Detcon TP-624D - Manual (Page 6)

Model TP-624D
TP-624D Instruction Manual Rev. 0.4 Page 2 of 40
Principle of Operation
Method of detection is by diffusion/adsorption. Air and H
2
S diffuse through a sintered stainless steel filter
(flame arrestor) and contact the heated surface of the metal oxide sensor film. As hydrogen sulfide gas
molecules react with oxygen ions on the film, there is a decrease in electrical resistance proportional to the gas
concentration. The heater film elevates the temperature of the sensor film creating convection and promoting
a quick response to changing gas concentrations. Electronically, the heater film is used to maintain a constant
temperature of the sensor film enhancing stability and repeatability. The sensor response is reversible and
results in continuous monitoring of ambient air conditions.
1.2 Modular Mechanical Design
The Model TP-624D Sensor Assembly is completely modular and is made up of four parts:
1) TP-624D Plug-in Transmitter
2) Field Replaceable H
2
S Gas Sensor
3) Connector PCB
4) Splash Guard
TP-624D Plug-in Transmitter
The Plug-in Transmitter Module is a microprocessor-based package that plugs into the connector board located
in the explosion proof junction box. Circuit functions include extensive I/O circuit protection, sensor pre-
amplifier, sensor temperature control, on-board power supplies, microprocessor, LED display, magnetic
programming switches, a linear 4-20mA DC output, and a Modbus RTU output. Magnetic program switches
located above and below the LED Display are activated via a hand-held magnetic programming tool, thus
allowing non-intrusive operator interface with the Transmitter Module.
Calibration can be accomplished
without declassifying the area. Electrical classifications are Class I, Division 1, Groups B C D.
Figure 2 Circuit Functional Block Diagram
Figure 3 Transmitter Module
PGM 1
PGM 2
MODEL TP-624D
HOUSTON, TEXAS
FLT 1 2 CAL
MicroSafe H2S Gas Sensor
ALM ALM
TM
I/O
Circuit
Protection
RS-485
4-20mA
Display
Micro-
Processor
Temperature
Control
Pre-Amp Analog 4-20mA Out
Power In
Power Supply
Plug-In
Sensor
Element
Modbus RTU
Relays Out