Systematic Errors in Engineering Experiments

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It comes from unpredictable changes during an experiment.

Errors in Measurement System

Systematic error always affects measurements the same amount or by the same proportion, provided that a reading is taken the same way each time. It is predictable. Random errors cannot be eliminated from an experiment, but most systematic errors can be reduced. When weighing yourself on a scale, you position yourself slightly differently each time. When taking a volume reading in a flask, you may read the value from a different angle each time.

Observational error

Measuring the mass of a sample on an analytical balance may produce different values as air currents affect the balance or as water enters and leaves the specimen. Measuring your height is affected by minor posture changes. Measuring wind velocity depends on the height and time at which a measurement is taken. Multiple readings must be taken and averaged because gusts and changes in direction affect the value.

Readings must be estimated when they fall between marks on a scale or when the thickness of a measurement marking is taken into account.

Forgetting to tare or zero a balance produces mass measurements that are always "off" by the same amount. An error caused by not setting an instrument to zero prior to its use is called an offset error. Not reading the meniscus at eye level for a volume measurement will always result in an inaccurate reading. The value will be consistently low or high, depending on whether the reading is taken from above or below the mark. Measuring length with a metal ruler will give a different result at a cold temperature than at a hot temperature, due to thermal expansion of the material.

An improperly calibrated thermometer may give accurate readings within a certain temperature range, but become inaccurate at higher or lower temperatures. Measured distance is different using a new cloth measuring tape versus an older, stretched one.

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Proportional errors of this type are called scale factor errors. Drift occurs when successive readings become consistently lower or higher over time. Electronic equipment tends to be susceptible to drift. Many other instruments are affected by usually positive drift, as the device warms up. The two main types of measurement error are random error and systematic error. Random error causes one measurement to differ slightly from the next.

Random errors cannot be eliminated from an experiment, but most systematic errors may be reduced. Bland, J. Martin, and Douglas G. Altman Cochran, W. Consider the Battery testing experiment where the lifetime of a battery is determined by measuring the amount of time it takes for the battery to die.

Practices of Science: Scientific Error |

A flaw in the procedure would be testing the batteries on different electronic devices in repeated trials. Because different devices take in different amounts of electricity, the measured time it would take for a battery to die would be different in each trial, resulting in error.

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Because systematic errors result from flaws inherent in the procedure, they can be eliminated by recognizing such flaws and correcting them in the future. Random errors result from our limitations in making measurements necessary for our experiment. All measuring instruments are limited by how precise they are. The precision of an instrument refers to the smallest difference between two quantities that the instrument can recognize. For example, the smallest markings on a normal metric ruler are separated by 1mm. This means that the length of an object can be measured accurately only to within 1mm.

The true length of the object might vary by almost as much as 1mm.

As a result, it is not possible to determine with certainty the exact length of the object. Another source of random error relates to how easily the measurement can be made.

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  • Suppose you are trying to determine the pH of a solution using pH paper. The pH of the solution can be determined by looking at the color of the paper after it has been dipped in the solution. However, determining the color on the pH paper is a qualitative measure. Unlike a ruler or a graduated cylinder, which have markings corresponding to a quantitative measurement, pH paper requires that the experimenter determine the color of the paper to make the measurement.