The BMI High Precision Level, and why it is terrible for picture hanging

As you might remember from a ways back, I have one of the BMI levels.  It used to be called the SuperRobust R1000, but it has a name change to the more descriptive BMI High Precision R1000.  Irrespective of the name, the level is the same.  (I have removed the rubber end for the following photo.  Oh, and excuse the image quality – I still haven’t found my camera charger since moving house, so have resorted to using my iPad!)

BMI High Precision

BMI High Precision

Inside, it has a number of rectangular sections for dimensional stability, and the bottom edge is machined flat.  Where it gains significant accuracy is the technology behind the vial.  This is where the R1000 in the spirit level’s title comes from.

Most levels have a vial radius of 200mm to 250mm.  Some even less.  So when the level is raised or lowered a little, the bubble moves a little.  For most builders, this has been the accepted norm, and that bubble is taken as a gospel indication that an object is level (horizontally or vertically).  When that bubble is at top dead centre, what else can it be?

But there are a number of problems with that.  For one, just how good is your eyechrometer in detecting a very small movement in a bubble, and moreso, in determining that the bubble is exactly centred.  Secondly, has the bubble had an opportunity to move?  With a combination of surface tension, and internal friction inside the vial, was there enough of a movement of the level for the bubble to move?  The smaller the vial radius, the steeper the sides, the less inclined is the bubble to move.

On the other hand, the BMI High Precision has a radius of 1000mm.  As in a metre radius.  Try drawing two circles, one 200mm radius, and one 1000mm radius, and see just how gentle the curve of the BMI level is.  What that means that if the level moves even a little bit, that bubble is off and running.  It is therefore very obvious when the object being measured is just off horizontal (or vertical).

radiius

This drawing was done in Illustrator, and was at 1:1 scale on screen when I captured it.  It may not be precisely 1:1 here, but pretty close, and you can see just how flat the R1000 level is compared to a typical vial.

When placed alongside two other levels I have, the difference is rather obvious.

3levels 3BubblesThese three levels are accurate.  But there is a significant difference between accuracy, and precision.  The bottom level has moved a little – probably a radius of 200mm.  The next has a radius of 250mm.  And the top one, the BMI has the metre radius, demonstrates how even being off level just a little is very apparent.  Equally, it is very obvious when you have achieved the ideal.

So with all this amazing precision, why is the BMI terrible for hanging pictures?  Because builders typically use the very imprecise levels, and what they build is not horizontal, or vertical.  Build a piece of fine furniture with square sides and try to fit it into the corner of a room.  Gaps everywhere.  So when I went to use the BMI to level some pictures that I was hanging last night, they were really level.  Standing back to admire my handiwork, and they looked wrong.  Not because the picture wasn’t level, but the entire house in comparison was well out.

Perhaps, if you are ever in the position of having a new home built, you can choose a builder based on the precision of the level they are prepared to use.  If they are using one such as the BMI High Precision, you can begin shortlisting!

2bmi

6 Responses

  1. When hanging pictures I tend to use a tape measure and measure down from ceiling then stand back and adjust by eye.

    • That is what I ended up having to do 😉

  2. Hanging steel work at a hospital project was only allowed 4mm over 5 meters as the plasters had a laser level ,most levels not up to it

    • Most couldn’t – I agree! The BMI has +/-0.3mm/m, so over 5m it will be out by 1.5mm at most. Even if you double that error rate, it would still be within tolerance.

  3. I don’t think it’s a little bit different between the first and third one. So what is the best accuracy? I have a right choice that I never use BMI for hanging anything in my house.

    • Hi Kennet – that is very much the point of the article – the BMI has a much larger radius of curvature of the vial, so a small amount of off-level results in a large (and therefore easier to see) movement of the bubble. If you can get the BMI bubble level, then you are really close to level, whereas the other spirit levels are “close enough is good enough”

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