STATUS SCIENTIFIC CONTROLS
FGD3 Infrared Gas Detector Heads
TD06/024
Issue:
7
Change Note:
1904
Page 5
2. Introduction
The FGD 3 Infrared Gas Detector Head incorporates the latest generation of compact
sensors incorporating infrared technology (see section 0).
Warning
– Infrared sensors will not detect Hydrogen gas.
There are two current detectors in use, one for Hydrocarbon gases and the other for
Carbon Dioxide.
The Hydrocarbon sensor can be set to either methane or general hydrocarbons. These
two settings use different calculations to convert the sensor signals in to gas readings,
it is important to select the correct gas type to give accurate readings.
The detectors use the industry standard 4-20mA current loop to convey the gas levels
detected to a control unit. This means that under
zero gas
conditions 4mA is drawn
from the supply, and under
full scale
gas conditions 20mA is drawn from the supply.
The current varies linearly for gas levels between zero and full scale.
The detector heads require a three-wire connection (see section 3.4). While the loop
current supplies the power required by the detector head electronics within the
detector head, a second supply must be provided to power the infrared sensor and its
associated circuitry.
Additional features of the latest generation of FGD3 Heads include:-
•
The terminal block for wire termination is located within an EMC enclosure
within the FGD3 Detector Head. This improves the instruments
’ immunity to
radio and electromagnetic interference.
•
The software has been redesigned to allow more data to be accessed by the
knowledgeable user.
An optional weather guard (Stock No. SS 475) is available for installations exposed to
the atmosphere or contaminants and is also suitable for use in other areas where
hosing down takes place. The weather guard is attached with tamperproof screws to
ensure that it is not inadvertently removed.
2.1 Background
The infrared sensors use the proven non-dispersive infrared principle (NDIR) to detect
and monitor the presence of gases. This technique relies upon the target gas having a
unique, well-defined absorption signature within the infrared region of the
electromagnetic spectrum.
