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QUALITY OF MEASUREMENT PART 1

QUALITY OF MEASUREMENT PART 1. LIMITATIONS OF YOUR EQUIPMENT. RESOLUTION. SENSITIVITY. STABILITY. RESPONSE TIME. Find the height of the oceans to 1 cm from 1000 km………..for the environmentalists, a Physics problem.

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QUALITY OF MEASUREMENT PART 1

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  1. QUALITY OF MEASUREMENT PART 1

  2. LIMITATIONS OF YOUR EQUIPMENT

  3. RESOLUTION SENSITIVITY STABILITY RESPONSE TIME

  4. Find the height of the oceans to 1 cm from 1000 km………..for the environmentalists, a Physics problem.

  5. 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” pulse:a very STABLE piece of equipment!

  6. f = ~1010 Hz T = 1/f = 10-10 s = 0.1 ns Resolution of departure/arrival of signal is to within 1 cycle.

  7. 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

  8. High RESOLUTION High STABILITY Low UNCERTAINTY

  9. 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.

  10. 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, it’s 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.

  11. 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.

  12. 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.

  13. 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.

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