Advanced Automotive Fault Diagnosis
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LÔØI NOÙI ÑAÀU, ledklok
| 3.1.3 Accuracy of test
equipment Accuracy can mean a number of slightly differ- ent things: ● careful and exact ● free from mistakes or errors; precise ● adhering closely to a standard Consider measuring a length of wire with a steel rule. How accurately could you measure it to the nearest 0.5 mm? This raises a number of issues. Firstly, you could make an error reading the ruler. Secondly, why do we need to know the length of a bit of wire to the nearest 0.5 mm? Thirdly the ruler may have stretched and not give the correct reading! The first and second of these issues can be dis- pensed with by knowing how to read the test equip- ment correctly and also knowing the appropriate level of accuracy required. A micrometer for a plug gap? A ruler for valve clearances? I think you get the idea. The accuracy of the equipment itself is another issue. Accuracy is a term meaning how close the measured value of something is to its actual value. For example, if a length of about 30 cm is measured with an ordinary wooden ruler, then the error may be up to 1 mm too high or too low. This is quoted as an accuracy of ⫾1 mm. This may also be given as a percentage which in this case would be 0.33%. Resolution, or in other words the ‘fineness’ with which a measurement can be made, is related to accuracy. If a steel ruler was made to a very high standard but only had markings of one per centimetre it would have a very low resolution even though the graduations were very accurate. In other words the equipment is accurate but your reading will not be! To ensure instruments are, and remain, accur- ate there are just two simple guidelines. ● Look after the equipment; a micrometer thrown on the floor will not be accurate. ● Ensure that instruments are calibrated regularly – this means being checked against known good equipment. Here is a summary of the steps to ensure a meas- urement is accurate: Step Example Decide on the level of Do we need to know that the accuracy required battery voltage is 12.6 V or 12.635 V? Choose the correct A micrometer to measure the instrument for the job thickness of a shim Ensure that the Most instruments will go out of instrument has been adjustment after a time. You looked after and should arrange for adjustment at calibrated when regular intervals. Most tool necessary suppliers will offer the service or in some cases you can compare older equipment to new stock 26 Advanced automotive fault diagnosis Study the instructions Is the piston diameter 70.75 mm for the instrument in use or 170.75 mm? and take the reading with care. Ask yourself if the reading is about what you expected Make a note if you are Don’t take a chance, write it down taking several readings tải về 7.07 Mb. Chia sẻ với bạn bè của bạn:
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