SOLUTION MANUAL www.1 FIND: Explain the hierarchy of standards. Explain the term standard. SOLUTION The term standard refers to an object or instrument, a method or a procedure that provides a value of an acceptable accuracy for comparison. A primary standard defines the value of the unit to which it is associated. Secondary standards, while based on the primary standard, are more readily accessible and amenable for use in a calibration. There is a hierarchy of secondary standards: A transfer standard might be maintained by a national standards lab (such as NIST in the United States) to calibrate industrial “laboratory standards”. It is costly and time-consuming to certify a laboratory standard, so they are treated carefully and not used too regularly. A laboratory standard would be maintained by a company to be used to certify a more common in-house reference called the working standard. A working standard would be calibrated against the laboratory standard. The working standard is used on a more regular basis to calibrate everyday measurement devices or products being manufactured. Working standards are more the norm for most of us. A working standard is simply the value or instrument that we assume is correct in checking the output operation of another instrument. Example: A government lab maintains the primary standard for pressure. It calibrates a an instrument called a “deadweight tester” (see C9 discussion) for high pressure calibrations. These form its transfer standard for high pressure. A company that makes pressure transducers needs an in-house standard to certify their products. They purchase two deadweight testers. They send one tester to the national lab to be calibrated; this becomes their laboratory standard. On return, they use it to calibrate the other; this becomes their working standard. They test their manufactured transducers using the working standard – usually at one or two points over the transducer range to assure that it is working. Because the working standard is being used regularly, it can go out of calibration. Periodically, they check the working standard calibration against the laboratory standard. See ASME PTC 19.2 Pressure Measurements for a further discussion. A test standard defines a specific procedure that is to be followed.2 FIND: Why calibrate? What does calibrated mean? SOLUTION: The purpose of a calibration is to evaluate and document the accuracy of a measuring device. A calibration should be performed whenever the accuracy level of a measured value must be ascertained. An instrument that has been calibrated provides the engineer a basis for interpreting the device’s output indication. It provides assurance in the measurement. Besides this purpose, a calibration assures the engineer that the device is working as expected. A periodic calibration of measuring instruments serves as a performance check on those instruments and provides a level of confidence in their indicated values. A good rule is to calibrate measuring systems annually or more often as needed. ISO 9000 certifications have strict rules on calibration results and the frequency of calibration.3 FIND: Suggest methods to estimate the accuracy and random and systematic errors of a dial thermometer. SOLUTION: Random error is related to repeatability: how closely an instrument indicates the same value. So a method that repeatedly exposes the instrument to one or more known temperatures could be developed to estimate the random error. This is usually stated as a statistical estimate of the variation of the readings. An important aspect of such a test is to include some mechanism to allow the instrument to change its indicated value following each reading so that it must readjust itself. For example, we could place the instrument in an environment of constant temperature and note its indicated value and then move the instrument to another constant temperature environment and note its value there. The two chosen temperatures could be representative of the range of intended use of the instrument. By alternating between the two constant temperature environments, differences in indicated values within each environment would be indicative of the precision error to be expected of the instrument at that temperature. Of course, this assumes that the constant temperatures do indeed remain constant throughout the test and the instrument is used in an identical manner for each measurement. Systematic error is a fixed offset. In the absence of random error, this would be how closely the instrument indicates the correct value. This offset would be present in every reading. So an important aspect of this check is to calibrate it against a value that is at least as accurate as you need. This is not trivial. For example, you could use the ice point (0oC) as a check for systematic error. The ice point is formed from a carefully prepared bath of solid ice and liquid water. As another check, the melting point of a pure substance, such as silver, could be used. Or easier, the steam point. Accuracy requires a calibration to assess both random and systematic errors. If in the preceding test the temperatures of the two constant temperature environments were known, the above procedure could serve to establish the systematic error, as well as random error of the instrument. To do this: The difference between the average of the readings obtained at some known temperature and the known temperature would provide an estimate of the systematic error.4 FIND: Discuss interference in the test of Figure 1.3 SOLUTION : In the example described by Figure 1.3, tests were run on different days on which the local barometric pressure had changed. Between any two days of different barometric pressure, the boiling point measured would be different – this offset is due to the interference effect of the pressure. Consider a test run over several days coincident with the motion of a major weather front through the area. Clearly, this would impose a trend on the dataset. For example, the measured boiling point may be seem as increasing from day to day. By running over random days separated by a sufficient