The
Trouble with Humidity:
Hidden Challenges of Humidity Calibration
A Case Study
One year after installation, numerous high-accuracy relative humidity
(RH) transmitters are removed from service and returned to the lab
for routine testing and re-calibration. A few days later they are returned. Unfortunately, they are now further out of calibration than ever before.
This scenario occurs more often than you think. Humidity
calibration is intricate and difficult, despite vastly improved calibration technology in measuring other parameters.
The following article, orginally published in Cal Lab Magazine, examines various calibration processes
and assesses the efficacy of each.
Changes in Humidity Measurement
Calibrating relative humidity instruments
accurately and efficiently is a daunting challenge. Today's lab
managers and QA personnel struggle with the need for humidity measurement that will stay within accuracy specifications for a reasonable length of time.
Humidity is a critical environmental factor in thousands
of processes and manufacturing facilities, but the accurate measurement of relative humidity
remains elusive. Tracking humidity is rapidly becoming a greater concern in regulated environments. Applications affected by humidity include:
- Pharmaceutical Research & Development
- Pharmaceutical shipping and storage
- Microelectronics manufacturing
- Medical Equipment manufacturing & storage
Many companies are scrutinizing their humidity calibration
methods to ensure adherence to required standards. However, calibration processes are often inefficient, incomplete
or incapable of providing reliable results. Others remain unaware
of the effectiveness of their processes and may be surprised to find out through a 3rd party audit that their system lacks the requisite accuracy.
Accuracy in Humidity Testing/Calibration
As an applications example, in the
photographic film, pulp and paper, semiconductor, and textile industries,
slight variations in relative humidity can have disastrous effects
on process efficiency and output. To ensure high product quality,
companies are incorporating their humidity discoveries into process
control specifications and into their monitoring procedures.
These
developments, combined with recent technology advancements, have led
the industry to demand ever higher levels of relative humidity accuracy. Many secondary relative humidity sensors today boast of accuracies
ranging from +/-1 to 4%RH.
This performance is impressive compared
to the state of the industry 20 years ago when horse hair hygrometers
at +/-10% RH were the norm. But what about calibration? If these modern
systems are capable of achieving such accuracy, it puts enormous pressure
on the higher-accuracy (typically 3 times or better) systems used
to calibrate them.
The Role of Humidity Calibration & Testing
All secondary relative humidity sensors are absolutely dependent
on accurate calibration in order to deliver their specified performance.
While the best relative humidity sensor may boast an intrinsic accuracy
of +/-1%, calibration errors can render them useless.
It follows that all secondary
relative humidity sensors require calibration to a higher standard,
not only during production, but also as part of ongoing maintenance
to correct for drift. Good calibration cannot save a bad sensor, of
course, but improper calibration will cause an otherwise excellent
system to produce erroneous and potentially damaging results.
A Unique Perspective on RH Calibration
As a manufacturer of relative
humidity data loggers since 1995, Veriteq Instruments has experienced,
first-hand, the complexities of the relative humidity calibration
process. Portable, self-contained recorders, data loggers uniquely
introduce the element of time to the measurement of humidity, an extremely
important factor in accurate calibrations.
The units work by producing
a time-based record of temperature and relative humidity that can
be graphically displayed on a computer. The advantage of such a record
is that it details the level of thermal and relative humidity stability
during calibration and, if multiple units are tested, may also indicate
the homogeneity of the test chamber.
Thus, data loggers make it easy
to determine when, or if, a steady state has been achieved. Conversely,
they also make it obvious when a calibration has been performed hastily
or in unacceptable conditions.
A Veriteq Innovation for Better Calibration
A recent example of the value of such a record involved a customer
preparing to use several data loggers for hygrothermal mapping of
a Clean Room. Prior to use, a calibration check was made by placing
the loggers in a specially-prepared, insulated and sealed chamber
overnight to ensure that the units read identically under, what was
assumed to be, identical conditions.
Based on this assumption, and
after completing the test, the customer concluded that the loggers
were not operating within their specifications - there was simply
too much variability in the readings between units.
The real problem became clear after closely examining the data logging
records. In these records a surprising level of instability in the
test chamber was apparent, despite the special precautions.
By installing
a small device to stir the air, the customer was able to improve uniformity
and the loggers reflected this by returning to their "in-spec"
condition. So what began as a test of data loggers using a chamber
- became a test of the chamber using data loggers.
The State of Relative
Humidity Calibration today
Veriteq's experience has also given it a strong sense of how the
industry is dealing with the humidity calibration burden. With this
experience, the company has discovered that relative humidity calibration
is often a neglected or misunderstood aspect in humidity monitoring.
An example of this lack of understanding is the popular misconception
that if an instrument has a "NIST-traceable" calibration
certificate, the unit is not only accurate but is also "validated"
for the user's application.
The truth is that many calibration processes themselves have not
been validated. Equipment is often assumed to operate in accordance
with the claims made in the sales brochures. Procedures are often
borrowed and not questioned or tested.
