QUALITY OF MEASUREMENT

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QUALITY OF MEASUREMENT PART 1
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LIMITATIONS OF YOUR EQUIPMENT
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RESOLUTION
SENSITIVITY
STABILITY
RESPONSE TIME
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Find the height of the oceans to 1 cm from 1000
km..for the environmentalists, a Physics
problem.
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Solution use a radar transmitter of 13 GHz. The
transmitter can be CALIBRATED using an atomic
clock, accurate to 1 part in 1012.
Work out the TIME PERIOD (time for one CYCLE) of
the transmission.
This is the time for a there and back pulsea
very STABLE piece of equipment!
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f 1010 Hz
T 1/f 10-10 s 0.1 ns Resolution of
departure/arrival of signal is to within 1 cycle.
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Resolution in DISTANCE Distance 106 m Change
in distance 10-2 m Resolution 1 in 108
Resolution in TIME Trip time 2 x 106 m/3 x 108
ms-1 10-2 s approx (2 way) Need a resolution in
trip time of 1 in 108 So time difference
resolution must be 10-2/108
10-10 s
0.1 ns
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High RESOLUTION
High STABILITY
Low UNCERTAINTY
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• Systematic errors
• Tracking these down makes your measurements
  ACCURATE as well as PRECISE.
• Pulse speed variation can be due to
• Electrons in atmosphere slowing pulse down /-
  0.5 m
• Water vapour in atmosphere /- 0.5 m
• O2 concentration affects pulse speed /- 2 m
• The BULGING Earth (it bulges at the equator).
• Winds, tides and atmospheric pressure.

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RESOLUTION
The smallest change that your instrument can
detect.
For a scale, this is the value of the SMALLEST
SCALE DIVISION. For a digital readout, its the
value of one unit in the last digit.
e.g a microammeter with a range of 100 ?A with 50
divisions has a resolution of 2 ?A or 2 of the
range.
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SENSITIVITY
This is the RATIO of the change of output to the
change of input. It refers to an instrument where
a scale is made to REPRESENT another range of
values.
e.g. a sensor measuring LIGHT uses a
VOLTMETER 6.0 V per lux. A CRO measuring p.d.
on a LINEAR scale on a screen 0.2 V per cm.
An AMPLIFIER can increase sensitivity.
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STABILITY
A stable piece of equipment means that the repeat
readings are close to each other.

• Examples of things that can affect stability
• Pointers that stick slightly
• Bad electrical contacts
• A clamp that is not rigid
• Clumsy use of a micrometer.

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RESPONSE TIME
The length of time a sensor takes to reach its
final reading following a sharp change in input.
Fast change sensors CRO necessary for HIGH
FREQUENCY fluctuations. LDR semi-conductor
Slow change sensors Light beam galvanometer
takes time for light beam to scan
scale. Ratemeter for radioactivity pointer shows
a steady AVERAGED rate.
Temperature sensors tend to be sluggish. Rapid
temperature fluctuations are hard to measure
reliably.