ASEN 2002 Experimental Laboratory 1: Temperature Measurement and an Blow Dryer Test

Transcription

1 ASEN 2002 Experimental Laboratory 1: Temperature Measurement and an Blow Dryer Test Assigned 6 September 2000 Individual Lab Reports due 3 October 2000 OBJECTIVES Learn the basic concepts and definitions associated with the temperature and temperature measurements. Develop awareness of sources of error in temperature measurements. Build thermocouples and use them for temperature measurement. Use temperature and power measurements to compute the efficiency of a commercial blow dryer. REQUIRED DELIVERABLES Attendance at every lab period is required. Instructions for weekly tasks and the individual report will be presented during the scheduled lab time. Prepare a written report of the results of your laboratory exercises. Use the guidelines for short and precise report style. WEEK OF Sept 4 1. Learn the basic concepts and definitions associated with temperature and temperature measurement. a) Read the background material at the end of this handout and the more detailed thermocouple discussion in National Instruments Application Note 043 ( b) What are sources of measurement error? 2. Identify quantities to be measured and required instruments. a) What are the quantities to be measured? b) What instruments will be used? c) Record instrument information including pertinent manufacturer information. 3. Build a thermocouple for an ice-water reference point and one for software cold-junction compensation. a) What is a thermocouple welder and how does it work? b) What are the range, precision, and readability of the voltmeter? (Know your equipment) WEEK OF Sept Perform simple measurements (Perform as a group and share data among group members). a) Use a water-ice reference and a hand-held voltmeter to record the room temperature. Use the voltagetemperature conversion from National Instruments Application Note 043. b) Use the same thermocouple to measure and record the body temperatures of your group. (Measure the temperatures in the bend of the elbow.) Is there a significant variation in your temperatures? Exchange 1

2 data with at least two other groups. Are there significant differences? Is there a gender difference? Should there be a gender difference? c) Use the same thermocouple to measure the temperature of ¼-liter of water as it is heated to boiling. Why is the boiling temperature higher or lower than 100 C? d) Now use a thermocouple with software cold-junction compensation. Repeat measurements ii) and iii). Explain any discrepancies; discuss accuracy of measurements. WEEKS OF Sept 18 & 25 The Instrumented Hair Blower is a modified Conair Model 068 Hair Dryer. This dryer has three modes of operation: High, Low and Cool. High delivers maximum heat output with the fastest blower speed. Low delivers less heat output with a lower fan speed. Cool (a misnomer) delivers heat output, which is even less than Low and rather should be called the Warm mode. In Cool mode, blower speed is set by the High/Low switch and can be either fast or slow. There are three heater elements in the dryer. Two are 20 ohm and one is 48 ohm in resistance. High mode energizes both 20 ohm heaters. Low mode energizes only one 20 ohm heater. Cool mode disables both 20 ohm heaters and energizes only the 48 ohm heater. The fan motor is DC powered by either a full-wave (FW) or half-wave (HW) bridge rectifier. Switching between FW and HW changes motor speeds. This is controlled by the High/Low switch. A thermostat in the power neutral line cycles to prevent overheating. A thermal fuse is also in the power line to prevent catastrophic overheating. Attached to the dryer intake vent is a TurboMeter, wind speed indicator. Air flow spins the TurboMeter fan. Internal electronics convert spin rate into units of Feet/ Min, MPH or M/Sec. Description of Instrumentation: Refer to Figure 1 (to be distributed). The Hair Blower module is attached to Labstation using one MS connector, P3. All data and instrumentation power flows through this cable and is processed by the Labstation SCXI 1141 module. The wind speed data from the Turbometer is a pulse train that is buffered and presented to Labstation for processing. The Turbometer must be placed in the M/S position for normal operation. Variations between the Turbometer and the Labstation data should be ignored. (The Labstation will be more accurate as it is now calibrated for reverse direction air flow thru the TurboMeter.). For the purpose of this experiment we consider the flow rate between inlet and outlet as constant. Power consumption is the product of applied voltage X series current ( P = V I). Applied voltage to the 20 ohm heaters ( 1 & 2) is the line voltage (120 VAC typ.). A voltage divider (95.3 KΩ and 2.75 KΩ ) reduces the line voltage by a factor so it is within the measurement range of Labstation. Corrections for this factor are made in software. Current is calculated by measuring the voltage across a 0.05 Ω precision resistor placed in series with the heaters. Software calculations convert the measured voltage to a current. Then a simple power calculation is made. Applied voltage to the 48 Ω (Cool), heater 3 is line voltage minus the fan motor voltage. Again a voltage divider (95.3 KΩ and 2.75 KΩ ) reduces this voltage by a factor Another 0.05 Ω resistor is placed in series with heater 3 to measure current. Again conversions and calculations are done in software to calculate power consumed in heater 3. 2

