Detcon DM-200 - Manual (Page 6)

Model DM-200
DM-200 Instruction Manual Rev. 2.0 Page 2 of 40
Construction of Electrochemical Sensor
Reference Electrod
Counter Electrode
Electrolyte Reserv
Sensing Electrode
Diffusion Barrier
Construction of Galvanic O2 Cell
Cathode
Anode
Electrolyte
Solution
Diffusion Barrier
Air Supply
Figure 2 Sensor Cell Diagram
The Method of detection is by an electrochemical reaction at the surface of an electrode called the sensing
electrode. Air and gas diffuse through the capillary diffusion barrier. The controlling circuit maintains a small
external operating voltage between the sensing and counter electrodes of the proper bias and magnitude so that
no current flows to or from the reference electrode while its potential is maintained at the correct fixed voltage
— usually ground. The electrochemical reaction creates a change in current flow from the counter electrode to
the sensing electrode. This change in current is proportional to the gas concentration and is reversible. The
quick response of the sensor results in continuous monitoring of ambient air conditions
.
Figure 3 Function Block diagram
1.3 Universal Transmitter Module (UTM)
The UTM is microprocessor based, and is packaged as a field replaceable module mounted in an explosion
proof enclosure. This facilitates easy replacement and minimum down time. Circuit functions include a PIC
microprocessor, 2-line alphanumeric display, magnetic programming switches, and a linear 4-20 mA DC
output. Field wiring is terminated on the bottom side of the UTM. The UTM also provides an easily
accessible LCD contrast adjust pot and a protective input fuse (refer Figure 5 below). Calibration can be
accomplished without removing the cover. The low power intrinsically safe electronics design provides for an
area classification, which is Class 1; Division 1: Groups B, C, D.