Humboldt H-3250 Length Comparators - Manual (Page 4)

With the test bar in place between the terminals, set the large hand on the
outer scale and the hand on the lower small scale (0.010" movement) on “0”.
The upper small scale (0.100" movement) should be set on “2”. If adjustment is
necessary, loosen the hex locking nut on the terminal elevating screw at the base
and adjust the height of the terminal until the hand on the upper small scale
(0.100" movement) is on “2”. Hold base terminal in place and tighten lock nut.
Operation
Use for measuring length changes of hardened cement paste, mortar and
concrete prismatic specimens to 0.0001-inch accuracy.
Specimens are placed between the two terminals on the Length Comparator.
These terminals are equipped with collars, and both are shaped to meet
the measuring studs cast into the ends of the test bars. The upper terminal
is attached to the end of the indicator spindle and is adjustable due to the
functioning of the indicator. The lower terminal is stationery, being attached
to the base with a threaded elevating screw through the base and held in
place with a hex lock nut. The lower terminal needs to be adjusted is to obtain
a proper reading on the indicator. This is done using the Invar Test Bar (see
Assembly and Set-up above).
In operating the Comparator, specimens should be placed into the instrument
with the Indicator retracted. The specimen is set against the lower terminal and
the indicator is released, very slowly and carefully, to allow contact of the upper
terminal with the gage stud of the sample.
Operation of Mechanical Dial Indicator
The outer scale around the circumference of the dial directly indicates in
0.0001" increments with a range of 0.4000". The scale may be rotated to set
zero at any indication of the needle pointer then locked with the set screw.
The two smaller scales with needle pointers on the face of the dial indicate the
number of revolutions of the large pointer. One full revolution of the gauge’s
large hand on the outer scale is equal to .010"
One full revolution of the hand on the small, lower scale is equal to .100"
Every full revolution of the hand on the small, lower scale is recorded on the
upper small scale, up to the maximum of 4. Refer to Figure 1 for an explanation.