Alcotest 7410 GLC User’s Manual
6
Theoretical Considerations
The Alcotest 7410 GLC measures the
alcohol concentration in the breath using an
electro-chemical sensor called a fuel cell.
This fuel cell is heated as part of the
“READY” process. When the “Ready” light
appears the subject blows into the device.
After a specified volume of breath (greater
than 1.2 litres at a flow of more than 6 litres
per minute) has been provided, a pump
supplies a small volume of this deep lung
breath to the fuel cell for analysis.
If the sample contains alcohol, a chemical
reaction occurs where the alcohol is
converted to acetaldehyde. This reaction
involves a release of electrons, resulting in
an electric current in the fuel cell. A
microprocessor evaluates this current to
determine the BAC. The BAC is displayed
in the LCD window. Normally the result is
provided in 10 to 25 seconds.
As indicated, there are two reaction
processes. The first reaction is fast and
allows the BAC to be measured quickly.
The second reaction is slow and must go to
completion before another test can be
conducted. The time interval before the next
test can be performed depends on the
magnitude of the previous reading. If it was
high, the build-up of acetaldehyde and
electrons is greater and it will take longer
for the second reaction to complete.
A schematic of these reactions is shown in
the figure below.
The time necessary before the GLC is
“Ready” for the next test is proportional to
the size of the previous reading. Normally,
such a recovery time can be expected to
range from as little as 20 seconds to as much
as three minutes depending upon the last
reading.