period of days, so as not to allow any one atmospheric front to impose a trend on the data, the effects of atmospheric pressure can be broken up into noise. The measured boiling point might then be high one test but then low on the next, in effect, making it look like random data scatter, i.5 FIND: How does resolution affect accuracy? SOLUTION The resolution of a scale is defined by the least significant increment or division on the output display. Resolution affects a user's ability to resolve the output display of an instrument or measuring system. Consider a simple experiment to show the effects of resolution. Under some fixed condition, ask several competent, independent observers to record the indicated value from a measurement system. Collect the results – this becomes your dataset. Because the indicated value is the same for each observer, the scatter in your dataset will be close to the value of the resolution of the measurement system. Data scatter contributes to the random error. As such, the output resolution of a measurement system forms a lower limit as to the random error to be expected. The resolution would not contribute to systematic error. Systematic error is an offset.6 FIND: How does hysteresis affect accuracy? SOLUTION Hysteresis error is the difference between the values indicated by a measurement system when the value measured is increasing in value as opposed to when it is decreasing in value; this despite the fact that the value being measured is actually the same for either case. A common cause of hysteresis in analog instruments is friction in the moving parts. This can cause the output indicator to 'stick'. In digital instruments, hysteresis can be caused by the discretization. If hysteresis error is ignored, the effect on any single measurement can be seen as a systematic error. On multiple measurements in any one direction, the effect can be to impose a 'trend' on the data set. The use of randomization methods can break up the trends incorrectly implied by hysteresis effects. Randomization makes systematic errors behave as random errors, which are more easily interpreted. If randomization methods are not used, the hysteresis effect behaves as a systematic error.7 SOLUTION This problem is open-ended and has no unique solution. We suggest that the instructor use this Problem as the basis for an in-class or small group discussion.8 FIND: Identify measurement stages for each device. SOLUTION a) thermostat Sensor/transducer: bimetallic thermometer Output: displacement of thermometer tip Controller: mercury contact switch (open:furnace off; closed:furnace on) b) speedometer Method 1: Sensor: usually a mechanically coupled cable Transducer: typically a dc generator that is turned by the cable producing an electrical signal Output: typically a pointer/scale (note: often a galvanometer is used to convert the electrical signal in a mechanical rotation of the pointer) Method 2: Sensor: A magnet attached to the rotating shaft Transducer: A Hall Effect device that is stationary but detects each sensor passage by creating voltage pulse Signal Conditioning: A pulse counting circuit; maybe also digital-analog converter (if analog readout is used) Output: An analog or digital readout calibrated to convert pulses per minute to kph or mph. c) Portable CD Stereo Player Sensor: laser with optical reader (reflected light signal differentiates between a "1" and "0") Transducer: digital register (stores digital information for signal conditioning) Signal conditioning: digital-to-analog converter and amplifier (converts digital numbers to voltages and amplifies the voltage) Output: headset/speaker (note: the headeset/speaker is a second transducer in this system converting an electrical signal back to a mechanical displacement) www.net d) anti-lock braking system Sensor: brake activation switch senses brakes 'on'; encoder counts wheels revolutions per unit time Signal conditioning: timing circuit Output: a feedback signal that pulsates brake action overriding the driver's constant pedal pressure e) audio speaker Sensor: coil (to which the input terminals are connected) Transducer: coil-magnet-speaker cone that acts as a miniature electrical dc motor responding to changes in current applied. Output: speaker cone displacement www.9 KNOWN: Data of Table 1.5 FIND: input range, output range SOLUTION By inspection 0.2 V The input range (x) is from 0. The output range (y) is from 0. The corresponding spans are given by ri = 99.8 V COMMENT Note that each answer has units shown. By themselves, numerical answers are meaningless. Always show units for data, for each step of data reduction and in all reported results.10 KNOWN: Data set of Table 1.5 FIND: Discuss advantages of different plot formats for this data SOLUTION: Both rectangular and log-log plots are shown below. Rectangular grid (left plot below): An advantage of this format is that is displays the data clearly as having a non-linear relationship. The data trend, while not immediately quantifiable, is established. A disadvantage with this data set is that the poor resolution at low x values makes quantification at low values difficult. Log-log grid (right plot below): An advantage of this format with this particular data set is that the data display a linear relationship of the form: log y = m log x + log b. This tells us that the data have the relationship, y = bxm. Because of these facts, resolution is equally good over the whole scale. A disadvantage with this format is that one must remember the data has been conditioned to look linear. We are no longer plotting x versus y. This is particularly important to remember when attempting to find the slope of y against x.11 KNOWN: Calibration data of Table 1.5 FIND: K at x = 5, 10, 20 cm SOLUTION: The data reveal a linear relation on a log-log plot suggesting y = bxm. That is: log y = log (bxm) = log b + m log x or Y = B + mX From the plot, B = 0, so that b = 1, and m = 1. Thus, we find from the calibration the relationship y = x1.18 100 Y[V] 10 1 We should expect that errors would propagate with the same sensitivity as the 0. Hence for y=f(x), as sensitivity 0.
