Electronics. Pharmaceuticals. Medical devices. Cosmetics. Foods and beverages. These are just some of the products requiring careful control of rela- tive humidity levels in controlled environments.
Failure to properly measure and control relative humidity
in the cleanroom can result in lower yields, increased scrap
and waste, contaminated product inadvertently reaching
consumers, customer lines down, increased liabilities and
decreased revenues—among other situations best avoided.
Carefully monitoring and controlling the relative humidity in a cleanroom is an absolute requirement—with no
What is relative humidity?
The amount of water vapor in the air at any given time is
usually less than that required to saturate the air. According
to Georgia State University1,the relative humidity is the
percent of saturation humidity, calculated in relation to saturated vapor density.
Relative humidity (RH) can be defined as “the amount
of moisture in the air compared to what the air can ‘hold’
at that temperature. When the air can’t ‘hold’ all the moisture, then it condenses as dew.”1 However, the argument
continues that, “The air doesn’t ‘hold’ water vapor in the
sense of having some attractive force or capturing influence. Water molecules are actually lighter and higher speed
than the nitrogen and oxygen molecules that make up the
bulk of the air, and they certainly don’t stick to them and
are not in any sense held by them.”1
Why is relative humidity important?
Particulate count. Temperature. Airflow. Humidity. These
five words are among the environmental factors that must
be measured and controlled in the cleanroom environment.
Sometimes the ‘stickiest’ of these is humidity. Measuring and
controlling it within prescribed parameters can be a challenge.
Too little or too much RH can impact much more than the personal comfort of cleanroom employees. Too little humidity can
be quite electrifying—creating issues of static build-up and discharge. Too much humidity brings its own woes: encouraging
the growth of bacteria and microbes, corroding sensitive metals
whether in products or equipment, and manifesting itself in
moisture condensation and water absorption. Then there’s photolithographic degradation. Photoresist processes are among the
most sensitive to humidity, and can be among the most costly
to control for, due to their tightly required parameters. The bottom line: any of these conditions can result in cost overruns,
scrapped products, and shortened equipment life. In short, the
diminution of cleanroom performance, which is costly in itself.
Some of these evils deserve a bit more discussion, as the
cleanroom facilities staff finds itself in a role not unlike that
of the carnival plate spinner—trying to balance sometimes
conflicting, and certainly not aligned, humidity requirements
for different control factors.
For employee comfort working in a controlled environment, the generally accepted ideal range for humidity levels
is 40 to 60 percent. The ‘bunny suits’ worn by cleanroom
employees exacerbate warmth and moisture effects so it’s
important not to discount employee comfort and the impact
that can have on productivity. High levels of humidity can
depress even the most energetic employee; levels that are too
low are clinically proven to increase the rate of respiratory
infections, skin issues, and general discomfort.
The Sticky Challenge of Relative
A host of evils lurk within the realm of humidity control. Maintaining the integrity
Target humidity and temperature control decisions impact costs in both
construction and operating budgets. (All images courtesy SMRT; Randall
Consistent accuracy—over an extended period of time—is the most important factor in selecting RH monitoring equipment.
Brad Hodges, PE
Relative Humidity = actual vapor density x 100% saturation vapor density c u apor u t v de t