INTERPRETATION OF CRYSTAL
ACTIVITY DATA FROM THICKNESS MONITORS
This technical advisement provides general guidelines regarding “Crystal Activity Data” seen on
thickness monitors, specifically, what “Activity” means and how it should be interpreted. Operators use
Activity Data as an indicator if a new quartz crystal sensor is “good or bad”. However,
as explained
below, Activity Data is a more useful tool for showing problems with crystal hardware.
WHAT DOES CRYSTAL ACTIVITY
MEAN?
The true "Activity" of a quartz crystal is its
amplitude of oscillation. The amplitude of oscillation is
correlated to the resistance at resonance measurement that we use at Fil-Tech to determine if a crystal
works properly.
The Activity specified on thin film monitors is the ability
of the crystal measurement circuit to conduct
current. The crystal measurement circuit includes the quartz crystal, sensor head, feedthrough,
and
cables. If there is poor contact between any of theses components the activity
reading will be low. The
activity can range from the best value of 650 to a minimum of 0 which would indicate a dead crystal.
Typically, a new crystal will register from 400 - 550, depending on the state of the sensor head
and other
associated hardware.
DIAGNOSIS AND SOLVING ACTIVITY
PROBLEMS
The use of the Activity reading as an indicator of whether
a new crystal is defective is marginal. Activity
is a more useful tool for showing problems with crystal hardware. If you are replacing
multiple crystals in
order to get a high activity reading, the odds are that the crystal is not the problem, but rather something
else in the “circuit” is faulty. Because the Activity is related to current flow, and based on Ohm’s
Law, any
additional resistance to the circuit would adversely impact the Activity value. A short list of
culprits
includes heavily coated sensor heads, damaged cables, electrical shorts of the feedthrough, dirty contacts,
and contact misalignment.
The two most troublesome and most common culprits causing
low Activity readings are contact
misalignment of the “double anchor” electrode pattern with the contact spring and flattened
crystal
contacts springs from normal “wear and tear”, causing high contact resistance. It has been
our experience
that crystal Activity can vary from crystal failure to 500 with a twist of the sensor head cap (rotating
the
crystal around the spring contact) even with very low resistance crystal of 10 Ohms. This behavior
suggests that the contacts were not mating properly with the electrode pattern of the crystal.
To determine if contact misalignment or flattened contact
springs, any of the other aforementioned
components are the source of your activity problems, follow Fil-Tech’s flow chart on the following
page
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