Biomedical Engineering Reference
In-Depth Information
Fig. 2
The experimental apparatus
a leakage. A 0.68 ml gelatine capsule was filled with N
2
O gas at a pressure of
2 bar, and placed into a syringe and this was compressed to release the trace gas
into the system. The pump was then activated, draining samples of the gas from the
abdominal lumen analogue box and transporting it into the gas sensor. Sampling of
the gas continued for up to 5 min following initial release of the gas. For each of
the tubes the experiment was repeated a number of times and the average values
recorded. In order to acquire all the data from the experiments a Pico Data logger
system was used. The output signals from the data logger were then converted to
ppm (concentration of gas in parts per million) in order to measure the increase of
N
2
O gas over the time sample for each experiment.
A closed system was used in all experiments, with the gas which was sampled
subsequently being returned via another flexible pipe into the abdominal lumen
analogue box. With a fixed volume of air in the system at the start of the tests
and a fixed quantity of trace gas injected, the aim with a closed system was to
check whether a common gas concentration equilibrium would be achieved given
enough time, say 2-3 min, regardless of the leak hole size or position used. In a
practical clinical setting such a closed system would not be used and instead it
is more likely that the inlet vent to the system would be drawn from either the
surrounding environment or a supply of medical grade CO
2
.
