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 measurement results.
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 range.
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%.
The 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.
Humidity Salt 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
The 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 levels.
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 humidity levels.