Final Design

High-Precision Humification Test Stand

The high precision humidification chamber, sponsored by Dr. Jooil Kim of the Scripps Institute of Oceanography, is a project designed to test the moisture correction factor embedded in SIO’s greenhouse gas analyzer (GHG) analyzer. The humidification chamber project focuses on varying the moisture content of an air stream which will then feed into the GHG analyzer. The moisture content produced by the chamber will range from 0.01% to 1% water to air molar ratio. As the moisture level varies, the GHG analyzer should produce steady levels of GHG measurements, showing that its moisture adjustment factor correctly adjusts for varying levels of moisture in the air stream.

Final Design incorporating the Test Stand, Piezo Fitting, Water Tank and connections

Manufacturing

Mixing Chamber Assembly

The overall design of the mixing chamber can be split into two components, the chamber itself and the piezo system. The mixing chamber is made of four removable parts:

2x End Caps
Central tube with a press-fitted baffle
Piezo Fitting
- Where the piezo atomizer is mounted to the side of the chamber
- Capable of atomizing water at varying rates via a potentiometer

Air flows in through the top of the chamber, which then passes through the baffle where a swirling flow is initiated. Just below the baffle the piezo is atomizing water through the second inlet and mixing occurs between the air sample and water vapor. By the time the mixture reaches the outlet at the bottom of the chamber, it would have mixed uniformly and exited the chamber based on diffusion simulation of this chamber geometry. This uniform air is then fed into the GHG optical analyzer.

Humidification Chamber Assembly connected to Piezo fitting

Electronics

Hardware

The main electronic component of our design is a piezo atomizer that vibrates at a high frequency in contact with water in order to create a water vapor stream. An OCESTORE 113 kHz Ultrasonic Mist Maker piezo atomizer was tested and implemented on the side of the chamber using a constructed circuit with a hand potentiometer that would allow for the piezo to be driven at various duty cycles to induce lower atomization rates.

Circuitry

The circuit is designed in two sections. The right section creates a signal to the piezo that allows it to vibrate at 113 kHz. The left circuit induces a variable duty cycle at a much lower frequency (1.6 kHz) in order to pulse the piezo atomization circuit (right). This allows the piezo to atomize water at a significantly lower rate than it was manufactured for, without compromising the physical limitations of the piezo. This circuit allows us to achieve an atomization rate of ~6.7 mg/min!

PCB Board for Operating Varying Piezo Atomizer

Atomization Mass Rate Measurements of Piezo Operating at Lowest Duty Cycle (5.8%)

Greenhouse Gas Analyzer tests

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.

Humidification Performance

The data collected from the Greenhouse Gas (GHG) Analyzer gives us insight not only into the accuracy of the piezo and mixing chamber, but also the accuracy of the analyzer's moister adjustment factor. From the plot below we can see a large rise in H2O readings from the GHG analyzer at the 500 second mark when the piezo system is turned on and begins atomizing. The Piezo does not reach the targeted range of .01% to 1% but does reach 11.2% at the lowest. When running it through the GHG analyzer, there is an issue with the maximum water content of the air stream being around 2.25% or 2.25*10^4 ppm as seen on the graph below. This means any excess atomization will result in condensation in the chamber and potentially mess with the results, especially during longer test runs. Future considerations for the sponsor when handing off the project to another team is to custom order a piezo with an extremely low atomization rate and high precision in order to yield accurate results for testing the LI-Cor GHG analyzer.

H2O level readings from GHG analyzer over time

The graph below displays a zoomed in version of the data collected. This section specifically shows the moment the potentiometer was turned to change the atomization rate to its lower limit. At around 1850 seconds a steep decline in water concentration is observed before it begins to level at around 2.26*10^4 ppm despite the piezo still atomizing at minimum capacity.

H2O level readings from GHG analyzer over time (zoomed in)

Page updated

Report abuse