Measuring water vapor in normal environmental conditions is a well-known application. Relative humidity (RH) is the common term for these measurements, and it is usually the best water vapor parameter to express what most people care about. RH is associated with human comfort, soggy potato chips, and the likelihood of getting a small shock from static electricity. But water vapor must also be considered in non-environmental applications, where its presence, or lack thereof, can be important. Devices for measuring water vapor in process applications are often different from those used to measure environmental conditions, and RH might not be the best parameter to express the amount of water vapor in a gas.
An example of this is tablet coating, which is a complex process with many variations. In most cases, tablet coating requires air that is conditioned so that it is suitable for a specific process. The air is used to dry tablets, so the temperature of the air influences the process, as does the water vapor content. Air that is saturated (100% RH) at any given temperature cannot hold any more water vapor and has no ability to dry anything. Raising the temperature of the air increases its water-carrying capacity, but this can also change the overall thermodynamics of the process.
Conditioned air for tablet coating is typically held at temperatures between 50°C and 90°C. If a hypothetical process requires air at 80°C and a dew point of 10°C, these conditions result in RH of less than 2% (you can download a free humidity calculator at http://forms.vaisala.com/LP=434). This presents some challenges for humidity measurement.
Normal RH instruments do not respond well at 2% RH. One solution is to remove the humidity probe from the airstream, letting the process air cool to ambient temperature in a sample system. The dew point remains the same, but RH increases into the measurable range. The downside to this solution is the added cost and complexity of a sample system, and the sample system may slow down the response to humidity changes in the process air.
A second solution is to specify a dew point sensor that can be directly inserted into the process air, tolerates 80°C, and has a measurement range that can cover all possible requirements. In both cases, the measuring device will need a remote probe, preferably made of stainless steel, and the hardware necessary to make a process connection.
Some measurement parameters can be expressed in two or more ways, e.g., temperature in degrees F or degrees C. Water vapor content can be expressed in dozens of ways. Dew point and relative humidity are common units for water vapor, but drying processes are often better understood when using mixing ratio. This is a dimensionless ratio of mass of water vapor per mass of dry air, such as grams of water per kilogram of dry air. When the mass flow rate of air is known, it is possible to calculate the absolute amount of water in the airstream. This is useful when it’s essential to know how much water is going into a process by weight and how much is coming out.
In short, instruments for measuring water vapor in any process air or gas must be carefully selected to match the process requirements. Selection criteria include the type of sensor incorporated in the device, associated hardware for correct installation, and availability of the most useful water vapor parameter. For more thorough guidance on selecting an instrument, see Vaisala’s application note entitled, “How to Choose the Right Instrument for Measuring Humidity and Dew Point” at http://tinyurl.com/d565llm.
About the Author
Jim Tennermann is the North American Life Science segment manager for Vaisala, a global leader in environmental measurement. He has 25 years of experience in sales, marketing and business development with various firms that manufacture measuring instruments and systems. He studied mechanical engineering at Syracuse University and has published many papers and articles on the practical use of measurement technology.