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ERRORS DUE TO ENVIRONMENTAL INPUTS

environmental input : apparently real input to ameasurement system caused by a change in theenvironmental conditions surrounding themeasurement system.

The static and dynamic characteristics specifiedfor measuring instruments are only valid forparticular environmental conditions, must bereproduced as closely as possible during

calibration exercises The magnitude is quantified by the two

constants: sensitivity drift and zero drift

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EXAMPLE

Suppose we are given a small closed box and told thatit may contain either a mouse or a rat. We are also toldthat the box weighs 0.1kg when empty. If we put thebox onto bathroom scales and observe a reading of 1.0

kg, this does not immediately tell us what is in the boxbecause the reading may be due to one of three things:

(a) a 0.9 kg rat in the box (real input)

(b) an empty box with a 0.9 kg bias on the scalesdue to a temperature change (environmentalinput)

(c) a 0.4 kg mouse in the box together with a 0.5 kgbias (real + environmental inputs).

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The magnitude of any environmental input

must be measured before the value of the

measured quantity (the real input) can be

determined from the output reading of an

instrument.

Difficult to avoid environmental inputs

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WEAR IN INSTRUMENT COMPONENTS

Systematic errors can frequently develop over

a period of time because of wear in

instrument components.

Solution: Recalibration

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CONNECTING LEADS

Common source of error : failure to take proper account ofthe resistance of connecting leads.

In resistance thermometer, thermometer is separated fromother parts of the measurement system by 100 metres. The

resistance of such a length of 20 gauge copper wire is 7,and there is a further complication that such wire has atemperature coefficient of 1m/C.

Connecting leads should: Adequate cross-section so that their resistance is minimized

Adequately screened if they are to be subject to electrical ormagnetic fields that could otherwise cause induced noise.

The routing of cables needs careful planning: changing theroute of the cables between the transducers and thecontrol room, the magnitude of noise can be reduced.

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REDUCTION OF SYSTEMATICERRORS

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The prerequisite for the reduction of

systematic errors is a complete analysis of the

measurement system that identifies all

sources of error.

Simple faults within a system can usually be

readily and cheaply rectified once they have

been identified.

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CAREFUL INSTRUMENT DESIGN

Careful instrument design : reducing the

sensitivity of an instrument to environmental

inputs to as low a level as possible.

Errors due to the way in which an instrument

is designed are not always easy to correct,

Solution: high cost of redesign or accept the

reduced measurement accuracy

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METHOD OF OPPOSING INPUTS

Compensates the effect of an environmental

input by introducing an equal and opposite

environmental input that cancels it out.

Example: Milivoltmeter

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