Precision/Accuracy

Using a metre rule, I will measure a distance of 0.5m between 2 points on the measuring cylinder. A metre rule is generally ‘readable’ to the nearest 1mm this means the precision is ±0.5 mm, hence its suitable for measuring this distance

I wrote the above for a piece of coursework. Is there anything I could improve?

Reply 1

Joinedup

20

Original post by GPODT

Using a metre rule, I will measure a distance of 0.5m between 2 points on the measuring cylinder. A metre rule is generally ‘readable’ to the nearest 1mm this means the precision is ±0.5 mm, hence its suitable for measuring this distance

I wrote the above for a piece of coursework. Is there anything I could improve?

you could calculate it as a percentage of the 0.5m and use that to quantify it's suitability for making measurements in that range.

Reply 2

GPODT

OP

16

Original post by Joinedup

you could calculate it as a percentage of the 0.5m and use that to quantify it's suitability for making measurements in that range.

Just to clarify, am I right in saying the precision is the smallest measurable value? So lets say if you have a voltmeter reading of 4.68 V then the precision is 0.01 ? That's what this video says anyway:

http://www.youtube.com/watch?v=1dTn2pt5PuA

But in my first post I stated that the precision is 0.5 mm.. Which one is correct?

Also, if the precision is 0.5mm does it mean the uncertainty is ±0.5 ?

Reply 3

GPODT

OP

16

Bump...

(edited 10 years ago)

Reply 4

Joinedup

20

you want to stick with calling it ±0.5 mm imo - it tells you the amount of resolution of your measuring instrument (1mm) and that the amount of error is likely to be equal in both the +ve and -ve directions.

I can't watch youtube on the phone and since the tellytubby upgrade, the site frequently logs me out again as soon as I log in.

if you've still got questions hopefully someone else will be able to answer them since I'm virtually unable to use the site anymore :frown:

Reply 5

Stonebridge

13

Original post by GPODT

Just to clarify, am I right in saying the precision is the smallest measurable value? So lets say if you have a voltmeter reading of 4.68 V then the precision is 0.01 ? That's what this video says anyway:

http://www.youtube.com/watch?v=1dTn2pt5PuA

But in my first post I stated that the precision is 0.5 mm.. Which one is correct?

Also, if the precision is 0.5mm does it mean the uncertainty is ±0.5 ?

The 4.68 ±0.01 of the voltmeter means you are saying the value is between a max of 4.69 and a min of 4.67
This is a digital readout so in effect you have to believe what the meter tells you.

With the meter rule you are doing a small amount of estimating using the judgement of your own eyes. This is not quite the same thing, is it?
If the metre rule has a 1mm scale then if you estimate the reading as 46mm you are saying it's not 47 or 45, it's probably max 46.5 and min 45.5
You are saying the reading is probably within that range.
That is ± 0.5mm

So both are correct.
There is no, one correct answer to how you express the precision in a reading with a metre rule. In most cases it's fine to give it as ±0.5mm
The uncertainty in a reading cannot be less than the precision. However it can be greater. The classic example is a stopwatch with a precision of 0.01s
If you time an object moving from a to b by pressing the button at a and then at b, you have a reaction time of possibly 0.1s, so the uncertainty in each time measurement is ± 0.1s
This means the time interval measurement has an uncertainty of ±0.2s

(edited 10 years ago)

Precision/Accuracy

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