From the Whelen Manual
XENON FLASH TUBE PROCEDURES
1. A xenon flash tube can be very photosensitive. One will flash normally
when exposed to an external light source, but may become
hard to fire when subjected to darkness.
2. They will become hard firing with age, or when exposed to a very
high temperature. A hard firing tube will sometimes operate with
the engine running, but will fail when operated on a low battery.
3. They can develop a leak through eggshelling of the glass, or a leak
can develop around the seal of the wire to the glass. This is caused
by hot and cold cycling of normal operating of the system.
4. They can go into self-ionization (continuously glow a light blue),
thus rendering the entire system non-operational until flash tube is
replaced. This most likely occurs when the input voltage is highest.
This can be checked by turning the system off. When turning the
system back on, it generally will operate normally for a few flashes
before going back into self-ionization.
ANY OF THE ABOVE MENTIONED CONDITIONS ARE REASONS FOR
REPLACEMENT OF THE XENON FLASH TUBE.
NOTE: Installing one new flash tube in any multi-head strobe light
system, will sometimes cause the remaining old flash tube to misfire
or skip. This signifies that the old flash tube is nearing the end of it?s
service life. However, to check the questionable flash tube, install it in
a system and apply a reduced voltage, approximately 20 percent, to the
input to the power supply. If the flash tube will operate at this reduced
level, it still has a great deal of service life in it.