BME 3511 Bioelectronics I - Laboratory Exercise #1A. Digital Multimeters

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1 BME 3511 Bioelectronics I - Laboratory Exercise #1A Digital Multimeters Introduction: Electrical measurements are essential techniques for trouble shooting electronic equipment/circuits. The three quantities of voltage, current, and resistance are the basis for most analyses of both constant (direct current DC) and time variant (alternating current AC) circuits. Frequency, capacitive reactance, and inductive reactance are three additional factors related to AC circuits. Multimeters are the most frequently used instrument to measure current, voltage, and resistance. Digital Multimeters (DMM) are convenient, accurate, portable, and durable. MCM DMM Model is an inexpensive, yet relatively accurate DMM that will be used in BME Note on Biomedical Electronics Lab Safety: Electric shock can be fatal; read and heed the BME 3511 Bioelectronics Safety Guidelines. In general, the undergraduate BME electronic laboratory experiments conducted in BME Teaching Laboratories, do not use voltages greater than 30 V (± 15 V); therefore, the chance of receiving an electrical shock is greatly reduced. However, all voltages do have the potential to burn materials and start fires, destroy electronic components, and present hazards to the person performing the operations. Common sense and an awareness of electrical circuits is important whenever you are working on these experiments. Objective: Become familiar with DMM functions and characteristics; and with the techniques for using a DMM to measure resistance. Understand and apply the concepts of measurement accuracy and measurement precision. Laboratory Equipment and Supplies: DMM Assorted Resistors Page 1

2 Background: The MCM DMM Model features a rotary switch that can used be to select various functions and range of measurement values. See Digital Multimeter DMM Model Functions, Ranges, Resolutions, and Accuracy (page 4). Refer to the DMM itself for examples of the function labels; starting at the top center and moving clockwise: Off, AC Voltage, DC Current, Square Wave Output, Battery Tester, Diode Test, Resistance, and DC Voltage. Note: There is no AC Current function. The DMM features also include three display symbols: Low Battery Negative Value Value Exceeds Selected Range - + The terms accuracy and precision are associated with scientific and engineering measurements. Accuracy describes the difference between the measurement of a quantity and the true value of that quantity. Precision is the degree to which repeated measurements under unchanged conditions show the same conditions. A measurement system can be accurate but not precise, precise but not accurate, neither, or both. For example, if an experiment contains a systematic error, then increasing the sample size generally increases precision but does not improve accuracy. The result would be a consistent yet inaccurate string of results from the flawed experiment. Eliminating the systematic error improves accuracy but does not change precision. A measurement system is designated valid if it is both accurate and precise. Related terms include bias (non-random or directed effects caused by a factor or factors unrelated to the independent variable) and error (random variability). The terminology is also applied to indirect measurements; that is, values obtained by a computational procedure from observed data. In addition to accuracy and precision, measurements may also have a measurement resolution, which is the smallest change in the underlying physical quantity that produces a response in the measurement. Page 2

3 Procedure: Measuring Resistance Values 1. Use the DMM, (see Measuring Resistance, page 5) to measure the values of a nominal 33 Ω, a 6800 Ω, and a 1.5 MΩ resistor. Use Table 1 of the Laboratory Exercise # 1 Report Form to record values and interpretation of each of the display symbols for each of the five DMM Resistance Measuring Ranges. 2. Use thee sets of three resistors of varying values: R1A, R1B, R1C, R2A, R2B, R2C, R3A, R3B, R3C; use the resistor codes to identity the nominal values. Record the nominal values in Table 2 of the Laboratory Exercise # 1Report Form. Measure the resistance five different times for each of the nine resistors and record the measurements in Table 2. (You should have recorded a total of 45 resistance measurements.) Transfer the measurement values to an Excel spreadsheet and calculate the mean (AVERAGE) and standard deviation (STDEV) for each resistor, record the calculated values in Table 2. Note: You may also use a pencil & paper, a calculator, or an other computer application program such as MatLab, Mathematica, SPSS, JMP to perform the statistical calculations. Safe guard your five resistors from Laboratory Exercise #1. You will need these same resistors for later laboratory exercises. You must annotate any references you consulted in answering the following questions. You may wish to use a word processor to complete your answers and attach the printout to your Laboratory Exercise #1 Report Form. 3. The MCM DMM Model is described as having a so called display. Explain the meaning/interpretation of a display. 4a. Use the target analogy to differentiate among accuracy, precision, bias, and validity. 4b. Use the flawed tape measure analogy to differentiate among accuracy, precision, bias, and validity. 5a. How would the accuracy of your measurements be affected if the number of measurements had been increased to 20 measurements for each resistor? Briefly explain your answer. 5b. How would the precision of your measurements be affected if the number of measurements had been increased to 20 measurements for each resistor? Briefly explain your answer. 6a. Briefly describe factors that might affect the bias in your measurements 6b. Briefly describe the factors that might affect the error in your measurements. 6c. Briefly describe a method for increasing the precision of your measurements. 7. Describe a procedure for making low-resistance measurements. 8a In checking for a short circuit, what DMM ohmic range should you select; what measurement value would you expect for a short circuit? 8b. In checking for an open circuit, what DMM ohmic range should you select; what measurement value would you expect for an open circuit? 9. Based on the results of this resistance measuring exercise, how many measurements do you suppose should be made in determining the resistive values of a prototype circuit? Briefly explain your answer. Page 3

