What does the +/- mean on Laser Level Accuracy?
With laser level accuracy all models have a quoted accuracy tolerance. This is nearly always quoted as a +/- mm over m (in the US may be quoted as Inches over Feet). For example +/- 2mm over 20m. But, what does this actually mean?
The 2mm over 20m in the above example is fairly obvious and means that the accuracy of the laser is within 2mm over every 20m from perfectly level. This is a linear thing so at 200m we are looking at calibration tolerance of under 20mm. So what about the +/-? This symbol means Plus or Minus for the example the laser may be shooting up to 2mm above (+) or below (-) perfectly level. I’ve had some situations where users believe that this actually means he laser’s tolerance is therefore 4mm in 20m, But this is not the case. I’ll explain why.
Lets now look at an example of a rotating laser. For the purpose of this example, we will assume that there is a tolerance of 3mm over 20m from perfect on the “X” axis. what determines whether its a +3mm higher or -3mm lower is which way round the laser is facing. If we face the laser’s “X” axis at a wall which is 20m away and mark the height of the laser. Next, rotate the laser unit by 180 degrees do the reverse side of the axis is now facing the same wall as before. Again we mark the height. There is a height difference between the two marks of 6mm. The level height is exactly between these to heights, so one side of the “X” axis is +3mm away from level and the opposite side of the “X” axis is -3mm below level.
This confirms that the “X” axis is out +/- 3mm over 20m, the laser is never more than 3mm away from level over 20m or 30mm over 200m. The laser is never 6mm over 20m out. The above example is a simple way you can check the accuracy of your laser level, remember you also need to check the “Y” axis as well, as this may well have a different result.
The principles of the process above hold true for line laser, dot lasers and even builder spirit levels, a reverse test against itself is generally the most reliable way of testing at home.
Laser Tolerances Conclusions
I think this hopefully explains why a plus or minus accuracy tolerance is quoted on laser levels. As you can see the laser is never more than 3mm out over 20m. One more point is that a stated manufacturers tolerance of +/- 2mm at 20m does not mean the laser is out that much. It’s a within figure, so calibration is better than the stated figure. How much better than the stated tolerance depends on the quality of the laser and the care taken when factory calibrated. Often brands that calibrate their lasers at the point of dispatch in the country of sale are ofter better. This is compared to relying on calibration from the country of origin after spending some weeks on a vibrating container ship.
For calibration purposes, there are other factors often at play such as bearing inconsistencies on pendulum based self-levelling lasers or Cone Issues on rotating lasers but those are another topic altogether.