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In the pharmaceutical industry, relative humidity (RH) must be
carefully controlled to ensure suitable manufacturing and storage
environments and to assure proper stability testing. If the
RH rises or falls above pre-established parameters, it can jeopardize
product quality, derail FDA submissions, increase liability
and damage a company’s reputation. And while many companies
rely on RH regulating control systems to monitor and
control humidity within their facilities, few realize just how sensitive
and prone to distortion RH sensors can be.
 Factors That Affect Sensor Functionality
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RH sensors are “air breathers.” Like tiny sponges, they “absorb”
water vapor from the air. To function properly, RH sensors
must maintain intimate contact with the environment.
Unfortunately, this exposure leaves sensors vulnerable to airborne
contaminants such as chemicals and cleaners, which can
coat or permanently damage a sensor’s surface, prevent it from
properly absorbing water vapor and ultimately distort its signal.
Even simple condensation can affect an RH sensor’s accuracy.
If the door to a high humidity environment is opened, for example,
condensation may form on the RH sensor inside. Long after
the RH reading appears normal, the sensor can remain wet internally
causing an offset in value and it may need to be removed
and dried before it can once again provide accurate readings.
To ensure RH remains within a given range, many companies
install RH system alarms. However, as I’ll demonstrate below,
these alarms provide a false sense of security. In reality, RH sensors
can drift – and RH can stray well outside pre-established
parameters – without any notification from the alarm system.
RH control systems work by measuring the current RH, comparing
it against a desired setpoint and automatically increasing or
decreasing humidity as necessary to achieve and maintain the setpoint.
RH sensors convey information via electrical signals that are
proportionate to the amount of RH detected. For every 10 percentage
points that humidity varies, for example, the sensor might
send a 1-volt signal. RH recorders, displays and system alarms
work by monitoring these same electrical signals. The alarm system
will alert users if they stray outside a pre-defined range (e.g.
35%-45%, or 3.5V-4.5V).
If an RH sensor has been damaged or contaminated, it might
send a signal of 4V, indicating 40% humidity – and the system
display would reflect this – when in reality the relative humidity
might be just 38%. As this sensor drifts over time, it may continue
sending a 4V signal, when RH is just 36%, then 34%, and
so on until the RH is well outside the pre-defined range of
35%-45%. And yet, because the RH alarm system relies on the
sensor signal, and the signal remains at 4V, the alarm system
will not be activated and the system display and recorder will
look normal.
Factors That Affect Sensor Functionality |
How can your company protect itself from the challenges inherent
in RH sensors? By incorporating redundancy and using
two RH sensors – one to regulate the RH and a second, independent
sensor to monitor the system. The second sensor will
verify the results from the primary sensor and highlight any irregularities
or discrepancies. And, while it’s technically possible
for both sensors to become distorted, the likelihood that both
will drift at exactly the same rate, and exactly the same time, is
miniscule. As long as there’s variation between the two, you’ll
know a potential problem exists.
In addition to verifying the results of your primary RH sensor, a
secondary sensor can provide continuous monitoring during primary
system failures. The secondary sensor will supply a valuable
record of RH levels throughout the system failure, enabling you to
identify potential problems that may have occurred as a result.
To maximize the benefit from your secondary sensor system,
choose one with an extended battery life. If your primary system
is knocked out due to power failure, your secondary system
may need to function on battery power for an extended period
of time. Some sensors have batteries that last 10 years while
others are unlikely to last through a long weekend.
To minimize malfunctions and down time, choose a secondary
RH system that’s easy to use, with a minimum number of
moving parts. Chart recorder’s pens, for example, can run out of
ink and recorder paper must be replaced regularly. A better option
might be a data logger, which is self-contained, has no
moving parts and can be installed in minutes.
For the most accurate results, place sensors as far apart as
possible within the monitored environment. This way, if contamination
or condensation occurs in one area of the space, the
chance of both sensors being affected in the same way is greatly
reduced. Also, because temperature variations can have a significant
impact on RH, the RH in an environment is much less uniform
than you might think. In a stability chamber operating at
40°C and 75% RH, for example, 1°C of nonuniformity will cause a
4% variation in RH. While the amount of water in the air may remain
close to constant, the RH may not. Positioning sensors at
multiple monitoring points gives you a much more accurate idea
of what’s happening throughout the environment.
While risk reduction through redundancy is common in many
other process control applications, the pharmaceutical industry
has been slow to adopt this practice. This may be due in part to
price – it used to cost several thousand dollars to set up a second
RH sensor system. Today, however, when a complete, self-contained
secondary monitoring system can be purchased for around
a third as much, there’s simply no excuse for not protecting your
company’s operations, products, clients and reputation by building
redundancy into RH regulating systems.
Written by Kevin Bull.
Originally published in the March 2006 edition of Pharmaceutical Processing Magazine. |
