Humidity is one of the
more difficult measurement problems for calibration laboratories. There’s a lot
more to calibrating Relative Humidity than simply buying equipment and
following generally accepted procedures. There are many complex techniques to
learn and a wide range of humidity systems and technologies, whether it is older
hair units, capacitive/resistive sensing elements, or wet-bulb/dry-bulb psychrometers
and chilled dew-point mirror systems. The proper equipment and proven
calibration techniques are important to achieving consistent and repeatable
A further difficulty is
that Relative humidity is a derived measurement from the temperature and the absolute
moisture content, or dewpoint, of the environment. Since most humidity
equipment on the market senses relative humidity rather than the amount of
water present, small temperature variations in the environment will introduce relative
humidity errors in the measurements.
Generating the various
humidity measurement points can be accomplished by using several types of
humidity generators that use two pressure systems or dewpoint temperature
systems. Those systems can generate humidity levels with high accuracy and
reliability however, they are relatively expensive to purchase.
One of the oldest methods
for generating humidity at different levels is by using Saturated Humidity Salt
Baths. Depending on the accuracies needed for the laboratory, Humidity salts
can be a cost-effective way of generating known humidity levels across the
Humidity Salt Fundamentals
When mixed with distilled
water, the chemical properties of certain salts produce a known relative
humidity value. Sodium Chloride, or table salt,
is the most widely used salt and will provide a reading of roughly 75% RH.
Other popular salts that are used to produce different humidity levels include Lithium
Chloride for 11%, Magnesium Chloride for 33%, Potassium Carbonate for 43%, and
Potassium Sulfate for 97%.
correct mixtures of the salts and distilled water (called a ‘slurry’) can maintain
a fairly constant humidity in a closed container. Typically, the slurry mixture
is placed with the device to be calibrated into a sealed jar, bag or other
clear container and the environment is allowed the proper time to reach the
specific humidity level. The indication on the device can then be read to
determine if the reading is within the equipment’s specifications. By repeating the procedure using several
different salts, calibration of the device can be made over the full range of
the device’s capabilities.
solutions will maintain their specific humidity values with accuracies of between
1%-2% RH depending on the salt used and the temperature of the environment, but
their ease of use makes them ideal for lower precision applications, such as
checking handheld or wall mounted electronic humidity devices.
Advantages of Humidity Salts
Humidity Salts are an inexpensive and efficient way of generating various humidity reference
points. The expiration dates of the salts are typically 2 or 3 years as long as
they are stored in a cool, dry environment out of direct sunlight.
Humidity Salt solutions
are highly portable and are excellent for field calibrations or for use in
areas where the removal of the measurement instruments to bring into the
calibration laboratory is difficult, time-consuming or otherwise not practical.
Disadvantages of Humidity Salts
Continual maintenance is required
for the Humidity Salts to sustain the proper humidity. Over time, water in the
slurry will either need to be added or removed to maintain the desired humidity
levels. The slurry will also need to be stirred prior to each use to ensure
that the materials do not separate. The Humidity Salt solutions can also be
highly corrosive and can react with any metal surfaces within the chamber.
Depending on the size of
the container used, it can take anywhere from 6 up to 20 hours for the humidity
conditions within the container to stabilize. In
larger containers without circulation, there will be slight variations in the temperature
at various levels as the air in the container is not a very efficient heat
transfer medium and therefore, variations in the humidity will exist at those
Although these factors can
add increased time, inconvenience and uncertainty to the process of calibrating
humidity instruments, Humidity Salts are still widely used in calibration
laboratories and will continue to be an efficient way of generating known