Introduction to Tiltmeters (Grades K-3)
Decker and Decker (1991) introduce tiltmeters by comparing them
more familiar tool, a carpenter's level. If a horizontal carpenter's
level is tilted up, the air bubble rises to the uplifted end. If a
horizontal carpenter's level is tilted down, the air bubble rises to the
other end (see figure in background information).
If volcanoes experienced large amounts of uplift and subsidence over
small distances, a carpenter's level would be a useful tool. The change
in angle would indicate the amount of uplift or subsidence. However, the
amount of uplift or subsidence is very small and requires geologists to
use very sensitive instruments. For example, the amount of uplift or
subsidence at the summit is on the order of millimeters. Geologists
measure these small changes in tilt in parts per million (ppm). A ppm is
equivalent to raising the end of a level one kilometer long by one
millimeter. A tiltmeter one km long might be an accurate tool but would
be very impractical to use. Instead of long instruments, geologists
designed more sensitive instruments. One type of tiltmeter consists of
three pots that are connected by water and air hoses. The water level in
each pot serves as a reference or datum. A pointer in the pot is raised
or lowered until it touches the surface of the water. The amount the
pointer moves is measured on a micrometer scale at the base of the pot.
Geologists can measure changes as small as one one-hundredth of a millimeter.