SF
6
-Aciditor
12
5.1 Working Principle
The
SF
6
-Aciditor
is a stand-alone device designed to quickly and accurately measure the
amount of sulfur dioxide (SO
2
) in SF
6
. Because the unit contains an automatic flow and
pressure control module, the user does not need to be concerned with setting the correct
flow rate, or compensating for variations in equipment pressure. All readings are displayed in
real-time on the touchpad display, in parts per million volume (ppm
v
).
The
SF
6
-Aciditor
has been particularly developed to be used on arced or otherwise stressed
SF
6
. Unlike conventional decomposition testers, which require disposable detector tubes, this
unit uses an electro-chemical sensor (catalytic principle).
The applied electro-chemical sensor incorporates a gold based, three electrodes and acid
electrolyte system.
The introduced SF
6
diffuses into the SO
2
-sensor. Here it reacts at the sensing electrode
through an oxidative process, according to the following equation:
Equation 1:
Sensing process
Sulphur Dioxide
(SO
2
): SO
2
+ 2H
2
O = H
2
SO
4
+ 2H
+
+ 2e
-
The Counter electrode acts to balance the reaction at the sensing electrode, by means of an
oxygen reduction forming water as a consequence.
Equation 2:
Counter reaction:
Oxygen
Reduction:
1/2O
2
+ 2H
+
+ 2e
-
= H
2
O
The two equations represent the overall cell reaction as follows:
SO
2
+ 1/2O
2
+ H
2
O = H
2
SO
4
In case of a SO
2
presence within the SF
6
under inspection, equation 1 changes its electro-
chemical potential and electrons are released consecutively. Thus a change of currency is
detected and converted to ppm
v
-values.
The applied sensor carries a transmitter board that includes a temperature compensation
and a calibration in the specified range.
