Detcon TP-424C - Manual (Page 5)

3.1 P RINCIPLE OF O PERATION
In operation TP-424C sensors are installed in industrial areas where hydrogen sulfide gas leaks are most like-
ly to occur. Method of detection is by diffusion adsorption. As hydrogen sulfide gas diffuses through the sin-
tered porous filter and contacts the surface of the heated sensor element, the metal oxide sensor film decreas-
es in electrical resistance non-linearly proportional to the concentration of gas. This change in resistance is
conditioned by a control transmitter circuit which, in turn, provides a linear 4-20 mA DC signal correspond-
ing to the range 0-100 ppm. As the gas leak subsides and the area clears, the sensor film returns to its original
resistance range. In application, sensor operation is a continuous reading detection/monitoring method.
3.2 C ONSTANT T EMPERATURE C ONTROL
A unique feature of the TP-424C solid state sensor is the RTD type heater film. The heater film serves two
important functions. First, it elevates the operating temperature of the sensor film to a level where most
efficient response and clearing characteristics are achieved. And second, it allows control circuits to control
surface temperature of the sensor film regardless of ambient temperature conditions. This constant temper-
ature method improves long term stability and overall accuracy, as well as improves sensor performance in
cold weather applications.
3.3 O NE M AN R EMOTE C ALIBRATION
Sensor assembly design incorporates a modular plug-in remote control transmitter which accommodates
all calibration adjustments: span (coarse and fine) and heater voltage. A metallic face plate protects against
damage and allows easy access to test points and trimming potentiometers. With the use of a digital volt-
meter, the design supports one man remote calibration.
3.4 A PPLICATION & I NTERFERENCE D ATA
Model TP-424C sensors are designed to detect and monitor hydrogen sulfide gas in ambient air in the
range of 0-100 parts per million. Minimum sensitivity and scale resolution is 1 ppm. Sensor specifications
reflect an operating temperature range of -40° F. to +175° F. While the sensor is capable of operating out-
side these temperatures, performance specifications are verified within the limit.
Sensor Placement/Mounting
Sensor location should be reviewed by facility engineering and safety personnel. Area leak sources and
perimeter monitoring are typically used to determine number and location of sensors. Hydrogen sulfide gas
is slightly heavier than air (approximately 1.18); therefore, the sensors are generally located 2 - 4 feet above
grade. The sensors should always be mounted in accessible locations to facilitate periodic maintenance.
Interference Data
Methane
25,000 = 0
Ethane 5,000 = 0
Hexane 5,000 = 0
Propane 5,000 = 0
Butane 5,000 = 0
Carbon Monoxide 800 = 0
Carbon Dioxide 5,000 = 0
Carbon Disulfide 14 = 0
Methanol 500 = 3
Isopropanol 500 = 1
Detcon Model TP-424C Hydrogen Sulfide Sensor PG.5
PRE-AMP
(SPAN)
CONSTANT
TEMPERATURE
CONTROL
SIGNAL
CONDITIONING
XMTR
POWER
SUPPLY
MOS
SENSOR
RTD
HEATER
I/O CIRCUIT
PROTECTION
SIGNAL OUT
POWER IN