How do manufacturers calibrate spirit levels? I have two spirit levels and one is way out. Can I recalibrate it?
The way I was taught to use a was always to check the level twice, the second time by turning the spirit level through 180 degrees. This allows you to factor out any errors in the position of the bubble. If there are errors you will get different results, and to find the true level, you average out the error in the two measurements.
If you are trying to calibrate a spirit level and have one that comes with a rotating dial for just this purpose, use a newly painted, smooth wall and fix a point of reference at one end. Now, place the level against the wall horizontally 鈥� according to the bubble 鈥� and draw a line from the point of reference to the level鈥檚 other end. Spin the level through 180 degrees, do the same again and you will end up with two diverging lines. Split the difference at the divergent end, and you can scribe a line from this new point back to the point of reference that is perfectly level. You can then use this line to recalibrate your level.
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I am also told that you can buy spare bubbles for spirit levels. You鈥檒l find them on the same shelf as left-handed screwdrivers and long weights.
Marco van Beek, London, UK
Where I live in Thailand the builders don鈥檛 have spirit levels. Instead they use a long piece of plastic tubing filled with water. When the tubing is allowed to hang in a shallow U shape and held against a wall, the surface of the water at each end will settle at absolutely the same level. The builders then use a 鈥減owder string鈥� 鈥� a length of string covered in coloured chalk 鈥� which is stretched between the two water levels, and with a swift twang a nice level line is drawn along the wall. You could hold your spirit level up to this line to calibrate it.
Ken S
Your correspondent should find a perfectly horizontal surface using a spirit level that is known to be correct. Put the suspect spirit level on the surface and adjust the angle of the bubble capsule. Finally verify the adjustment by turning the readjusted, suspect level through 180 degrees.
Andrew Fogg, Sandy, Bedfordshire, UK