DESCRIPTION
The Delphian Determinator (Fixed Point Infrared) sensor is a smart infrared hydrocarbon gas detector. It has
no moving parts. It detects hydrocarbons up to 999% of
their Lower Explosive Limit (LEL). It can determine which
hydrocarbon gas is being detected and transmit a 4-20
mA signal proportional to the LEL concentration of that
gas or vapor. The Determinator displays the correct LEL
as well as the gas detected on its five digit LED display. It
can be set to show up to three gas alarm levels. An op-
tional four-relay alarm module (SLAM) can be connected
directly to the Determinator so that local alarms can be
actuated in the event of a gas alarm or failure. The detector head is designed so that all normal operations,
including checking calibration and changing alarm lev-
els can be done in the field without declassifying the area.
The Determinator is designed to be as failsafe as pos-
sible. The built-in self test continuously monitors the hardware and software. Minor fault conditions are displayed
on the LED display. Four informational and six possible
critical malfunctions are transmitted on the 4-20 mA line.
Standard RS 422/485 output is also available. Using a
separate module, the Determinator can be interfaced with
most digital protocols including HART and Fieldbus.
DETERMINES GAS
Delphian’s patented multiple active channel system allows the sensor to recognize and identify multiple hydro-
carbon gases. The sensor detects hydrocarbon gases
only. It cannot detect and will not be affected by hydrogen or other non-hydrocarbon gases. When complex
mixtures of hydrocarbon gases are present, especially
in substantial concentrations, it will display its best estimate of the correct LEL.
ROUTINE MAINTENANCE
The Determinator is a very low maintenance instrument.
It does not require span calibration. Zero point adjustments are required infrequently.
PRINCIPLE OF
OPERATION
A beam of infrared energy is directed to a detector through gas which is drawn into the sampling
cell by a patented convection mechanism. The
beam is filtered to match the absorption of hydrocarbon molecules. If a hydrocarbon gas is
present in quantities over 5% LEL, it will absorb
energy from the beam and the detector will register a drop in the total transmission of infrared
energy. This drop in transmission can be directly
related to the concentration of gas.
SENSOR
The Delphian Determinator (Fixed Point Infrared)
Combustible Hydrocarbon (CHC) sensor uses infrared light to probe for the presence of hydrocarbons.
The detection process is independent of the fact that
the gases of interest are combustible, therefore the
presence of oxygen is immaterial.
THE SCIENCE BEHIND THE
SENSOR
All CHC gases have one or more hydrogen (H) atoms chemically bonded to a carbon (C) atom. The
bond between the H and the C is elastic and the distance between the two atoms can oscillate. The oscillation is activated when energy, with a wavelength
of about 3.3 micrometer (3.3 μm, 0.00013 inch),
strikes the molecule. The precise wavelengths that
interact with any one gas are unique for that gas.
The energy at 3.3 μm is qualitatively no different from
visible light, except that that the human eye can not
“see” the light. For this reason we call this kind of
energy "infrared light."
Infrared-based CHC gas sensors differ from catalytic bead sensors in the variation range of the transfer factors for different gases relative to methane gas.
Catalytic beads exhibit transfer factors between 0.6
and 4.0. For the common IR sensors with one refer-
ence and one active channel the transfer factors can
easily vary in a range of 100 to 1. Therefore it is essential to know which gas is present at the sensor in
order to obtain a reliable gas concentration in terms
of % LEL.
The Delphian FPIR sensor consists of a gold-surfaced “light pipe" with an opening at the bottom and
the top. The atmosphere containing the combustible
gas enters the tube through a flame arrestor at the
bottom opening and exits through an opening at the
top. The gas diffusion is enhanced by a heater at the
base of the pipe. A miniature infrared source at the
bottom directs infrared into the pipe. At the opposite
end of the pipe from the source is an infrared sensitive receiver with four custom filters. Each filter cuts
a narrow slice out of all the wavelengths coming from
the infrared source. Three filters transmit energy in neighboring slices of wavelengths that interact with
CHC gases. One filter transmits only energy that does
not interact with CHC gases. The signals from the
three detectors behind the interacting filters are called
the active channels, and the fourth signal from the
detector behind the non-interacting filter is the reference channel.
Most hydrocarbons exhibit characteristic ratios of interaction with IR energy in the three wavelength slices.
The microprocessor in the Delphian sensor evalu-
ates the signals and deduces from the ratios of the
three active channels which hydrocarbon has entered
the light pipe. Once the gas has been identified, the
% LEL is computed for that gas.
WARNING: The IR-based sensor can accurately
measure the concentration of only one CHC gas at
one time. A mixture of two or more CHC gases of
concentrations above 5% LEL may be either misinterpreted by the instrument as a different gas than
either of the present gases, or, more likely, the instrument may not identify the gas at all. In this case
the microprocessor computes a % LEL gas concen-
tration based on a gas that it believes provides the
highest margin of safety (worst case). This kind of
uncertainty can only occur if similar concentrations
of different gases are present, such as 10% LEL of
methane and 8% LEL of butane. A background of
multiple gases, each in concentrations of less than
1% LEL will not severely impact the identification of
a CHC above 15-20% LEL concentration.
CONSTRUCTION
The sensor and conduit box are explosion proof
The conduit box is epoxy coated to prevent corrosion. The internal sensor components, as well as all
connectors exposed to the atmosphere, are gold
plated
All components are designed to plug into an Interface Module. This module makes wiring the Detector
Head easy as well as permitting rapid replacement
of components
Sensors are designed to withstand “hot plugging” in
and out of the Interface Module while power is supplied
All connectors are unique and are keyed to prevent
incorrect connections
All working electronics are encapsulated to prevent
deterioration from dust and humidity.
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