3 Fan motor power consumption is difficult to monitor and calculate. It is AC power converted to DC power using bridge rectifiers. You may assume this is an experimental constant with known value as follows: Low Speed- 17 watts High Speed - 43 watts Set up procedure: Refer to Figure 2 (to be distributed). Note: This setup normally is provided by TA s but should be checked by students prior to doing experiment. 1. Connect MS connector P3 to Labstation. Note choice of Side A or B. 2. Connect +5VDC from Labstation fixed supply (small box unit - under top shelf) to Extra 6 (+). 3. Connect Ground from fixed supply to Analog ground. 4. Connect Extra 6( - ) to Buffer circuit input. 5. Connect Buffer circuit output to CTR1 SRC (+) using RG cable. 6. Connect RG cable from CTR1 gate to CTR0 out. 7. Turn ON +5VDC and observe lighted LED display on power supply and on TurboMeter. 8. Locate and initiate appropriate Labview VI s. WS Lund 8/29/00 Tasks: With your own thermocouples measure T at inlet and T at exit Carefully describe the positions where you measured.(compare different locations and discuss the findings). Compare different thermocouples from other groups, discuss accuracy. What are error-generating features/issues. Operate blow dryer in several modes: cool-mode, hi- and low- modes. Plot the instantaneous power consumption for each mode. Determine the rate of energy transfer to the air during the drying process for each mode. Determine the efficiency of the blow dryer for each operating mode Think of any other discovery learning experience. 3

4 BACKGROUND Temperature and Temperature Scales We usually think of temperature in terms of the adjectives hot and cold, i.e., hot associated with a higher temperature than cold. We also know from common experience that a form of energy called heat flows in a direction from hot to cold. Two bodies in thermal equilibrium are at the same temperature so there is no heat flow between them. This is essentially a statement of the zeroth law of thermodynamics. Since substances behave predictably with respect to temperature, the physical behavior of a substance can be used to create a temperature scale. Two examples of temperature scales are the Fahrenheit and Celsius scales. Each of these is a two-point scale, based on the freezing and boiling points (the ice and steam points) of water. Note that the ice point is defined as an equilibrium mixture of ice, liquid water, water vapor, and air at 1 atm pressure. The steam point is defined as a mixture of liquid water and water vapor without air. The freezing and boiling points are arbitrarily assigned a value of 32 F and 212 F, respectively, for the Fahrenheit scale and 0 C and 100 C, respectively, for the Celsius scale. The Rankine (R) and Kelvin (K) temperature scales are the absolute Fahrenheit and Celsius scales, respectively. The zero point of both absolute scales corresponds to the lowest temperature possible. Note that T(K) = T( C) and T(R) = T( F). The relations between the common Celsius and Fahrenheit scales and the corresponding Kelvin and Rankine absolute scales are: T(K) = T( C) T(R) = T( F) T(R) = 1.8 T(K) T( F) = 1.8 T( C) + 32 Thermometers Fluid-in-Glass Thermometer A thermometer is a device used to measure temperature. The molecular structure of matter responds predictably to temperature changes. This is manifested through changes in certain variables such as volume, electrical resistance, thermal expansion, etc. The thermometer uses the predictable changes in physical variables to measure temperature. For instance, the mercury-in-glass or alcohol-in-glass thermometers that were often used to measure body temperature or the daytime temperature in the shade, are based on the principle that over certain temperature ranges, these liquids predictably expand (contract) with an increase (decrease) in temperature. The bulb of the in-glass thermometer is immersed in the medium for which it is measuring the temperature. The measurement is made once the fluid inside the bulb is in thermal equilibrium with the medium. The location of the meniscus in the capillary tube depends on the volume change of the thermometer fluid. An increase in temperature above some initial value causes the fluid to expand and the meniscus to rise; for a temperature decrease, the meniscus will fall. An instrument is calibrated by comparing its output to a known standard or known input. The thermometer is calibrated for the Celsius, for instance, by assigning marks on the tube associated with the height of the fluid at the ice and steam points. Then a scale of 100 equal increments between these points corresponds to the Celsius temperature scale. Because the glass also expands, the calibration actually corresponds to the difference in expansion of the thermometer fluid and the glass capillary. Thermocouples A transducer is a device that transforms physical variables into corresponding electrical signals. When two dissimilar metals are brought into contact, an electromotive force (emf) will be generated, this thermoelectric effect is the Seebeck effect and the resultant emf is called the Seebeck voltage. For certain materials, over specific temperature ranges, the Seebeck voltage can be accurately predicted with a polynomial approximation. This predictable behavior is the basis of the thermocouple, a transducer that uses the thermoelectric properties of dissimilar metals in contact to measure temperature. 4

5 metal 1 junction metal 2 voltmeter Unless the terminals of the voltmeter are made of exactly the same material as the wires that are attached to it, a Seebeck voltage is generated and must be accounted for also. To get around this problem, the leads and the terminals can be of the same material and a second junction is created and kept at a known reference temperature. As shown below, the reference junction is often put into an ice-water mixture that is at a known reference temperature. More sophisticated methods use hardware or software to compensate for the additional voltage at the reference junction. A detailed thermocouple tutorial is provided in National Instruments Application Note 043. metal 1 measuring junction metal 1 voltmeter metal 2 reference junction ice-water 5

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