1.4 Measure instruments and kind of errors.
Before addressing the issue of measure instruments, it’s important to highlight that these are devices or elements able to determine the magnitude a certain event is represented and they are based on patterns or measure standards that have been previously established, which make possible obtained values interpretation or even comparison with values of reference.
Two important characteristics are considered when choosing a measure instrument, which are its precision and sensibility. They are indicated by the maker or producer and can change depending on the kind of instrument and the measured variable, this variation represents an error range that might be considered acceptable according to the operator’s judgement
The easiest way to classify them is:
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Analog and digital
Analog instruments are those who use a direct signal to process and calculate the measured variable, that is to say, they indicate the measured variable’s value continuously and it’s directly transferred to a measure scale using only physical properties of materials. Most instruments have a needle to represent measures. On the other hand, digital instruments also use physical properties of materials, but in order to transform them into electric signal which will be transmitted afterwards to some other device or electronic element that allows its interpretation.
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Interaction with the measured variable
Another of the common classification for measure instruments is based on the type of variable that will be measured. Among them, we can find:
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Electrical measure instruments
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Temperature measure instruments
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Level measure instruments
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Pressure measure instruments
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Length measure instruments
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Mass measure instruments
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Speed measure instruments
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Volume measure instruments
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Time measure instruments
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Classification according to their function.
Mentioning instrument classification according to their function, we can identify
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blind instruments: which are those that don’t possess any visible indication of the measured variable, that means they should be adjusted to work limits due to a characteristic known as hysteresis.
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indicator instruments: which are those that give us a visible reading of the measured variable and they are composed by an rating and a graduated scale to visualize variable’s range.
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Recorder instruments allow to count the times a certain event happened. Moreover, transmitters allow to transfer certain magnitudes’ values to other devices and transducers o converters allow us to obtain a variable’s representation in another magnitude. Besides, there are controllers and receptors.
Error
Error is the measure’s deviation that practical measures present of a process variables related to theoric or ideal measures, as a result of imperfections in devices and useless variables that affect the process.
kind of errors
No measure can be done with a perfect accuracy, but it is important to describe which the real accuracy is and how do different errors in measures generate.
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Spurious mistake: They are the ones we made by mistake or distraction.
Attribution BIAS: It is a systematic mistake where real demand is constantly upper or lower than demand’s prognostics
Parallax errors: This kind of error has to do with the position the operator takes for measure readings. It originates in the lack of perpendicularity between the observer’s sight and the respective scale.
Scale mistakes: It’s normally accepted as scale error that value from the smallest division on the measure device (sometimes division’s half or other fractions are considered as an error).
Process erros (assembly): It happens due to the chosen measure processes. Several of a process error reasons happen because of the operator. For example: lack of visual acuity, neglect or disregard, fatigue, disturbance, etc.
Calibration errors: Those are the ones that make instruments give upper or lower readings all the way through the scale. They repeat constantly and affect the result in a single sense (increasing or decreasing the measure).