What are some of the instruments volcanologist's use and what do they do?

Question

What do they use to aid in predicting the behaivior of volcanos?

Answer 1

Broad band seismiograph used to record seismic activities, tilt meter to see the land height differences, geophone to record the low frequency noise mostly heared before volcanic eruption.

Answer 2

Seismometers are used to measure the frequency of earthquakes.  The number, intensity and frequency of earthquakes can alude to movement of magma.

High temp thermometers are used to measure steam vents, lava, hot springs etc...

Also things like pH meters and ion detectors to measure acidity and presence of sulfur ions and such to see if the chemistry of the outgasing magma is changing...

Those are just a few...