Plumb line

[George]

would you trust it over a spirit level in relation to vertically trying to get something straight?

I do.

[dennis]

Yes, assuming you know how to use a plumb line.

[Dave Plowman (News)]

Of course - both work on gravity. But spirit levels by their nature are not that accurate - and also only as accurate as the one reading them. Less chance of user error with a plumb line.

[George]

Well it ain't rocket science Dennis,I'm sure you like going into technical mode more often than not.

[dennis]

Why does it need a technical explanation rather than user instructions? Anyway if you want to go into rocket science I can manage that.

[Roger Mills]

Absolutely! If something is (say) 1/8" out of vertical over a 6' length you can easily see it with a plumb line - whereas the bubble of a spirit level would hardly move from centre for that amount of error.

[Cicero]

================================== You might find it even better if you use it on a plumb board. This is a piece of 4" x 1" timber about 4 feet long with a notch at the top for the string and a hole near the bottom roughly the shape of your plumb bob. I don't know if it has a proper name other than 'plumb board' and I can't find a picture but they used to be very common.

Cic.

[George]

Spot on that man,my sentiments entirely. The use of a spirit level on an 125 year old house is pointless when trying to install a window frame it needs a line of sight or plumb line to make it look near enough level in the vertical plain. In my view what ever discrepency that shows up on a spirit level gets amplified threefold on whatever you're trying to level up vertically. Obviously a spirit level has to be used at sometime or other but there's no substitute for going with your sight or plumb line.

[The Medway Handyman]

If there is a technique to it I'd like to lnow about it.

[dennis]

Well you have to be sure its swinging freely to start with. If its pined to a wall and the weight is rolling about on the wall it may not stop exactly vertical, this is probably what the plumb rule is to stop BTW.

[George]

Well Dennis a 4" nail wouldnt go amiss so long as you havn't pinned it in far too much.

[dennis]

Did I say it wouldn't? Are you trying to be a pain or is it natural?

[George]

Certainly not,you started it.

[dennis]

Started what? You are the one with the flippant replies.

[Tanner-'op]

And keep it out of strong air currents - I've used a plumb line with a 6lb lump of lead as the 'bob' and still had it swinging in the bloody breeze!

A little trick here. - If you have problems with strong air currents when using a plumb line - you can drop the 'bob' into a container of water (or oil) to ease the problem.

Not strictly true as providing there is a heavy enough weight in the 'bob' (and the walls flat) it will still hang vertically.

this is probably what the plumb rule

No it's not, it's to allow you to obtain and accurately mark a straight line on vertical surfaces, along with plumbing and straightening such things as door frames - that's why a plumb rule is usually around 6' to 6'6" (in old money) - using it one-handed.

BTW, I'm not being pedantic, just experienced in using the damn things over many years!

Tanner-'op

[dennis]

There are a few things that it helps with beside what I said, including:-

reducing parallax errors eliminating the holes from 4 inch nails giving you a solid edge to run the pencil along rather than some floppy string

None of which contradict what you said.

[Tanner-'op]

Dennis,

You appear to be a little 'touchy' over the use of such a simple tool - I'd hate to think what sort of a reaction one would get if we were discussing the use of theodolites, dumply levels, water levels, the boxwood rule as opposed to the steel rule, the chalk line, chains - and heaven forbid (to the purists of measurement) the decidedly inaccurate flexible steel and cloth tape measures!!!

I think I'll pass on that thread. ;-)

Tanner-'op

[George]

:-)

I've just had a nice walk.

[Dave]

So what would have been the answer to my problem many years ago?

I was always tasked to do symmetry and alignment checks on all the new development aircraft. It was done so that the aircraft could be set up again and measured if it exceeded stress limits in flight or landing.

This consisted of levelling the centre module of the aircraft by using the telescope using a rule dropped from 3 known points (this was done to create a Z line and then positioning 2 theodolights around the aircraft so as to produce X and Y lines. We took an awful lot of measurements from plumb bobs, It was accurate to about 0.5 mm. Without going into detail, we found that if we took a break for lunch and came back and did a cross reference check, to see that no one had taken a look to see what we had been doing and disturbed the tripods, we found that some measurements had changed. We set up again and took another set of measurements and carried on. Later, we found out that it was the state of the tide that affected the concrete rafts. The floor was being tilted by the tides. The hangar floor was very close to sea level there.

Dave

[dennis]