpose temperature probe is needed to cover the range from -100oC to +500oC. Select a thermal-couple and design a circuit to provide an output voltage proportional to temperature such that the voltage is -1.00V when the probe is -100oC and +5.00V at 500oC. EE323 Measurements of temp, strain, pressure, motion Page 6 of 40

7 4. The Resistance Temperature Detector. Resistivity of metal is a positive function of temperature. Constructed with fine wire of platinum, nickel, germanium. Mounted in enclosed case for protection from environment, strain, Rt = Rn(1 + t) R n = nominal resistance at 0 O C,Ω α= resistance coefficient, Ω/Ω/0 O C EE323 Measurements of temp, strain, pressure, motion Page 7 of 40

8 Use Bridge measurement, lead resistance important RTD Lead resistances EE323 Measurements of temp, strain, pressure, motion Page 8 of 40

9 5. Thermistors Resistance changes with temperature. Negative coefficient, non-linear Fast response, high sensitivity Small, encapsulated beads 1 T = A + B(ln R) + T= temperature, K 3 C(ln R) A,B,C = curve-fitting constants Use bridge measurement to detect small temperature changes (0.01 O C). EE323 Measurements of temp, strain, pressure, motion Page 9 of 40

10 EE323 Measurements of temp, strain, pressure, motion Page 10 of 40

11 Ex: A thermistor is to be used as a liquid sensor. When the level of the liquid reaches the height of the sensor, the pump being used to fill the tank is to be shut off by means of a 12V relay. Design a circuit to interface the thermistor to the pump relay. The liquid is at room temperature. The thermistor has an operating range of - 25 o c t0 110 o C. R 0 =100Ohms at 25 o C. EE323 Measurements of temp, strain, pressure, motion Page 11 of 40

12 6. Semiconductor temperature sensors Diode junction voltage as a function of temperature -2.2mV/ O C, low current, measure voltage. Low cost, limited range. i D = I s (e v V D T 1) 7. IC sensors Low cost, IC form Current or voltage output proportional to temperature. LM135, 2-terminal Zener diode breakdown voltage (+10mV/K) EE323 Measurements of temp, strain, pressure, motion Page 12 of 40

8. Radiation pyrometer Non-contact, measure remotely Detect infrared radiation from the source Converted absorbed radiation into voltage or

13 8. Radiation pyrometer Non-contact, measure remotely Detect infrared radiation from the source Converted absorbed radiation into voltage or current Used in high temperature, inaccessible location, harsh environment EE323 Measurements of temp, strain, pressure, motion Page 13 of 40

14 Ex: A remote control unit has a transmitting infrared LED with a peak radiant intensity of 150mW per steradian. The photodiode receiver has an active area of 10 square millimeters and it is 3m away from the remote control. The photodiode has a sensitivity of 0.6A/W and a dark current of 30nA in the photovoltaic mode. What is the incident radiant power (irradiance) on the photodiode? What is the expected photocurrent? Design a circuit which applies a 10V reverse bias to the photodiode and which creates a TTLcompatible low output voltage in the absence of input light and a TTLcompatible high output for the incident power expected at 3m. EE323 Measurements of temp, strain, pressure, motion Page 14 of 40

15 II. MEASUREMENT OF STRAIN A small force is applied to the length of a block, length changed (not permanently deformed). Elasticity As long as the material remains elastic, the change in length is proportional to the applied force (Hooke's law): F l EE323 Measurements of temp, strain, pressure, motion Page 15 of 40

16 Divide the change in length by the original length of the block, l F l Strain (µε): = l l EE323 Measurements of temp, strain, pressure, motion Page 16 of 40

17 Effect of the force on the strain is reduced by the area of the block: F A l l Young's modulus, E, N/m 2, is a property only of the material. Young's modulus is a measure of the stiffness of a material, (resist a change in length when loaded). Modified Hooke's law: F = A l E l Stress (force per unit area on a given plane within a body). = F A σ = stress, N/m2 EE323 Measurements of temp, strain, pressure, motion Page 17 of 40

18 Stress-strain relationship: σ = Eε EE323 Measurements of temp, strain, pressure, motion Page 18 of 40

19 1. The strain gage Resistance of wire (or a metallic bar): R = l A The bar undergoes a compression force, Length decreases Area increases. Volume un-changes. This causes the resistance of the bar to decrease: R R = (l l) A + A EE323 Measurements of temp, strain, pressure, motion Page 19 of 40