4 Digital Multimeter DMM Model Functions, Ranges, Resolutions, and Accuracy Measurement Range Resolution Accuracy Notes Resistance 200 Ω 0.1 Ω 2K Ω 1 Ω 20K Ω 10 Ω ± (2.5% +5) 200K Ω 100 Ω 2M Ω 1000 Ω Input Resistance 1M Ω DC Voltage 0.2 VDC 0.1 mv 2.0 VDC 1.0 mv 20 VDC 10 mv 200 VDC 100 mv 300 VDC 1 V ± (2.5% +2) DC Current 2 ma 1 ua 20 ma 10 ua 200 ma 100 ua ± (2.5% +10) AC Voltage 200 VAC 0.1 V 300 VAC 1.0 V ± (2.5% +15) Input Resistance 500K Ω Frequency HZ AC Current NA Battery Test Range Internal Resistance Maximum Current 1.5 V 60 Ω 25 ma 9.0 V 1800 Ω 5 ma Out-of-Range / Negative Value - Source: MCM DMM Model Operating Manual Page 4

5 Measuring Resistance Using the MCM DMM Model Warning To avoid damage to the meter or to the device under test, disconnect circuit power and discharge all the high-voltage capacitors before measuring resistance. Measurement Procedure The resistance measurement positions are: 200Ω, 2000Ω, 20kΩ, 200kΩ and 2000kΩ. To measure resistance, connect the meter as follows: 1. Set the rotary switch to an appropriate measurement position in Ω range. The rotary switch should be placed in the desired position prior to connecting leads. This position should not be changed while the leads are connected. 2. Connect the test leads across with the component being measured. The measured value shows on the display. 3. When resistance measurement has been completed, disconnect the connection between the testing leads and the circuit under test. Notes If the value of resistance to be measured is unknown, use the maximum measurement position (2000kΩ) and reduce the range step by step until a satisfactory reading is obtained. The test leads can add 0.1Ω to 0.2Ω of error to resistance measurement. To obtain precision readings in low-resistance measurement, that is in the range of 200Ω, short-circuit the input terminals beforehand and record the reading obtained (call this reading as X). X is the additional resistance from the test leads. Then use the equation: Measured Resistance Value (Y) - (X) = Precision Readings of Resistance. For high-resistance measurement (> 1M Ω), it may require several seconds to obtain a stable reading. If Ω reading with shorted test leads is not 0.5 Ω, check for loose test leads, or incorrect positioning of the function selection switch. The LCD displays / indicating an open-circuit for the tested resistor or the resistor value is higher than the maximum range of the meter. Source: MCM DMM Model Operating Manual Page 5

6 BME 3511 Bioelectronics I Laboratory Exercise #1 Report Form Digital Multimeters I affirm that I personally participated in the collection and analysis of the data for this laboratory exercise and that I personally contributed to the completion of this laboratory report. Student Name: Signature: Date: I affirm that I personally participated in the collection and analysis of the data for this laboratory exercise and that I personally contributed to the completion of this laboratory report. Student Name: Signature: Date: Grade: Grader Comments:

7 BME 3511 Bioelectronics I Laboratory Exercise #1 Report Form 33 Ohm Nominal Value Resistor DMM Resistive Range Display Value Interpreted Value K 200k 2000k 6800 Ohm Nominal Value Resistor DMM Resistive Range Display Value Interpreted Value K 200k 2000k Table 1 Interpreting DMM Displayed Values 1.5M Ohm Nominal Value Resistor DMM Resistive Range Display Value Interpreted Value K 200k 2000k Nominal Value (Ohms) Tolerance (%) Lower Bound (Ohms) Upper Bound (Ohms) R 1A R 1B R 1C R 2A R 2B R 2C R 3A R 3B R 3C Measurement #1 #2 #3 #4 #5 Average Standard Deviation DMM Accuracy (Ohms) Range of Values (Ohms) Table 2 Recorded DMM Measured Resistances and Calculated Average Values