Swann AMI Hydrogen QED Operator's Manual page 10

AMI Hydrogen QED
Product Description
Communica-
tion Interface
(optional)
Faraday
Verification
Measuring
principle
8
 USB Interface for logger download
 Third signal output (can be used in parallel to the USB interface)
 RS485 with Fieldbus protocol Modbus or Profibus DP
 HART interface
The Faraday verification is used to check the sensor periodically.
The intervals can be freely programmed in the menu operation.
Hydrogen is measured by an amperometric method. A small volt-
age, called polarization voltage, is applied between two electrodes.
If a hydrogen molecule hits the positively charged platinum elec-
trode (anode) it will be oxidized. This means that two electrons are
removed under the influence of the positive voltage and the catalyt-
ic properties of the platinum. This leaves two (charged) protons and
two electrons behind.
(1) H2 ––> 2 H
The two electrons are transported by the polarization voltage
source to the negatively charged silver-silver chloride electrode
(cathode), causing the following reaction:
-
(2) AgCl + 2 e
Finally the two kinds of ions produced will combine into a small
amount of hydrochloric acid:
(3) 2 H+ + 2 Cl
If the hydrogen concentration in the sample increases, more hydro-
gen molecules will hit the platinum anode within a given time.
Therefore, more electrons will be transported within a given time,
which corresponds to an increased electrical current. This current
can be measured by the electronics. It is directly proportional to the
concentration of hydrogen in the sample. Formula (2) indicates that
in the course of the hydrogen measurement the silver chloride is re-
duced to elemental silver. If all silver chloride has been converted to
silver the hydrogen sensor has to be refurbished in the factory.
However, there is enough silver chloride provided to keep the sen-
sor operational during two years under normal conditions.
The actual SWAN hydrogen-sensor design is based on the well-
known Clark principle. Clark-type hydrogen sensors have been
successfully in use for many years.
+ -
+ 2 e
-
––> Ag + 2 Cl
-
––> 2 HCl
A-96.250.791 / 040219