20 The change in resistance: R = GF l R l GF = constant called the gage factor, (dimensionless) EE323 Measurements of temp, strain, pressure, motion Page 20 of 40

21 Metallic strain gages are made from very thin conductive foils with a conductor folded back and forth to allow a long path for the conductive elements while maintaining a short gage length. The back-and-forth pattern is designed to make the gage sensitive to strain only in the direction parallel to the wire and insensitive in the direction perpendicular to the wire. Gages exhibit some transverse sensitivity, but the effect is generally small. Foils are made from special alloys such as constantan, a combination of 60% copper and 40% nickel. Standard foil resistances are 120Ω and 350Ω; some gages are available with resistances of up to 5000Ω. Bonded strain gages: made with a carrier adhesive on an electrically insulating. The gage is attached to the specimen with a special adhesive Unbonded strain gages consist of pre-tensioned wires assembled in some type of fixture. EE323 Measurements of temp, strain, pressure, motion Page 21 of 40

2. Measurement with strain gage Change in resistance within the elastic region is an extremely small quantity (i.e., requires sensitive instrumentation).

22 2. Measurement with strain gage Change in resistance within the elastic region is an extremely small quantity (i.e., requires sensitive instrumentation). Wheatstone bridge circuit is used to measure tiny change. Strain gage measurements are complicated by temperature (heating and specimen expansion and contraction). EE323 Measurements of temp, strain, pressure, motion Page 22 of 40

23 One arm Wheatstone bridge (quarter-bridge configuration) Dummy gage does not experience the strain; included for temperature compensation. Balance adjust the bridge (zeroed) with no strain. The gage is then subject to strain, output voltage is observed 4V = r GF(1 + 2V r ) EE323 Measurements of temp, strain, pressure, motion Page 23 of 40

24 V r (dimensionless) represents the difference in the ratios of the output to input voltage between the strained and unstrained condition: V V out out r = ( ) strained ( ) unstrained Vin Vin V The equations take into account the non-linearity of the bridge. This error is very minor at the low levels. Initially balanced (V out, unstrained = 0V): 4V = GF(V out in ) EE323 Measurements of temp, strain, pressure, motion Page 24 of 40

25 Half-bridge configuration (two active gages) o Best applied to measuring bending beams o Causes temperature effects, change the resistance of both gages in the same way (i.e., to be canceled). 2V = GF(V out in ) EE323 Measurements of temp, strain, pressure, motion Page 25 of 40

26 Full-bridge configuration (active gages in all four arms). o Two strain gages in opposite diagonals are in tension and the other two are in compression. o Temperature compensation can be included. V = GF(V out in ) EE323 Measurements of temp, strain, pressure, motion Page 26 of 40

27 EE323 Measurements of temp, strain, pressure, motion Page 27 of 40

28 III. MEASUREMENT OF PRESSURE Pressure = force per unit area P = F A (N / m 2 = Pa) In a static fluid, pressure increases in proportion to the depth, density of the liquid, and additional pressure acting on the surface (for example atmospheric pressure). P = gh P= pressure at the bottom of a liquid, Pa ρ=mass density, kg/m 3 h= height of the liquid, m With gases, pressure is exerted equally in all directions. EE323 Measurements of temp, strain, pressure, motion Page 28 of 40

29 1. Pressure transducers Principle of balancing an unknown pressure against a known load. Common technique is to use a diaphragm to balance the unknown pressure against the mechanical restraining force keeping the diaphragm in place. Diaphragm: a flexible disk that is fastened on its periphery and changes shape under pressure. A spring may be used to push against the diaphragm and provides a load. The amount of movement of the diaphragm is proportional to the pressure (i.e., the displacement of the diaphragm) EE323 Measurements of temp, strain, pressure, motion Page 29 of 40

30 Requires converting mechanical motion of the pressure-sensing element into an electrical signal. Conversion techniques: potentiometric, reluctive, capacitive, and strain gage methods. EE323 Measurements of temp, strain, pressure, motion Page 30 of 40

31 Potentiometric method converts the displacement of the sensor into a resistance (simple, less expensive, mechanical wear and is electrically noisy. Reluctive method changes the inductance of one or two coils by moving some part of the magnetic circuit. The coils are electrically connected in a bridge circuit. In capacitive transducers, the motion changes the capacitance of an internal capacitor (electrically connected into a bridge circuit). High-frequency response (due to low mass), quick response to changes in pressure. A strain gage can be used as a sensing element by bonding it to the diaphragm. Pressure on the diaphragm introduces strain, which is sensed by the gages and converted to an electrical resistance. Gages are bonded on both sides of the diaphragm and connected in a full- or halfbridge arrangement. Transducers may also contain temperature compensation and zero-balance resistors. EE323 Measurements of temp, strain, pressure, motion Page 31 of 40

