At 11:20 AM 11/11/97 -0800, Stephen York wrote:
>> Who, a long time ago, took a class in making glassware
>> for chemistry, and can still probably size, curve, and
>> seal a tube(if absolutely necessary B^)).
>> Oh, and who paid $3 for the level.
>I think they are filled with alcohol, and it is said that if left
>in dealer's glass display cases on a hot sun day, you start to
>lose the alcohol. My dad's old Stanley level was his father's originally,
>and either my brother or I broke one of the vials when we were
>kids. The other two are fine, and the thing is as accurate as my
>new aluminum 4 ft model.
Alas, Steve, that doesn't say much, as many of the newer levels aren't
accurate. Next time you're in a hardware store, set a bunch of em down
together and see the variation.
And the old ones ain't gauranteed to be aligned either. I was using an old Stanl
ey on a fireplace surround, and I kept getting different readings. I discovered
that the vial I was reading with was loose in the plaster of paris, and every ti
me I set it down, the vial would rotate a little......
We talked about levels and vials quite a while ago, and I posted some stuff I ob
tained from Starrett.....It started when I noticed that one of the level manufac
turers had posted a note "OUR LEVELS ARE NOT STRAIGHT" on their product.
here it is again.......>
from the Starrett Exact Level Division
As you probably know the origins of the level date back to the Egyptians using p
apyrus reeds and water (water seeks its own level) to build their structures. Th
e fluid used commonly in the early stages of vial development was mineral spirit
s, hence the term "spirit level". While it is still used somewhat today, the mos
t common fluid is isopropyl alcohol. It is used as its characteristics allow for
extremes in temperature. The bubble will "grow" somewhat during cold - actually
the fluid contracts, and the bubble will "shrink" in hot weather i.e. the fluid
expands. Vials are manufactured in a controlled environment with the bubble siz
e determined at 72 degrees. Sensitivity of a vial is defined as the angle thro
ugh which a vial must move in order to move the bubble 0.100 inches. A precision
ground vial is produced from a straight length of glass tubing. The tubing is
placed in a holding fixture and mounted on a custom built machine which does th
e internal grinding. This machine rotates the fixture and moves the fixture back
and forth on its longitudinal axis over a steel bar which has a continuous curv
e end to end. This bar is kept coated with a grinding powder/water mixture. By i
ts movement on this bar, the glass tubing is ground internally with a barrel sha
ped curve. The vials are tested on a very accurate sine bar to verify the intern
al curvature. If the results are out of spec, the curve in the bar is adjusted.
The vials are made by hand from the ground tubing. They are then marked with the
specified graduation pattern, filled with the proper fluid, sealed and finally
tested again to ensure they meet the sensitivity requirements. Sensitivity range
s can get quite impressive, with
10 seconds not being uncommon. This would equate to one 1/8th division move
equaling 0.0005 inches per foot.
A 360 degree plastic vial can be manufactured basically one of two ways. The fir
st is to internally machine a barrel shaped curve in a length of plastic tubing.
After machining the vial to the specifications, (a normal specification for thi
s type vial would be a sensitivity of 35 - 45 minutes) the vial is then filled w
ith a predetermined amount of fluid so as to leave enough air to create the "bub
ble". The vial is then capped and ultrasonically welded to seal the end. The via
ls are then tested again to ensure they meet the necessary requirements. This
method provides the greater degree of accuracy, than the alternative method, but
is also usually 3 times as costly. The alternative method is to injection mold
the vial cylinder. This is done by fabricating a mold and injecting semi-solid a
crylic into the cavity. The barrel is created by inserting a spinning collapsibl
e mandrel into the part while still in the semi-solid state. By varying the spee
d while inserting and removing, the barrel shape is created. The inherit problem
with this method is that the conditions must be perfect as the plastic can beco
me distorted or oblonged when the mandrel is removed, and the plastic is allowed
to cool. This is however the most common vial found in the homeowner type level
which contain the 360 degree vial system.
The curved or bent vial is manufactured either from glass tubing or plastic via
the injection molding process. Glass vials are manufactured several different wa
ys. One method is to cut soda lead glass or "Pyrex" tubing into desired lengths
on a custom built machine. The glass is heated and the ends are "sealed" during
this process much in the way glass blowing is done. A small hole is then either
drilled in the end or side. The lines are then put on the vials, again using a c
ustom built machine. After this the vials are sent through an oven to cure the c
eramic ink used to put on the lines. The vials are then placed on trays which ha
ve been machined to the desired curvature of the vials. In this process, the cur
vature of the vials determines the sensitivity, since the inside of the vial is
straight, as opposed to the barrel shape in the aforementioned vials. They are n
ow placed back into the furnace at a determined temperature and time. The glass
is heated to a semi-solid state to allow the vial to conform to the curvature ma
chine in the trays. The vials are then filled via a vacuum and the volume of flu
id is adjusted to allow for the correct bubble size. The vials are then sealed a
nd tested for the correct specifications. Curved plastic vials are manufactured
much in the same way as described in the injection molded 360 degree plastic via
l. However, the main difference here again is that the curvature determines the
sensitivity and not the barrel located inside the tube. This therefore eliminate
s the distortion that is associated with the injection molded 360 degree vial.
From a purely manufacturing point of view, there a pro's and con's to each typ
e. The precision ground vial is a necessity for any type of extremely accurate w
ork These type vials are used to calibrate the vial setting apparatus' used to s
et the majority of all the levels manufactured. The curved vial generally provid
es a greater "accuracy" (not sensitivity) than the 360 vial, although the machin
ed 360 vial is very close. The curved vial must be hand set, making sure that th
e crown of the vial is up, and must contain two vials in each location to be abl
e to use both working edges of the level. The 360 degree vial can be hand set, b
ut is generally used because it can theoretically be inserted in any direction a
nd should be accurate. This speeds the manufacturing process greatly, thus lendi
ng itself to the "economy" style of level. Just a note; two 360 degree vials in
a single location (i.e. plumb or level) is redundant and serves to only prove th
e theory that the accuracy of injection molded
360 is questionable. If it is truly a 360 degree vial, why put in two?
The answer is that the level will not read true in all four possible positions.
Therefore by adding the second 360 degree vial the margin for error is lessened.
However by doing this, confusion is created. The two vials will have close to t
he same reading, but be off slightly. Which one is correct? Usually the bottom v
ial of the two is correct. Usually. I should also note that while stating that t
he injection molded 360 degree vial is not quite the quality of a machined 360 d
egree or bent vial, that they are usually more than sufficient for the average g
uy (or gal). The accuracy specifications are the same for both vials of equal se
nsitivity. We always try to use the common sense approach of "if you find a 2
x 4 that straight".
hope you enjoyed that.....