Evaluation copy. Build a Temperature Sensor. Project PROJECT DESIGN REQUIREMENTS

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1 Build a emperature Sensor Project A sensor is a device that measures a physical quantity and converts it into an electrical signal. Some sensors measure physical properties directly, while other sensors use conversions or calculations to determine the value. Sensors are usually categorized by the type of property that they measure. A good sensor is sensitive to the property under investigation, but should have limited influence on the property being measured. For example, a temperature sensor should be small in size and made from a material with good response. Inserting a very large or very cold thermometer into a hot liquid will cool the liquid somewhat as heat is transferred to the measuring device. Sensors usually need to be calibrated. Sometimes the calibration relationship is a simple linear one and other times it is more complex. emperature sensors are often built from electronic components called thermistors. A thermistor is a device whose resistance varies with temperature (the name comes from a combination of the terms thermal and resistor ). ypical thermistors are made from ceramic semiconductors or from platinum wires wrapped around ceramic mandrels or spindles. hermistors usually have negative temperature coefficients (NC), meaning the resistance of the thermistor decreases as the temperature increases. Depending on the material and fabrication process, the typical operating range for thermistors is 50 C to 50 C. he small size of most thermistors results in a rapid response to temperature changes making them very useful for control systems requiring quick feedback. hey are very rugged and better able to handle mechanical vibration or thermal shock than other temperature sensors. hermistors have excellent interchangeability due to their low cost, precision, and tolerance over a temperature range. hermistors are used extensively in many applications, including automobile engines, digital thermostats, rechargeable battery packs, circadian rhythm devices, and fluid-flow measurements. Evaluation copy PROJEC DESIGN REQUIREMENS In this Project, you will build and calibrate a temperature sensor. First, you will construct a voltage divider circuit using a thermistor. hen you will write a LabVIEW program to convert the raw voltage reading of the thermistor into Celsius temperature units. Your program should Engineering Projects with NI LabVIEW and Vernier Vernier Software & echnology P -

2 Project simultaneously collect data from a commercial sensor to verify the readings from your homemade sensor. he temperature values from your homemade sensor and the commercial sensor, as well as the thermistor voltage readings, should be displayed on the front panel. MAERIALS SensorDAQ, LabQuest, or LabQuest Mini LabVIEW computer USB cable Vernier Surface emperature Sensor Vernier Breadboard Cable thermistor 5 k resistor breadboard PROJEC SEUP Construct a voltage divider circuit. Wire the resistor and thermistor to the Breadboard Cable (see Figure ). Figure Wiring diagram for temperature sensor 2. Insert the BA connector on the Breadboard Cable into Channel on the interface. Connect the commercial sensor to the interface. Connect the Surface emperature Sensor to Channel 2 on the interface. 2. Connect the interface to the computer. If you are using a LabQuest, make sure it is turned on. PROJEC BACKGROUND INFORMAION In this Project, you can determine the temperature of a thermistor if the resistance of the thermistor is known. he resistance-temperature relationship of a thermistor is nonlinear, but can be approximated using the Steinhart-Hart model. his model gives the absolute temperature of a thermistor (in Kelvin) as a function of its resistance. K 0 K lnr K ln R 3 2 P - 2 Engineering Projects with NI LabVIEW and Vernier

3 Build a emperature Sensor he resistance of the thermistor (R ) can be measured indirectly by placing it in a voltage divider circuit with a known resistor (R ) as shown in the diagram below. Figure 2 Voltage divider circuit his is a common circuit design used to measure resistance. he output voltage, V out, is related to V in as follows: V out V in where V in is the voltage supplied by the interface. In this Project this voltage comes from the pin labeled +5V on the breadboard connector. o start with, use the typical value for your interface. ypical values depend on the interface used: SensorDAQ=5.08 volts, LabQuest=5.3 volts, and LabQuest Mini=5.29 volts. V out is the measured voltage from your homemade sensor, and R is the value of the resistor placed in series with the thermistor (R ). From the equation above, you can algebraically solve for the thermistor resistance. V out R R Vin V out his equation can be simplified, because the circuit resistance is known (R = 5,000 ). R in R out R Vout 5000 V V You can configure the Vernier Analog Express VI to read the raw voltage from your homemade sensor. When the Analog Express VI is placed on the block diagram, a configuration window appears. he commercial sensor plugged into Channel 2 is automatically identified; however, since you are building a custom sensor in Channel, you will need to manually configure this channel. Under Configured Channels, select the Manual option and then click the Add Channel button at the top of the window. You will be choosing the 0 to 5 V option under CHANNEL in the Activate Channel window. Note: Do not change the values for the K0, K, and K2 coefficients in the configuration window of the Analog Express VI. R Engineering Projects with NI LabVIEW and Vernier P - 3

