A simple, accurate, field-portable mixing ratio generator and Rayleigh distillation device

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 80cm³, 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 10W, 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.