This Small Business Innovation Research (SBIR) Phase I project aims to develop a micro-electro-mechanical-systems (MEMS) based moisture (dew point) measurement instrument. Measurements of trace moisture are needed in a variety of different industrial sectors, including semiconductor manufacturing, pure gas supply, atmospheric and climate research, aerospace, petrochemical processing, power generation, air filter and purifier manufacturing, and supply of reference standards for other trace gases. Most of the commercially available moisture sensors lack the required accuracy, sensitivity, and responsiveness for many existing applications, or are too costly. The proposed approach utilizes micro-electro-mechanical resonant balances coupled with a cooling element. The instrument will measure dew point using the standard chilled surface technique to deposit a layer of moisture on the surface of the resonant balances. Mass of the deposited moisture causes a shift in the resonant frequency of the micro-resonator that indicates reaching the dew point. Because the nanobalances are capable of weighing as little as a few femto-grams of deposited moisture, only a very small amount of gas needs to be chilled to deposit sufficient water to indicate the dew point. Preliminary prototype assembly and reliability and response time characterizations will be performed in this Phase I effort.
The broader impact/commercial potential of this project is development of more accurate, responsive, stable, robust, and versatile moisture measurement instruments than currently available instruments at the same price range. Chilled mirror dew point sensors generally provide higher accuracy compared to other categories of moisture sensors. Such devices however suffer from long response times and vulnerability of their optical sensing mechanism to debris and contamination. The proposed MEMS based technology combines the advantages of the chilled mirror sensors along with faster response and improved robustness provided by the MEMS resonant nanobalances. The reduced chilling needs allow for faster response to changes in moisture content, reduced power consumption for the cooler and novel uses such as battery powered spot sampling systems. An important immediate market for the proposed instrument with great national significance is the measurement of moisture in natural gas. In cold climates excess water in the gas pipeline can freeze the pipe shut, rendering the pipeline useless until the frozen point can be located and cleared. The battery powered, spot checking, robust operation and low cost nature of the proposed instrument will allow widespread use in the upstream Natural Gas pipeline system to measure moisture at points of custody transfer.
Natural gas is an important strategic resource and a relatively clean source of energy (cleanest among fossil fuels). With the recently developed and utilized advanced fracturing techniques allowing extraction of natural gas from shale, there has been a natural gas boom in the United States over the past few years that is expected to continue well into the future. Over 500,000 natural gas wells currently exist in the United States with over 38,000 new wells drilled each year. At a natural gas wellhead, as the gas leaves the well it contains water from the gas reservoir. As it moves through the pipeline system, the water vapor can condense and corrode the pipes, water can leak in or out of the pipeline, and worst of all, in cold weather, water may freeze in the pipeline rendering it useless until the frozen point can be identified and cleared. Under high pressures and in presence of water, methane can form methane hydrates. Methane hydrates are very similar to ice in appearance, but are in fact methane molecules entrapped by water molecules bonded by hydrogen bonds. Under the right conditions, methane hydrates can be stable at temperatures above freezing. Therefore, it is necessary for water to be removed from the pipeline at various water separation and dehydration points. At each of the dehydration points, the water content of the outgoing gas needs to be monitored to make sure that the dehydrator is working properly and the amount of moisture in the gas is below the potentially problematic level. The objective of this SBIR project is to develop a Micro-Electro-Mechanical-systems (MEMS)-based moisture sensor instrument that can be mounted at the Natural Gas wellhead as well as other points along the gas distribution network (e.g. points of custody transfer). The instrument must be able to operate off of solar power as no power is generally available in the locations where the moisture must be measured. The innovation in the proposed approach to measuring moisture is the use of a micro scale mechanical resonator capable of measuring as small as a few femtograms of mass in a chilled mirror type dew-pointer. The very high mass sensitivity of the microresonator allows for the accurate sensing of a layer as thin as a few angstroms (a few molecules thick) of water vs. the hundreds of nanometers thick layer that is required by a mirror type optical sensor. The small sample size allows much faster response and the nature of the microresonator allows automatic re-zeroing of the sensor before each measurement. This re-zeroing (taring) makes for an extremely accurate sensor and cannot be accomplished by competitive devices. Under the Phase I award, a sensor prototype was assembled and connected to a custom-made dew point generator setup. The test setup was automated using a computer interface and the sensor was continuously run for long periods of time performing dew point measurements in 1 to 5 minute timeframes. The measured dew point values were found to be consistent and in very good agreement with the predicted values based on the dew point generator settings. Dew points over a wide range of 16ºC (17,900ppmV) to -41ºC (115ppmV) were successfully and accurately measured by the assembled prototype. The MEMS resonant mass balances were found to be surprisingly robust. Accelerated tests were performed on two resonators running continuously for an overall period of over 3 months performing over 50,000 measurement cycles without showing any sign of fatigue or performance degradation. Other aspects of the proposed effort including performance characterization of the MEMS resonators under high pressure and customized easy-connect electrical connections to the resonators were also performed showing promising results.
