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The Determinator
infrared gas monitor

Principle of Operation    |   Determines Gas    |    Routine Maintenance    |     Theory of Operation
4-20 mA Signals and Error Codes    |    Construction    |    Part Numbers    |    Options
Why the Determinator is better than other fixed point infrared gas monitors
Determinator Part Numbers   |   How to Specify an Infrared System   |  The Remediator
Calibrating the Determinator
The Determinator - Installation    |   The Determinator - Setup

*****  FM and CSA Approved *****

The Delphian Determinator (Fixed Point Infrared) sensor is a smart infrared hydrocarbon gas detector. It has no moving parts. It displays gas concentrations up to and above 100% LEL including over-range (above 120% LEL) concentrations up to 999% 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 optional 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 levels can be done in the field without declassifying the area. 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.

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)

The Determinator is a very low maintenance instrument. It does not require span calibration. Zero point adjustments are required infrequently.

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.


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.

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 m, 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 m 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

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.

4-20 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.
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Standard Configuration includes: 
conduit box, interface module 
and processor module:
With aluminum sensor:
With stainless steel sensor:

Splash/Dust Guard
—for severe environmental and operational conditions.
Lightning arrestor —where high-energy transient suppression is necessary.
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.

Why the Determinator is better than other FPIR gas monitors:

  • Altitude adjustment: 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 pending).

  • Competitive models are often switch selectable between a few gases, but for all gases except the one selected, they compute their LEL incorrectly.

  • Virtually immune to the water condensation and temperature/humidity variations which cause competitive models to give inaccurate readings.

  • 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 reprogrammed.

  • Diagnostic signals: 4-20mA feedback with10 different diagnostic codes.

  • The Determinator is designed to handle rapid temperature changes.

Glossary of Gas Detection Terms  |   How to specify a Delphian infrared system

Copyright 2016 Delphian Corporation,  220 Pegasus Avenue, Northvale, N.J., U.S.A.