Combustible Gas Detectors are used in a wide variety of industrial plants to detect gas leakage or buildup before it reaches explosive levels.  These detectors rely on thermal oxidation of any combustible gas or vapor on the surface of a heated catalyst. In other words, they simply burn the gas and measure the heat released in the combustion process.

While this type of sensor is generally stable and reliable, it is also subject to poisoning by silicon vapor, lead compounds, and other environmental contaminants.  To detect loss of sensitivity, most users test combustible sensors either weekly or biweekly to ensure response. The procedure is simple, but requires manpower to perform the test.

ATI’s Auto-Test Combustible Gas Transmitter eliminates the need for frequent response checks. This transmitter performs an automatic gas test on the sensing element every 24 hours. The result is a combustible gas detector system with a level of reliability beyond that of any other system available.  This daily sensor response verification is as close to a fail-safe detection system as you can find, with the added benefit of greatly reduced manual testing.

Combustible gas sensors are made up of two matched sensing elements, one active and one passive. Both are electrically heated and form two legs of a Wheatstone bridge circuit. When combustible gas contacts the sensor, the active element catalyzes the oxidation of gas, heating the active element and changing its resistance. The passive element remains unchanged, resulting in a change in the bridge circuit that is proportional to the gas concentration.  As long as the active element remains in good condition, the sensor will respond rapidly to the presence of combustible gas.

The Auto-Test sensor is actually a combination of a gas sensor and an electrochemical hydrogen gas generator (patent pending) integrated into an explosion-proof stainless steel housing. When activated by the electronic transmitter, the generator produces hydrogen that diffuses to the gas sensor through the flame arrestor protecting the sensor. If the sensor responds properly, the transmitter will show a “Pass” indication. Should the sensor not respond properly, an “Auto Test Fail” message will activate at the transmitter and the 4-20 mA output from the transmitter will drop to 3 mA, providing for remote fault indication.

Combustible Gas Transmitters for the detection of methane                                    (or Specify Gas) shall be provided to monitor ambient gas concentration in                                      (Specify location).  Each gas transmitter shall consist of an explosion-proof electronic transmitter with close coupled combustible gas sensor.  For outdoor locations or for all units supplied with the Auto-Test generator, a splash guard shall be supplied for the sensor.  Gas transmitters shall be ATI Model A12-17 3-wire type with integral LCD display or approved equal.

Gas transmitters shall measure combustible gas concentrations in the installation area using poison-resistant catalytic bead type sensors.  Sensors shall be housed in a 316 stainless steel housing with flame arrestor to meet explosion-proof requirements.  Sensors shall be contained in a housing that also incorporates an electrochemical hydrogen generator.  The hydrogen generator shall be used to provide an Auto-Test function that will automatically test combustible gas sensors for response every 24 hours.  During the sensor functional test, the transmitter output shall be held at 4 mA.  In the event that the Auto-Test detects a sensor failure, the output of the transmitter will go to 3 mA to provide remote trouble indication.

The Combustible Gas transmitter electronic module shall be designed as plug-in unit that can be easily removed for service without disconnecting field wiring. Modules shall be interchangeable, allowing immediate exchange of transmitter electronics in the event of a fault in a module.  An LCD display on the front of the transmitter shall display gas concentration in % LEL (Lower Explosive Limit), and shall also provide an operational interface for service personnel. Four magnetic controls shall allow operators to perform calibration functions, lock the 4-20 mA output, or manually vary the output signal for test purposes. The entire transmitter electronic unit shall be housed in an explosion-proof enclosure with window, and the magnetic controls shall be operable through the enclosure window without removing the cover.

For transmitters equipped with the Auto-Test sensor verification system, the electronic transmitter shall also keep a test log and display the results of each test on the LCD display. The log shall be accessible to operators through an information sequence activated by the front magnetic switches.

Gas transmitters shall operate from power supplies of 12-40 VDC, and shall be capable of driving external loads up to 1000 ohms with a standard 24 VDC supply. Transmitters and sensor shall be designed to meet explosion-proof specifications for Class 1, Group B, C, & D, Class 2, Groups E & F; and Class 3 locations.

OPTIONAL:

Combustible gas transmitters shall be provided in a dual condulet version to allow the sensor only to be mounted near the ceiling while the transmitter display and controls are mounted at a more convenient elevation.  Flexible tubing shall be permanently installed to allow calibration gas to be fed to the sensor from the transmitter location.  This system will allow one man calibration, even when the sensor is not easily accessible.