Table of Contents
Advertisement
4. THEORY OF OPERATION
4.1 Electrochemical Sensor Elements
The sensor portion of the QTS-6100 is a compact City Technology electrochemical sensor. These sensors
are micro-fuel cells that are completely sealed so as to be maintenance free and stable for long periods. The
measurement is a gas-in-liquid chemical reaction rather than a surface area measurement. With no surface
area to coat, these sensors retain their sensitivity to gases such as H
air.
The cell consists of a diffusion barrier, O-ring seal,
electrolyte reservoir and three electrodes; Sensing,
Counter, and Reference (see Fig. 1). The target gas enters
the cell through a diffusion barrier.
process of the measurement is one of oxidation where
one molecule of the target gas is exchanged for one
molecule of oxygen.
molecule to the Counter electrode, generating a DC
microamp signal between the Sensing and Counter
electrodes.
This signal is linear to the volume
concentration of the sensed gas rather than the partial
pressure.
These sensors will not respond to normal
variations in atmospheric pressure.
In some cases, biasing is required to maintain a voltage differential between the Reference and Sensing
electrodes in order to facilitate the necessary reaction in the cell. The transmitter electronics will provide
the necessary bias voltage when configured for one of these sensor types. Most sensors produce a small
amount of "baseline" current in clean air.
transmitter.
This oxidation at the electrodes causes wear, and limits the life of the sensor. Typical life for these sensors
is two to three years in normal operation. This will vary somewhat from sensor to sensor, with some
working lifetimes exceeding four years. This wear also changes the characteristics of the sensor, requiring
regular re-calibration. It is recommended that these sensors be re-calibrated every three months or as
necessary.
*CAUTION*
Replacement sensor elements which are not bias types are shipped with a tiny spring
of wire shorting the Sense and Reference Electrodes.
prior to installing the element to the sensor assembly.
B71050-021-000-RB.doc
JUL. 18, 2007
Analog Transmitter/Sensor
The chemical
The reaction drives the oxygen
QTS-6000 Series
S even after prolonged exposure to clean
2
Figure 1 - Toxic Cell Construction
This is adjusted out with the zero potentiometer on the
4
Electrolyte Reservoir
Counter Electrode
Refereence Electtrode
Sensing Electrode
O-Ring Seal
Capillary Diffusion Barrier
(Schematic Drawing)
This spring must be removed
ELECTROCHEMICAL TRANSMITTER
PRODUCT MANUAL
Advertisement
Table of Contents
