How accurate is my folding ruler?
How do you recognise an accurate quality folding ruler?
How accurate a folding rule is is determined by its accuracy class.
If the folding rule has been tested in accordance with EC Directive 2004/22/EC (since 2016 2014/32/EU), this can be recognised by the accuracy class at the beginning of the metre scale.
A Roman numeral is often stamped on the first link of the metre rule. This is the EC accuracy class.
What is the accuracy class and how is it calculated?
There are three EC accuracy classes. They determine how accurately the folding rule must measure to a certain length and to what extent a deviation in the measurement is permitted.
The formula for the error limit is made up as follows:
a + b*L
L refers to the length to be measured rounded up to the nearest full metre.
a and b are taken from the table:
Table: Determine the error limits
| Accuray Class | a (mm) | b (mm/m) | c (mm)- Area |
|---|---|---|---|
| III | 0.6 | 0.4 | 0.3 |
| II | 0.3 | 0.2 | 0.2 |
| I | 0.1 | 0.1 | 0.1 |
If, for example, 1.80m is to be measured, the formula for class III is as follows:
0.6+0.4*2 = 1.4 mm
The metre rule may therefore deviate by up to 1.4 mm over this length in order to be classified in Class III. If it is more, this class does not apply as the folding rule becomes too inaccurate; if it is less, the metre rule may move up in the accuracy class.
III is the usual standard for folding rules made of wood or of plastic. Our LongLife® metre rules are marked and tested with the EC accuracy class.
Let us now take the example and calculate the error limit for class II:
0.3+0.2*2 = 0.7mm
The maximum deviation is 0.7 mm. This class is usually assigned to metre rods made of metal.
However, our LongLife® Plus Composite can also boast EC accuracy class II – the only plastic metre rule in this class.
How is the class tested?
With measuring instruments, we check the accuracy of our LongLife® folding rules internally. Of course, these instruments also tested externally in accordance with the Measuring Instruments Directive. So they are also CE-tested and calibrated and you can be sure that our LongLife® folding rules measure accurately.
To summarise: Our LongLife® metre rules are CE-approved and are in accuracy class III, which means that our folding rules deviate by a maximum of 1.4mm. Our LongLife® Plus Composite metre rule is even more accurate. With an error tolerance of just 0.7 mm, the LongLife® Plus Composite is characterised by accuracy class II.
Error tolerance of the accuracy classes at 2m:
I: +/-0.3 mm
II: +/- 0.7 mm
III: +/- 1.4 mm
