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
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 reference 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 evaluates 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 concentration 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.
about the infrared sensor
The sensor and conduit box are
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
All working electronics are encapsulated to prevent deterioration from
dust and humidity.
mA Signals and Error Codes
The sensor displays all detected
warnings and failures.
The following messages are transmitted on the 4-20 mA signal line:
>23.2 mA Over-range (>120%LEL)
20 mA Full scale
4.0 to 23 mA %LEL displayed
negative LEL is a sensor failure indicated by 0.2 mA (acetylene can
cause this failure)
2.3 mA CPU POST successful, no output
2.1 mA Off-line
1.9 mA Zero adjust failed - off-line
1.7 mA Zero adjustment in progress -offline
1.5 mA Voltage too high to function
1.3 mA Sensor missing or not connected
1.1 mA Optics dirty - light path blocked
0.9 mA Lamp failed
0.7 mA Low power line voltage
0.5 mA Sensor Module circuit fault
0.3 mA Processor Module circuit fault
<0.2 mA No power, system fault
NOTE: In the event 1-5 mA option is chosen only the
high and low gas alarms and system fail is transmitted.
click to see enlargement
Standard Configuration includes:
conduit box, interface module
and processor module:
With aluminum sensor:
With stainless steel sensor: 364-850-02
Splash/Dust Guard—for severe environmental and operational
Lightning arrestor —where high-energy transient suppression is
SLAM—local alarm relay module (relays for 3-gas alarms and one
for fail) connects to Interface Module.
Reclamation Adaptor —to allow the sensor to be mounted
externally to a duct or area outside of the sensor's temperature range.
the Determinator is better than other FPIR gas monitors:
altitude can make up to a 25% difference in readings. Delphian's patented
altitude adjustment corrects this problem.
Will provide correct LEL and linearization
simultaneously for a wide range of CHC gas/vapors consistently (patent
Competitive models are often switch selectable between a
few gases, but for all gases except the one selected, they compute their LEL
Virtually immune to the water condensation and
temperature/humidity variations which cause competitive models to give
Bright, large five-digit LED display allows user
to easily see gas alarms, the gas being detected and any fault conditions.
Easy to change alarm settings in the field,
without removing the cover.
No moving parts.
Smaller footprint than most competitive models.
Uses the same conduit box and wiring as Delphian's
catalytic bead sensor, so field replacement of existing catalytic
sensors is easy.
Rugged construction and encapsulated electronics protect
it from water, corrosion and normal vibration.
Response time and recovery of the Determinator is much
faster than that of competitive units we have tested. Design of light
Pipe (patent pending) allows more rapid response and recovery times.
2 year warranty.
Smart sensor: sensor is programmed with the latest
gas information so field electronics don't need to be changed or
Diagnostic signals: 4-20mA feedback with10
different diagnostic codes.
The Determinator is designed to handle rapid
Glossary of Gas Detection Terms |
How to specify a Delphian infrared
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