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UWE Bristol Engineering showcase 2015

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Introduction<br />

The detection of Volatile Organic<br />

Compounds (VOCs) has, for several<br />

decades, been a huge area in sensor<br />

research. Exposure to these<br />

compounds can lead to various health<br />

issues and are widely found across<br />

various work places, making them<br />

important to detect. It is known that<br />

many VOCs are found in the exhaled<br />

breath, urine, faeces and skin<br />

emanations of human beings; and that<br />

there is evidence to suggest that the<br />

presence of specific volatiles in these<br />

are related to cancer.<br />

The system to be created is essentially<br />

a mimicry of the mammalian olfactory<br />

system, using a sensor to mimic the<br />

nose, and an analysis of results along<br />

with pattern recognition to mimic the<br />

brain.<br />

The reaction that occurs upon the<br />

surface of a heated metal oxide when<br />

it comes into contact with a volatile<br />

causes the conductivity of the MO to<br />

change. This phenomenon can be<br />

measured, and used to detect VOCsin<br />

the air.<br />

Cataluminescence is the emission of<br />

light during the catalytic oxidation of<br />

organic compounds, in this case the<br />

metal oxide sensor serves as a catalyst.<br />

Thom Clark<br />

BEng (Hons) Aerospace <strong>Engineering</strong> (Design)<br />

The Fabrication of a Gas Sensing Device<br />

Combining Measurements of Cataluminescence<br />

and Conductivity change<br />

Initial Design<br />

The old chamber is shown above, as<br />

can be seen, it consists of a bolted on<br />

lid, no means of securing other than<br />

fixings that were added after it was<br />

made and the inside is full of sharp<br />

corners, not desirable for any flow.<br />

After a few stages of configuration, the<br />

design consisted of a cylindrical inside,<br />

screw on lid, linear air inlet-outlet, and<br />

built in bolt holes for fixing. The sensor<br />

sits on the end of the bolt, which is<br />

screwed into the chamber, opposite<br />

this is the inlet for the gas<br />

chromatography column.<br />

Computational Fluid Dynamics<br />

A small amount of basic fluid<br />

dynamic analysis was undertaken<br />

to determine the optimum<br />

arrangement for the chamber.<br />

It was found to be an arrangement<br />

that encouraged circulatory flow,<br />

when the sensor bolt was<br />

horizontal, which was incorporated<br />

in the manufactured design.<br />

Other Equipment<br />

To measure the light emission from<br />

the CTL, a photomultiplier tube was<br />

required, a PMT is a device that<br />

detects photons, and amplifies<br />

them using the photoelectric effect<br />

to emit a current.<br />

The module sourced as part of this<br />

project was the Hamamatsu H7828<br />

photon counting unit, which was<br />

connected to a pulse counter to<br />

record its emissions.<br />

Pulse Count Reading (kHz)<br />

100<br />

90<br />

80<br />

70<br />

60<br />

50<br />

40<br />

30<br />

20<br />

10<br />

0<br />

The Atmospheric Chamber<br />

The second chamber was designed to<br />

detect trace amounts of VOCs in the<br />

atmosphere, it differed from the other<br />

one with the sensory bolt screwed in<br />

through the lid, positioning it opposite<br />

the photocathode of the PMT once<br />

the chamber was screwed onto it.<br />

Equipment Testing<br />

The experimental equipment was<br />

tested using an old, basic sensor<br />

chamber, adapted to fit the PMT.<br />

Pulse Response of Nitrotoluene Vapour<br />

against Time<br />

0 20 40 60 80 100 120<br />

Time from sample injection (s)<br />

This graph shows the difference in<br />

pulse response after the injection of<br />

three different concentrations. Of<br />

course the highest concentration<br />

showed the best response.<br />

Saturated<br />

Diluted<br />

Air<br />

Project Supervisor<br />

Professor Norman Ratcliffe<br />

Project summary<br />

This project involved the design of two sensor<br />

chambers to detect the presence of Volatile Organic<br />

Compounds, using a combination of the<br />

measurements of the conductivity change of metal<br />

oxides and the emission of cataluminescence (CTL).<br />

The starting point for design was an old gas sensor<br />

chamber designed for use in the applied sciences lab<br />

2G5; hence certain criteria were to be fulfilled in the<br />

new design, based upon the experiences of the old<br />

chamber’s use.<br />

Project Objectives<br />

The main objectives for this project were:<br />

• To design two sensory chambers that combine the<br />

detection capabilities of CTL and conductivity<br />

change; one for medical diagnostics, connected to<br />

a gas chromatograph and one for detecting VOCs<br />

in the atmosphere<br />

• To source the required apparatus, such as a<br />

photomultiplier tube module<br />

• To improve upon the previous chamber’s design,<br />

making it easier to use in experimentation<br />

• Perform aerodynamic analysis upon the flow<br />

inside the chamber, identifying undesirable flow<br />

effects and attempting to reduce them<br />

Project Conclusion<br />

Both chambers were designed with what is thought<br />

are optimum designs, based on findings from various<br />

bits of investigation throughout the project and the<br />

results from CFD. However, it was planned from the<br />

start of the project that the chambers would be<br />

manufactured and then tested to validate their<br />

design. Due to unfortunate circumstances neither of<br />

the chambers were manufactured in time, meaning<br />

that the experimental analysis and comparison with<br />

the old chamber could not occur. Despite this, the<br />

design met the desired criteria and was improved in<br />

several ways.

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