Accurately measuring relative humidity is the initial step to
control. Selecting the most appropriate sensors and trans-
mitters to monitor and measure RH is the first item of
business in ensuring trouble-free operations. The second
critical factor is ensuring the selected equipment performs
in a stable manner over the long term. The third factor in
the holy trinity of RH monitoring is proper placement of
the monitors; it’s important to avoid background envi-
ronmental conditions that can impact accuracy. While
RH monitors probably aren’t the first thing most facilities
professionals would name among their top concerns, their
failure can create huge headaches. A quick overview of
each of these factors:
• Equipment selection: Required accuracy specifications are
the first items to consider and delineate when compar-
ing RH monitoring equipment. This is the time to review
process and product requirements, narrowing the field of
potential equipment under consideration. However, equip-
ment selection based solely on performance claims can fall
short. “Accuracy” can be in the mind of the salesperson—as
a qualitative term in the ambiguity of measurement.
• Accurate performance over the long term: Monitoring
relative humidity is a never-ending requirement—a marathon, not a 5k. Consistent accuracy, consistent accuracy,
consistent accuracy—over an extended period of time.
This is the most important factor in selecting RH monitoring equipment.
Chemical vapors lurk in controlled environments and
pose the biggest threats to stability over the long haul.
Because vapors can keep water molecules from the sensor,
they can cause inaccurately low RH readings when the
humidity is high. Conversely, they can also cause falsely
high readings in low humidity settings. Features on some
monitors, including a heat-driven chemical purge function, protect some RH monitors.
• Monitor placement: RH monitors are extremely sensitive
to factors in the surrounding environment—temperature,
heat generation, isolated moisture—any of which can produce inaccurate measurement.
RH is ruled by temperature; requiring careful consideration
when placing and installing RH sensors. Some factors to con-
• Install devices away from heat sources, include heat
generating equipment. It’s important to avoid false
readings created by temperature variations localized
to heat radiated from isolated equipment. While good
airflow throughout the controlled environment can
minimize this impact, discrepancies around equipment
• Make sure the RH measuring device doesn’t become
its own worst enemy. Some, when contained in enclosures, will heat up, resulting in an erroneous RH reading. Segregating the humidity sensing element from
the monitor’s electronics will circumvent this problem.
• Given that the RH monitor is measuring moisture, avoiding environments that falsely increase or decrease detection of RH is critical. A few places to avoid when placing
monitors: humidification systems – including ultrasonic
and steam injection, and too close to cooling coils. While
integrated filters shield the sensors from water, microclimates can be created from water in the filter, creating an
Maintaining the integrity of the RH measurements is an
ongoing effort, requiring careful and consistent calibration.
Whether manufacturing to GMP rules in the pharmaceutical industry or developing parameters in new product
development, careful calibration will ensure accuracy.
Maintaining a data log of RH readings verifies cleanroom
conditions—particularly important if product integrity
While the measurement and control of relative humidity can
literally be a ‘sticky wicket’ the central role it plays in process
and product integrity, as well as equipment life and employee
comfort, demands careful attention. A carefully designed
monitoring program is the best insurance policy to avoiding
operations and quality problems.
Brad Hodges, PE, LEED AP, CxA, is Principal, Science,
Technology and Industry Group at SMRT Architects and
Engineers ( www.smrtinc.com). He has more than 20 years of
experience engineering controlled environments and labs for
clients in life sciences, electronics, pharmaceutical, education,
government, and healthcare sectors. Brad can be reached at
Measuring and controlling RH within prescribed parameters in a cleanroom
can be a challenge.