INFICON Extrima Ex-certified Hydrogen Leak Detector - Manual (Page 10)

10
EN
INFICON - Operating Instructions EXTRIMA
Theory
The Extrima detector is based on micro-
electronic sensor technology known as
GAS-FET technology. The sensor is a field
effect transistor in an integrated circuit.
The gate electrode of the transistor is
made of a hydrogen absorbing metal al-
loy (metal hydride). When this device is
exposed to hydrogen the gas molecules
adsorb on its surface, dissociate into hy-
drogen ions (protons), and diffuse rapidly
into the gate metal. The absorption of
hydrogen ions affects the work function
(surface potential) of the metal, which gi-
ves the same effect as if the gate voltage
of the transistor was changed.
Only hydrogen ions can diffuse into the
metal. This excludes cross sensitivity from
substances that do not contain hydrogen.
Also, the dissociation of hydrogen from
other molecules is very inefficient, a fact
that makes these sensors practically in-
sensitive to other substances. The only,
relatively common, substance being
detected with comparable signals is H
2
S,
hydrogen sulphide. This gas is, however,
extremely toxic and has a very strong and
distinct smell. It is therefore never present
in interfering concentrations in normal
working environments.
The electrical output signal from these
sensors is not at all as stable and repea-
table as, for example, sensors for physical
parameters such as temperature, pres-
sure, etc. Therefore the output signal
must undergo signal interpretation in
order to give reliable measurements.
This is done by a microprocessor in the
instrument, which also controls the
sensor temperature with high accuracy,
and other sensor diagnostics in order to
ensure functionality. It also automatically
compensates for background gas.
3. Working principle