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Feedback control systems manage many parameters
within the pharmaceutical industry.
They’re used in heaters, refrigerators,
freezers, and stability chambers, among other
applications. Pharmaceutical companies also rely
on them to control temperature and relative humidity
(RH) and to maintain suitable storage,
manufacturing, and stability testing environments.
What many companies don’t realize is
that feedback control systems can mask sensor
drift. Because the drift usually won’t show up on
system displays or chart recorders or be detected
by system alarms, you may not know there’s a
problem until stability testing, product quality,
or patient health has been jeopardized.
Though the consequences of sensor drift can
be serious, the solution is surprisingly simple.
An independent monitoring system with an independent
sensor separate from the feedback
control system sensor should be installed. This
secondary sensor will monitor and verify proper
operation of the system independently and detect
potential drift early on.
Feedback control systems rely on sensors to
emit a signal proportionate to the parameter to
be controlled (e.g., RH). The system compares
this signal with a desired setpoint (e.g., 50% RH)
and automatically increases or decreases its output
to eliminate the gap between the signal and
the setpoint. Many systems use a display or
recorder, and some feature alarms that indicate
when the measured parameter falls outside an
established range, but all of these methods are
routinely connected to the same control sensor.
In time, if the feedback control sensor is exposed
to contamination or degradation, its output
signal may become distorted or drift. Because
signal drift usually occurs gradually and
incrementally, however, it will not be evident
on the system display or chart recorder, nor will
it trigger system alarms.
Although sensor drift may occur too slowly
to be detected, it can occur faster than most calibration
cycles. Because sensors drift without
any obvious system changes or indications, you
likely won’t be aware that a problem exists until
your operations or products have been compromised.
Here’s how.
Assume that the desired RH in a stability
chamber is 50% and that the system must remain
accurate within 5%.
Assume that for
every 10% RH, the feedback control system sensor
sends a corresponding electric signal of 1 V.
So, if the RH is 50%, the sensor sends a 5-V signal.
Should the signal be greater than 5.5 V or
less than 4.5 V, the alarm will be activated.
If the feedback control sensor drifts, it may
send a 5-V signal when the chamber’s RH is
actually 48%. The drift may worsen over time
until the RH is far outside the acceptable
range. Yet, because the sensor signal remains
at 5 V, the display will continue to indicate
50% RH, and no alarm will activate. The sensor
failure will have been masked, and you’ll
never know.
Though this type of drift occurs in many
types of feedback control sensors, it’s particularly
prevalent in RH measurement.Unlike most
other sensors, the internal structure of an RH
sensor must be in direct contact with the environment,
leaving it vulnerable to dust, airborne
chemicals, and other contaminants. Even small
levels of contamination can cause significant
and permanent drift.
Many industries routinely build in sensor redundancy,
but this practice is surprisingly uncommon
within the pharmaceutical industry.
If your company uses single-sensor feedback
control systems, it’s vital that you understand
the inherent risks and identify the potential effects
of an undetected sensor failure on your
organization, operations, and reputation.
Written by Kevin Bull.
Originally published in the March 2006 edition of Pharmaceutical Technology Magazine.
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