DETERMINES GAS
Delphian’s patented multiple active channel system allows the sensor to recognize and identify multiple hydrocarbon 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.
List of gases that the Determinator can recognize  (pn 364-335)

LOW 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 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.    More about the infrared sensor

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.

click to see enlargement

PART NUMBERS
Standard Configuration includes: 
conduit box, interface module 
and processor module:
With aluminum sensor:
      364-850-01
With stainless steel sensor:  364-850-02