The Control consists of two compartments, one wet and one dry.
The
wet compartment contains a hydrometer which responds to etchant
density.
Hydrometer motion is transmitted by a pivoted beam through a isolating
elastomeric diaphram into the dry compartment. A moveable balance
weight
on the beam mechanically sets the operating solution density (control
point)
at which the beam is horizontal. Optical sensors in the dry side
frictionlessly
detect pivot beam position and thus hydrometer position. The position
sensor
is an optical encoder which passes between light emitting
diode-phototransistor
pairs to cut the infrared beams passing between them. During operation
the etchant continuously flows into the bottom of the wet inner
chamber,
over the dam and out the wet chamber drain. Changes in etchant density
cause the float to move up or down. A drain fitting on the bottom
allows
the inner chamber to drain when the etchant supply turns off. The gas
bubbles
in the solution leave the wet chamber through a vent pipe on the bottom
of the Control unit. The encoder can measure deviations as much as
±1.9
degrees Baumé from the set point. When the etchant density stays
above the control point for a few seconds the replenishment pump is
turned
on and continues to run until the etchant density returns to the
control
point. The delay in pump turn on minimizes relay and pump wear due to
excessively
rapid cycling. Pump lights on Monitor and Power Module inform the
operator
when power is delivered to the pump terminal. If the replenishment pump
has turned on to correct the out of tolerance situation the alarm will
wait 5 minutes before it sounds to give the system a chance to correct
automatically. The alarm also sounds if the pump runs continuously more
than 5 minutes. The operator can silence the alarm for 5 minutes while
the situation is worked on.
The design and materials of the Model 140
Etchant
Control have been specifically tailored to withstand the attack of
ammoniacal
etchant and give accurate data in spite of the effervescent gas bubbles
it normally contains. Similarly, the Model 141
have been specifically designed to withstand the attack of 30% nitric
acid.
All Etchant Control/Monitor models can hold the density of the etchant in its measuring chamber within ±0.1 degree Baumé or ±0.001 g/ml of the control point if the etcher replenishment feed pump and sump mixing pumps are large enough to keep up with the work load.
Model 140 & 141: with the unit correctly standardized, the control point value will read correctly ±0.1 degrees Baumé or ±0.001 g/ml. Other displayed values above and below the control point will be accurate within ±2% of reading.