|GRIFFIS, T - University Of Minnesota|
Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2010
Publication Date: 12/15/2010
Citation: Baker, J.M., Griffis, T.J. 2010. A simple, accurate, field-portable mixing ratio generator and Rayleigh distillation device. Agricultural and Forest Meteorology. 150(12):1607-1611.
Interpretive Summary: Infrared gas analyzers are now in common use throughout the world for measuring CO2 and H2O exchange between the surface and atmosphere in diverse ecosystems. Analyzers can be routinely calibrated for CO2 using tanks of known CO2 concentration, but this cannot be done for H2O, because stable water vapor standards cannot be stored in compressed tanks. Instead, analyzers are periodically manually calibrated, limiting the frequency with which calibrations can be done, particularly at remote sites. Consequently, the H2O data from these systems have received little use because scientists are less confident in their accuracy. To address this, we have developed a simple, inexpensive portable device that can be connected to a tank of dry air to provide an airstream with a desired water vapor mixing ratio. The unit is controlled by a datalogger that can be programmed to routinely and automatically route air of known mixing ratio to an analyzer. Field and laboratory tests have shown it to be accurate and reliable. This system provides a means for improving the accuracy of evapotranspiration (ET) measurements at remote sites, and scientists using the data from these locations will have greater confidence in their results.
Technical Abstract: Routine field calibration of water vapor analyzers has always been a challenging problem for those making long-term flux measurements at remote sites. Automated sampling of standard gases from compressed tanks, the method of choice for CO2 calibration, cannot be used for H2O. Calibrations are typically done manually with a dew point generator, and as a consequence are done less frequently, degrading the accuracy and utility of H2O flux data. To remedy this, we have designed, built and tested a field-portable mixing ratio generator, also including features that facilitate its use in water vapor isotope research. It consists of an aluminum cell, with a usable capacity of 80 cm3, that is coupled to a Peltier heater/cooler and encased in insulation. The temperature of water in the cell is monitored with a thermocouple and a pressure transducer is used to measure the cell pressure. A data logger uses this information to compute the mixing ratio in the cell and control the polarity and duty cycle of the power input to the Peltier block in order to drive the system toward the desired mixing ratio and to maintain it. Incoming air enters through a bubbler and the exiting air is directed to the gas analyzer. A separate fitting mates to a syringe, allowing easy filling and draining of the cell. The cost of the components, exclusive of the data logger, is approximately 1100 USD. Power consumption is low, on the order of 10 W, facilitating use at remote sites. As a consequence, the response time of the instrument is relatively slow, but could be substantially improved for situations where dynamic response is more important than power consumption. Testing has shown that the unit is accurate over a broad range of mixing ratios, able to compensate for changes in ambient pressure, and stable for long periods of time. In addition, it has been used to generate Rayleigh distillation plots, demonstrating its utility for routinely checking the performance of water vapor isotope lasers.