32 Sensing element is a stain gauge EE323 Measurements of temp, strain, pressure, motion Page 32 of 40

33 IV. MEASUREMENT OF MOTION Motion can be rectilinear-along a straight line or it can be circular-about an axis. The measurement of motion includes displacement, velocity, and acceleration 1. Displacement transducers Contacting or noncontacting. Electrical output signal can be either a voltage or a current. Potentiometric displacement transducers are simple, can be designed to measure large displacements, (subject to wear and dirt and electrically noisy). Displacement can be converted into an electrical quantity using a variable inductor and monitoring the change in inductance. EE323 Measurements of temp, strain, pressure, motion Page 33 of 40

34 A related displacement transducer is the linear variable differential transformer (LVDT). The sensing shaft is connected to a moving magnetic core inside a specially wound transformer. As the core moves off-center, the voltage in one secondary will be greater than the other. The transducer has excellent sensitivity, linearity, and repeatability. EE323 Measurements of temp, strain, pressure, motion Page 34 of 40

35 EE323 Measurements of temp, strain, pressure, motion Page 35 of 40

36 Noncontacting displacement transducers include optical and capacitive transducers. Photocells can be arranged to detect light through holes in an encoding disk or to count fringes painted on the surface to be measured. Optical systems are fast, but noisy. Fiber-optic sensors make excellent proximity detectors for close ranges. The major disadvantage is limited dynamic range. Capacitive sensors can be made into very sensitive displacement and proximity transducers. The capacitance is varied by moving one of the plates of a capacitor with respect to the second plate. The capacitor can be used to control the frequency of a resonant circuit to convert the capacitive change into a usable electrical output. EE323 Measurements of temp, strain, pressure, motion Page 36 of 40

37 2. Velocity transducers Velocity = rate of change of displacement Can be determined by using a displacement sensor and measuring the time between two points. V=x/t=dx/dt Direct measurement with certain transducers that have an output proportional to the velocity to be measured (either linear or angular velocity). Linear velocity transducers: coil forming a simple motor by generating an emf proportional to the velocity. Angular velocity: tachometers provide a dc or ac voltage output. DC tachometers: speed and direction of rotation. AC tachometers: output frequency proportional to the rotational speed. EE323 Measurements of temp, strain, pressure, motion Page 37 of 40

38 3. Acceleration transducers Measured by use of a spring-supported seismic mass mounted in a suitable enclosure. A=dv/dt=d 2 x/dt 2 EE323 Measurements of temp, strain, pressure, motion Page 38 of 40

39 Damping is provided by a dashpot. Relative motion between the case and the mass is proportional to the acceleration. A secondary transducer such as a resistive displacement transducer is used to convert the relative motion to an electrical output. The mass does not move (ideally) when the case accelerates because of its inertia; in practice it does because of forces applied to it through the spring. The accelerometer has a natural frequency, the period of which should be shorter than the time required for the measured acceleration to change. Accelerometers used to measure vibration should also be used at frequencies less than the natural frequency. EE323 Measurements of temp, strain, pressure, motion Page 39 of 40

40 An accelerometer that uses the basic principle of the LVDT can be constructed to measure vibration. The mass is made from a magnet that is surrounded by coils. Voltage induced in the coils is a function of the acceleration. Another type of accelerometer: a piezoelectric crystal in contact with the seismic mass. The crystal generates an output voltage in response to forces induced by the acceleration of the mass. Piezoelectric crystals are small in size and have a natural frequency that is very high (measure high-frequency vibration). The drawback to piezoelectric crystals is that the output is very low and the impedance of the crystal is high, (subject noise problem). EE323 Measurements of temp, strain, pressure, motion Page 40 of 40

CHAPTER 9: TEMPERATURE, PRESSURE, STRAIN AND MOTION MEASUREMENTS

CHAPTER 9: TEMPERATURE, PRESSURE, STRAIN AND MOTION MEASUREMENTS I. MEASUREMENT OF TEMPERATURE 1. Introduction In industrial-process control, temperature is the most frequently controlled and measured