Scripps High-precision system for humidifying dry air streams in greenhouse gas analyzers

Sponsor: Dr. Jooil Kim

Background

Collecting data accurately is important to The Keeling and Weiss Labs at UCSD’s Scripps Institute of Oceanography, especially in providing information about the evolution of climate change. As more data gets collected the tools get better and come with new features that must be tested rigorously. One such new data acquisition feature that is to be tested by this project's sponsors is a moisture content adjustment factor on a greenhouse gas (GHG) optical analyzer.

Making sure this adjustment factor is working correctly is important because optical analyzers such as LI-COR have a really hard time measuring gas molecules that have too much water saturation. Most gasses in the air have a distinct light absorption spectrum that can be identified in a dry gas stream with relative ease. Water covers an extremely large absorption spectrum resulting in large amounts of noise in the data. Thus, being able to verify the accuracy of the moisture adjustment factor is significantly important.

The wavelength of H2O is incredibly similar to that of the atmosphere. The more saturated the air samples collected are, the less accurate the data may be.

The LI-COR Greenhouse Gas Analyzer is a portable device that allows researchers to collect and analyze data in seconds.

Primary Goals for this project included:

● Create a mixing chamber and piezo system to steadily atomize water to moisturize dry air (0%) to a range of 0.01% - 1%

● Achieve moisture detection with a resolution of 0.01% (or less)

● Mixing chamber is able to feed a steady air mixture into an optical GHG measurement instrument

Design Solution

The Humidification Chamber and Piezo Fitting

Air is fed into the top of the humidification chamber via tubing and is forced into a swirling motion by a baffle.
Water is fed into the piezo fitting, where is atomized into water vapor and enters the humidification chamber.
The air and the water vapor mix inside the chamber.
The mixture exits via tubing connected to the bottom of the chamber and is fed into the GHG analyzer.

The Piezo Circuit

The piezo circuit works in two steps. The left side of the circuit uses a 555 timer with added diodes, to induce a duty cycle on the piezo. The duty cycle can be controlled using a 5k ohm hand potentiometer and can be lowered down to 5.8%. The frequency of this cycle is ~1.6 kHz to be compatible with the second half of the circuit, the requirement being that it must be significantly lower than the vibration frequency of the piezo device. The second half of the circuit also utilizes a 555 timer injunction with a MOSFET and inductor to allow for the piezo device to physically vibrate at a frequency of 113 kHz, its resonant frequency. The vibration of the piezo is what drives the water atomization when in contact with water.

IMG_2724.MOV

Test run of the high-precision system for humidifying dry air streams in greenhouse gas analyzers. In this video the potentiometer in the circuit is adjusted from minimum resistance to maximum, causing the piezo element to change atomization rate from its maximum output to its minimum. The water vapor being fed into the chamber becomes fainter as this occurs and can be seen in the video. (Viewing in 1080p strongly recommended)

Summary of Results

The data below is from the Li-Cor GHG Analyzer connected to the high-precision system for humidifying dry air streams. The analyzer initially is fed just bone-dry air (0% moisture content) and displays steady CO2 and H2O levels. The piezo is then turned on in pulses and adds moisture to the air sample, which raises the H2O content. The piezo is then quickly shut off to allow for the moisture content to return to a dry stream. Quickly raising and lowering the moisture level like so tests the analyzer's moisture adjustment factor. If the adjustment factor on the Li-Cor is perfect, there should be no change in the CO2 readings when the moisture content is added. However from the data, there is a deviation of ~2 ppm of CO2 when the water content spikes. The amount of deviation also increases as the air stream reaches saturation, up to 0.6%, showing that the adjustment factor does not completely counteract the affects of moisture content in the air.

CO2 and H2O levels measure in the Licor GHG Analyzer: Each peak in H2O levels represents when the piezo was turned on and added moisture to the dry air samples. If the moisture adjustment factor of the analyzer was working as expected, there would be no change in CO2 levels and the deviation would remain close to zero.

Executive Summary

Executive Summary.pdf

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