4 Project Figure 3 Configuration window for manual setup of a custom sensor he Analog Express VI can be used in your program to measure the raw voltage of your homemade sensor after it has been configured and closed. With these measurements you can solve for the resistance of the thermistor and substitute that value into the Steinhart-Hart equation to solve for the temperature. Since the Steinhart-Hart equation calculates temperatures in Kelvin units, you should subtract degrees from your final answer to display your temperature values in degrees Celsius as shown in the equation below. his will allow for an easier comparison to the commercial sensor. We recommend using the Formula Express VI (found in the Express Arithmetic & Comparison function palette) to build the equation. K 0 K ln R K ln R PROJEC IPS. he Steinhart coefficients (K0, K and K2) for your thermistor can be found from your manufacturer s published data. For the thermistor we recommend, the coefficients are: K0 = , K = , and K2 =.33342E A good data collection rate for the Surface emperature Sensor is 0 samples per second. 3. he Analog Express VI collects data for a fixed amount of time. For an indefinite data collection length, check the Repeat option in the Analog Express VI s Set iming configuration window. 4. We recommend using the Vernier Surface emperature Sensor for this Project; however, the Vernier Stainless Steel emperature Probe will also work. Be aware that its response will be much slower due to the stainless steel casing on the probe. he steel acts as a heat sink P - 4 Engineering Projects with NI LabVIEW and Vernier

5 Build a emperature Sensor causing the sensor to gain and lose heat more slowly. After a short period of time, however, the Vernier Stainless Steel emperature Probe should read approximately the same temperature as your homemade sensor. 5. Refer to Appendix E for additional information about the Vernier Surface emperature Sensor and Breadboard Cable. PROJEC ROUBLESHOOING If the thermistor value does not match the temperature reading of the commercial temperature sensor in Channel 2,. Measure the voltage supplied by the interface (V in ). You can measure the voltage using a digital multimeter or a Vernier Differential Voltage Probe. Substitute the actual voltage into your equation and see if that improves the calibration. 2. Make sure the calculations in the program are correct. Run these calculations as a standalone VI with some test values for the Raw Voltage. For the thermistor we recommend, a Raw Voltage of 3.3 should give a emperature Out value around 0 C; a voltage of.7 should give a temperature around 30 C; and a voltage of 0.2 should give a temperature around 00 C. hermistors with different Steinhart coefficients will have different test values. 3. Measure the resistance (R ). You can measure the resistance using a digital multimeter. Substitute the actual values into your equation and see if it improves your calibration. 4. Check the Steinhart coefficients (K0, K and K2) for your thermistor against your manufacturer s published data. Engineering Projects with NI LabVIEW and Vernier P - 5

6 Project CHALLENGE DESIGN REQUIREMENS Note: Do not attempt the Challenge until you have completed the Project Design Requirements. Write a LabVIEW subvi to give the user the ability to display temperature values in Kelvin or in degrees Celsius or Fahrenheit. Modify your original Project program to change the units simultaneously on both your homemade sensor and on the Vernier Surface emperature Sensor. he user should be able to switch between units while the program is running. CHALLENGE BACKGROUND INFORMAION One way to make a subvi is to convert an existing section of code. Add the equations for unit conversions to your original Project program. o convert Celsius temperature units to Kelvin, add o convert Celsius degrees to Fahrenheit degrees, use the following equation. Fahrenheit =.8 Celsius + 32 Use the Positioning tool to select the section of the block diagram you want to turn into a subvi and then choose Create SubVI from the Edit menu. he selected code is automatically turned into a functional subvi that can be saved and reused. P - 6 Engineering Projects with NI LabVIEW and Vernier

7 Vernier Lab Safety Instructions Disclaimer HIS IS AN EVALUAION COPY OF HE VERNIER SUDEN LAB. his copy does not include: Safety information Essential instructor background information Directions for preparing solutions Important tips for successfully doing these labs he complete Engineering Projects with NI LabVIEW and Vernier manual includes 2 projects as well as essential teacher information. he full lab book is available for purchase at: Vernier Software & echnology 3979 S.W. Millikan Way Beaverton, OR oll Free (888) (503) FAX (503) info@vernier.com