Water Vapor Measurement in Process Gases
The Trials & Tribulations of Moisture Measurement
In our everyday lives we encounter situations where industrial processes can pollute and contaminate our drinking water and/or bodies of water which are sources of food and recreation. However in an ironic role reversal, in many industrial processes water can be the contaminant. Removing this water (typically in vapor form) can be vital to the quality of the process/product, but can also be very costly.
Water vapor is the third largest component of our atmosphere (after Nitrogen and Oxygen); and because it is a small polar molecule it is able to get even into places that otherwise seem well sealed.
If water vapor is a contaminant in a process, it must be removed. But since it is costly to remove it (the lower the desired level the costlier it gets) it becomes essential to determine accurately and quickly the water vapor content of a particular process. In addition, alarms for excessive water vapor content are required to prevent the decline of product quality. Thus some of us get tasked with the challenge of making that measurement and must take into account many considerations. In a short article we can't cover all of them or even explore the details of the ones we do mention, but we can become aware of what to research elsewhere. Following are some important points:
1. Like most endeavors our expectations must be in line with reality. It is important to cast off hopes that this measurement is like the measurement of temperature or pressure. Let me assure you it is not: in price for the equipment, in speed of response, and especially in the uncertainty of the measurement. Equipment for measuring water vapor in gas is much more like a laboratory analyzer than like a temperature or pressure transmitter. Yet we have to make the measurement in a real process and deal with field conditions. Choosing the most appropriate technology for the particular measurement is not a trivial task and it is further confused by a marketplace littered with unrealistic claims.
2. The terminology of water vapor measurement is wide-ranging. The proper terminology for water vapor is "Humidity", however commonly many refer to it as "Moisture" (which actually should mean liquid water). Traditionally "Humidity" references imply higher contents of water vapor such as found in the atmosphere (in the many 1000's of ppmv) and some use "Trace Moisture" to mean very low amount of water vapor (in the ppm or ppb levels). I will continue to use water vapor in hopes of avoiding confusion. I have encountered the following references in water vapor measurement discussions - some of which refer to the technology used or the units of measure: Moisture Analyzer, Dewpoint Analyzer, Dew Point Meter, Dew pointer, Hygrometer, Chilled Mirror, Humidity Meter, Hygrometrics, Phychometrics, These are not necessarily all interchangeable.
3. The units of measure are commonly determined by the particular industry and its historical measurements. Some of the units of measure one may encounter are:
a. Partial water vapor pressure - uB or mB or mmHg or Pascal etc
b. relative humidity - RH%
c. vapor concentration (density) - lbs of water per million standard cubic feet (lbs/mmscf), or g/m3
d. Dew point or frost point temperature - °C or °F etc
e. ratio by volume - ppmv or ppbv
f. ratio by mass (more commonly used for solids or liquids) - ppmw
Understanding these units of measure is intricate and making accurate conversions challenging, in addition some conversions require knowing the pressure while others the temperature of the gas. There are several formulas for converting units as well as several enhancement factors for the medium and none of them produce the same results, even though they are close enough for most practical applications the differences can be a source of confusion.
4. Some have preconceived notions of what the water vapor content should be in their process, or how well the process is sealed. One must keep an open mind that these notions could be erroneous. Water molecules will travel against the flow (and pressure gradient) of gas from the exhaust of a process, water molecules will permeate to some degree many if not most materials and even some metals, water molecules will tenaciously cling to almost any surface for a prolonged time. One can see that these facts in conjunction with the possible 20,000 ppmv content of water vapor in the atmosphere could cause your process to have different water vapor content than you think.
5. Strategically positioning the measurement equipment can have significant affect on the process if something goes wrong. Should it be positioned near the source of the dry gas in order to provide an early warning of a failure in the supply system; or should it be near the end-use point as to also detect leaks in the distribution system. To measure in-situ or extractive? To measure at pressure or atmospheric? All of these considerations will have an impact on the quality and long-term reliability of your measurement.
6. There are many other important considerations but I will mention only one more: Carefully consider how often to re-calibrate the equipment. A general rule of thumb is to initially re-calibrate in one year, ask the calibration lab for as-is-data which can be used to determine when should you calibrate again. Consider if the process will have to be interrupted to facilitate calibration. When choosing a vendor inquire about the calibration turnaround time and traceability.
I will leave you with an old chemistry class reminder, water vapor is a gas and it behaves according to Dalton's Law of partial pressures; it will permeate into sealed environments if the partial water vapor pressure in the atmosphere is greater than in the sealed environment - even though the sealed environment is at a total pressure much greater than atmospheric. For many of us this is somewhat counter intuitive but we cannot let intuition override science.
To determine the content of water vapor, you must be determined. Happy Measuring
This article has been kindly supplied by :
Author: Bedros Bedrossian
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