  Dark noise is a dominant background in photomultiplier tubes (PMTs), which
are commonly used in liquid-filled particle detectors for single-photon
detection to see the results of particle interactions. A major contribution to
dark noise is thermionic emission from the photocathode. The dark noise of
Hamamatsu R7081-100 PMTs is characterised in a temperature and purity
controlled water tank, with the thermionic emission contribution isolated. The
results suggest that the intrinsic dark rate of PMTs does not depend on the
medium, but does follow Richardson's law of thermionic emission. There are
external contributions to the overall observed PMT count rate identified, but
the intrinsic PMT dark rate in water matches that measured in air.
