MAE 334_09 Lab 1 Page 1 of 15 Department of Mechanical and Aerospace Engineering MAE 334 - Introduction to Instrumentation and Computers Laboratory 1 Static and Dynamic Calibration of Thermocouples Objectives This introductory experiment will give you a first experience in our Instrumentation and Systems Lab. You can expect to become familiar with the most basic concepts of using transducers for measurement and the acquisition of data with a computer based data acquisition system. You will use the National Instrument’s software product called Virtual Bench which is a part of the LabView data acquisition package. Data is gathered through an analog to digital converter (ADC) which is also a National Instrument’s product. The data acquisition system will be reading the output voltages of a temperature sensor called a thermocouple which you will calibrate and use to estimate several water and air temperatures. Specifically, the objectives of the experiment are: (1) to become familiar with thermocouples as very common transducers for measuring temperature; (2) to carry out a formal static calibration of a thermocouple as a typical transducer, in order to explore the linearity of a typical calibration curve and to evaluate the accuracy of measurements that can be made using the calibration; (3) to perform a dynamic calibration of a thermocouple considering a first order dynamic model of its transient behavior and to explore the factors which influence the dynamic response of thermocouples. Background Thermocouples are probably the most widely used industrial and laboratory temperature sensor because they are simple, relatively inexpensive, and reasonably linear. The thermocouple outputs a small electromotive force (emf or voltage) which is proportional to the temperature difference between the two junctions of the sensor. This small voltage can be readily conditioned and sampled by a data acquisition system. TheMAE 334_09 Lab 1 Page 2 of 15 Figure 1. The lab setup showing the thermocouples, thermometer and data acquisition inputs on the right physical phenomenon which creates the voltage produced by the thermocouple is referred to as the Seebeck effect – where a voltage is produced when two dissimilar materials are connected together at junctions which are at different temperatures. See your text1 for details (Section 7.5 or 8.5 depending on the edition). You will also want to read over the material in our “Further Background” section at the end of this write-up. The equipment used in this experiment includes an accurate dial thermometer, a commercial thermocouple probe and a simple “homemade” thermocouple of copper and constantan. You will use the digital multimeter (DMM) and the data logger of National Instrument’s Virtual Bench. There is also a collection of Styrofoam cups for you to use in providing multiple temperature sources for your calibrations and transient experiments. Figure 1 shows a typical lab station. Getting Started Start the PC operations by logging in with your user name and password. Of course, if the PC is powered off, turn it on first. After you have logged in, you should find that your Engineering home directory maps as drive N on the laboratory PC. You will want to save your data here at the end of your lab sessions. You can also use the “My 1 Figliola, R. S., Beasley, D. E., Theory and Design for Mechanical Measurements, John Wiley, New York.MAE 334_09 Lab 1 Page 3 of 15 Documents” folder on the PC during your lab which will normally store directly into your Engineering home directory. You can use the “Temp” directory on the laboratory computers for temporary storage but don’t rely on it as being a permanent storage space for you from lab to lab. USB flash disks are ideal to carry your data home or to other computers. Always make sure you have backed up your data files. Some of our saddest experiences are with students losing data. Also, don’t forget to Logoff when you have finished your lab session to close your login (and access to your account). Part 1 – Static Calibration Setting Up the Virtual Bench DMM for Thermocouple Measurements Place the dial thermometer on top of the aluminum input box for the analog to digital converter (ADC). This is to estimate the reference junction temperature which is approximately the lab air temperature. Note the thermocouple probe and ADC cable connections. The thermocouple probe should be connected to ADC input channel 0 and a ground connection should be in place between the digital output ground and the ADC input ground of channel 0. Please note that the “first” input channel is usually referred to as channel zero (this makes sense in a computer world). But sometimes the first channel is also referred to as channel 1. This may seem confusing but it is a continuing issue so you should just “get used to it”. Click on the desktop icon for the VirtualBench. Then look for the VirtualBench DMM (Digital Multi-Meter) in the options window that opens. Click on this and when the DMM opens click on Edit, Settings, General Tab. Set the Channel to 0 and the Sample Rate to Slow. See Figure 2. Figure 2. The DMM General Settings WindowMAE 334_09 Lab 1 Page 4 of 15 Under the Edit, Settings, Temperature Tab (Figure 3), set the Temperature Sensor to "T Thermocouple" (for copper-constantan), the Temperature Units to "Celsius", the TC Cold Junction Source to "Constant" and the Cold Junction Temperature (Degrees) to your dial thermometer measurement. If your thermocouple measurements show obvious offset as you begin your calibration runs, this Junction Temperature may be adjusted. Keep in mind that the ice bath is the very best known temperature in this experiment and it’s always 0 C. In the main DMM window (Figure 4) set the Range to Auto, the Mode to Volts DC and, when ready, click the Run Continuously button. The DMM is switched over to a direct temperature reading mode by clicking on the thermometer icon as shown in Figure 5. Making the Temperature Measurements 1. Obtain a supply of crushed ice and tap water including a cup of hot water. With the ice and hot and cold tap water, produce five cups