Detcon 1000 - Manual (Page 6)

Mdl 1000 H2S E-Chem / Inst. Air
Model 1000 H2S E-Chem w Instrument Air Rev. 1.1 Page 2 of 28
Detcon MicroSafe™ Model DM-624, toxic sensors are non-intrusive “Smart” sensors designed to detect and
monitor for H
2
S gas in the ppm range. A primary feature of the sensor is its method of automatic calibration,
which guides the user through each step via instructions displayed on the backlit LCD. The sensor features
field adjustable, fully programmable alarms and provides relays for two alarms plus fault as standard. The
sensor comes with two different outputs: analog 4-20mA, and serial RS-485. These outputs allow for greater
flexibility in system integration and installation. The microprocessor-supervised electronics are packaged as a
plug-in module that mates to a standard connector board. Both are housed in an explosion proof condulet that
includes a glass lens window that allows for the display of sensor readings as well as access to the sensor’s
menu driven features via a hand-held programming magnet.
1.2 Electrochemical Sensor
The sensor is an electrochemical cell. Each cell consists of three electrodes embedded in an electrolyte
solution all housed beneath a diffusion membrane. Sensitivity to specific target gases is achieved by varying
composition of any combination of the sensor components. Good specificity is achieved in each sensor type.
The cell is diffusion limited via small capillary barriers resulting in long service life of up to 3 or more years
depending on the application.
NOTE: This particular sensor cell is unique in that it has no cross-sensitivity to alcohol vapors
and thus is a good choice for measuring H
2
S in natural gas pipelines.
Figure 2 Construction of Electrochemical Sensor
DM-624 Electrochemical Sensor Principle of Operation
Method of detection is by an electrochemical reaction at the surface of an electrode called the sensing
electrode. Air and gas diffuse through the stainless steel filter and 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
sample gas conditions.
1.3 Microprocessor Control Circuit
The control circuit is microprocessor based, and is packaged as a plug-in field replaceable module, facilitating
easy replacement and minimum down time. Circuit functions include a basic sensor pre-amplifier, on-board
power supplies, microprocessor, back lit alpha numeric display, alarm status LED indicators, magnetic
programming switches, an RS-485 communication port, and a linear 4-20mA DC output.