The end result is that calibrations
are often performed:
- using reference standards that are not traceable or accurate enough
to support the intended accuracy of the calibration
- far too hastily, preventing the essential stabilization of conditions
necessary to calibrate properly
- without the use of adequately thermally-controlled chambers or
in areas where thermal stability is poor
- with unacceptable uniformity of conditions or, at the very least,
without air mixing provisions
- using methods or procedures that entail too much handling, are
out-of-date, or otherwise incomplete
- without written or defined procedures that can affect the repeatability
of results
Why is Relative Humidity Measurement so Difficult?
The basics seem elementary enough: Humidity refers to air-borne water
or vapor, which is water in a gaseous state. Relative humidity is
simply the measure of the amount of water vapor in the air compared
to how much it can possibly hold at that temperature. From a theoretical
perspective, these concepts are easy to understand - but on a practical
level, the process is not only complicated but even counter-intuitive.
Wide dynamic range
Relative humidity is more difficult to measure than most water-related
or atmospheric properties such as temperature, pressure, flow, volume,
mass or level. The complexity begins with the broad range of moisture
conditions relative humidity sensors must operate in.
For example,
a sensor rated to measure 1 to 90% RH from -40 to 70°C. must perform
in humidity conditions ranging from 1,000 parts per billion to 200,000,000
parts per billion. The dynamic range this represents is 200,000:1,
a figure that would challenge the linearity of most sensors.
If it weren't for temperature
Relative humidity is troubling because it is a temperature-dependent
variable. Its value can change significantly with even slight variations
in temperature and without any change in water content.
For example,
a 1°C. variance in temperature at 20°C and 50 % RH can introduce
an error of +/-3 % RH, an enormous variance in a calibration process.
At 90 % RH, even a 0.2°C. variance will result in a +/-1% RH error.
These temperature effects highlight the importance of thermal stability,
a condition that is often difficult to achieve in a calibration environment.
Air is harder to measure than a liquid
Perhaps the most significant calibration challenge is that relative
humidity testing must be carried out in air (or gas), unlike a temperature
calibration, for example, that can be performed using a liquid bath.
The advantage of a liquid bath is that it has a high heat transfer
capability and can create a reliable and uniform test point. Not so
with air.
Relative humidity sensors measure only the water vapor in the immediate
layer of gas contacting the surface of the sensor, a fact that emphasizes
the need for stable and homogenous environmental conditions. Air is
a poor thermal conductor and the temperature at any given point can
be affected by thermal currents and temperature gradients that make
such conditions not only difficult to achieve, but time-consuming.
Whereas a vigorously stirred liquid bath can be brought to temperature
relatively quickly, a humidity calibration environment may take hours
to stabilize, even when generated by an expensive primary system.
This slow rate of change, and the need to take time for conditions
to equilibrate at a constant level, are anathemas to modern processing,
with its high emphasis on speed and efficiency.
Sensor Non-Linearity
All secondary relative humidity sensors are non-linear devices with
temperature dependencies. Most hygrometric sensors work by changing
their electrical properties with variations in humidity and temperature.
Because of this non-linearity and temperature dependency, it is necessary
to validate sensor operation at multiple RH and temperature points,
ideally covering the range of the intended application. This requirement
is not only difficult to achieve without expensive equipment but is
also time-consuming.
How can you be Certain of Your Humidity Calibration?
Unless questioned or challenged, many flawed humidity calibration
processes may remain unchanged indefinitely. To avoid this, companies
need to conduct periodic reviews of existing equipment and procedures
and to look further at the effects and results of their calibration
processes. The following questions will prove helpful in carrying
out these reviews:
Reference standards
Are only NIST-traceable primary or transfer standards used? Have
they been validated? Do the standards have an uncertainty sufficiently
low to justify the final calibration accuracy claimed? Have all the
elements of uncertainty been considered in determining the reference
standard's accuracy statement?
Procedures
Have the procedures been validated? Do they clearly specify adequate
times to allow for thermal and humidity stability? Are there provisions
to ensure uniformity of the chamber environment? Are multiple relative
humidity and temperature calibration points tested? Are the procedures
written, followed and periodically reviewed?
Instruments and sensors
Are the calibrated instruments trusted by those who use and rely
on them? Do they need calibration more often than expected? Are the
as-found calibration conditions frequently out-of-spec? Do the instruments
require significant adjustments for each calibration? Are the instruments
or sensing devices reasonably interchangeable or are there "unexplained"
differences?
Products and processes
Are there recurring problems with product quality or efficiency that
relate to temperature or humidity sensitivity?
Summary
Relative humidity calibration, at first glance, can appear simple
and basic. In reality, it is a complex and time-consuming process
filled with potential pitfalls. Despite the importance accurate relative
humidity can play in an organization, many are unaware of the pivotal
role calibration plays and still more are unaware of the key factors
in a successful calibration.
With quality and regulatory pressures
continually increasing, it is important for companies to fully understand
and evaluate their processes to help prevent unpleasant surprises.
For more information on our relative humidity monitoring and calibration
solutions, please contact Veriteq.
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