Consumers often must make choices for materials and tools, even tradespeople on a project. The mechanism that moves us one way or another is a mysterious balancing act that offsets some given price for some perceived or real advantage.
In an ideal world, rational behavior would drive our desires and our decisions and we would simply maximize utility. But we don't always act rationally, especially when it comes to purchasing tools. A plumb bob and laster transit-level both do the job, but one is costs considerably more than the other. Which would you choose?
When reading a level, the horizontal reading is the "level" reading, while the vertical reading is referred to as the "plumb." In construction and remodeling, there are times when precision is very necessary. When finding most vertical lines around the house, a standard four-foot level will suit your needs just fine.
However, let's say you have to plumb something that is 20 feet high; that would be impossible to do with a four-foot level. Allowing for a standard deviation (the accurate tolerance of such device) of, say, an eighth of an inch for each four-foot reading, the total line could be off by a full inch. You will need something way more accurate, so, in this case, you would turn to either a laser or plumb bob, both of which are extremely accurate.
A plumb bob will find an absolute positive, true vertical reading. When suspended from a string, the weight of the plumb bob pulls the string line into a perfectly vertical position. The point of the plumb bob will always be directly below the point from which the string hangs because gravity is pulling all things to the center of the earth, hence the line is perpendicular to the earth. (For those geometry and physic whiz-kids out there: Yes, the plumb bob is not really perpendicular because the earth is round, but for our purposes, the perpendicular accuracy is well within our necessary constraints.)
A plumb bob is a weighted, tapered cylindrical tube that comes to a point. It usually is made of a heavier stable metal that resists deformation since pings and dents could cause a tilt of sorts. To use a plumb bob, first attach the string through its body. Most bobs have a removable tip to pass a string through. Hang it from the point to which you want to determine a vertical point directly below. Allow the bob to dangle until it is just slightly above the floor or other horizontal plain.
It will swirl a bit in a pendulum-like manner, finding its ultimate point; the circumference of the swirling ellipse is actually getting tighter on each orbit. Eventually, the bob will come to look like it's hanging still -- that will be your plumb point guaranteed. From this vertical base line, you can pull all sorts of measurements including using a perpendicular measurement to achieve a true horizontal line.
Many masons, stair builders and cabinet hangers use plumb bobs everyday because they can depend on their accuracy and, frankly, they're easy to use. But some tradespeople prefer the laser transit-level because in addition to plumb readings, it can provide horizontal lines.
Laser transit-levels are not as easy to use , but once you know how to use them correctly they are extremely accurate. It takes a little longer to set up a laser transit-level in a room; you have to fiddle with it a bit and station it somewhere in the line of sight for whatever you're doing.
Some laser transit-levels need to be mounted to a wall, others have a tripod, which you open and station in the floor. The laser transit-level shines a beam onto your project; some actually spin, providing a virtually continuous stream of light, while others need to be directed. Like the plumb bob, you will still need to transfer relative marks.
Care must be taken when using laser transit-levels because laser light is harmful if shone into one's eye. In fact, the laser transit-level will come with laser-protective eyeglasses that you, and others working with you, should use.
So there you have the tale of two devices used to find level and plumb readings. As you would have probably guessed, the plumb bob is a lot cheaper than a laser transit-level. A really good bob will cost you less than $10. A really good laser transit-level will set you back several hundred bucks. Rational behavior, indeed! I don't own a laser transit-level but for some reason, deep inside me, I want one. Go figure.