Fixed Gas Detection: Choosing Sensors, Placement & Calibration

Choosing a fixed gas detection system comes down to three decisions: which sensor technology can reliably see your target gas, where detectors must go to catch a release early, and how the system will be calibrated and maintained. Combustible gases call for detection well below their explosive limit, while toxic gases call for ppm-level detection tied to exposure limits. Get those choices right and the controller, alarms and outputs follow naturally.

What is a fixed gas detection system?

A fixed gas detection system is a permanently installed network of gas detectors connected to a controller or plant control system that triggers alarms and protective actions. A typical system includes:

• Detectors or transmitters at each monitoring point, usually with 4–20 mA or Modbus outputs.
• A controller, PLC or networked monitoring system that displays readings and manages alarm levels.
• Alarm devices such as horns, strobes and annunciators.
• Protective actions like starting ventilation, closing valves or shutting down equipment.
• Calibration and maintenance tools, including certified calibration gases.

What is the difference between toxic and combustible gas detection?

Combustible gas detection protects against fire and explosion, so it measures relative to the lower explosive limit (LEL). Toxic gas detection protects people from exposure, so it measures in parts per million against occupational exposure limits.

For reference, the LEL is about 5% by volume for methane, 4% for hydrogen and 2.1% for propane. Many facilities set a first alarm around 20% LEL and a higher alarm somewhere around 40–60% LEL, but setpoints should come from your hazard analysis, applicable codes and insurer requirements.

Which gas sensor technology should you choose?

Match the sensor to the gas, the environment and the maintenance your site can support.

Catalytic bead (pellistor) sensors

A catalytic bead sensor uses a heated, catalyst-coated bead paired with an inert reference bead in a bridge circuit. Combustible gas oxidizes on the active bead, raising its temperature and resistance in proportion to concentration.

• Strengths: compact, economical, responds to most combustibles including hydrogen, and tolerates dusty or dirty air.
• Watch-outs: needs oxygen to work; can be poisoned by silicones, halocarbons and other contaminants; and span loss may go unnoticed without regular calibration.

Delphian’s SafeCAL gas monitor uses catalytic sensing with one-touch magnetic calibration and is rated for −40 °C to +85 °C. Delphian also pairs catalytic sensors with its patented SensorSaver constant-temperature transmitter circuit.

Point infrared (IR) detectors

A point infrared detector measures how hydrocarbon gas absorbs infrared light at specific wavelengths, compared against a reference wavelength that compensates for optical changes.

• Strengths: immune to catalyst poisons, works in low-oxygen atmospheres, and monitors its own optical health, which reduces calibration effort.
• Watch-outs: cannot detect hydrogen or other non-hydrocarbon combustibles, and response varies by hydrocarbon, so calibrate for the gas you expect.

Delphian’s Determinator uses three active optical channels and one reference channel, reports in % LEL with a 4–20 mA output, and is FM and CSA approved. The Remediator reports hydrocarbons in percent by volume.

Electrochemical sensors

An electrochemical cell produces a small current proportional to the concentration of a specific gas reacting at its sensing electrode. It is the workhorse for toxic gases and oxygen.

• Strengths: ppm-level sensitivity, good selectivity, low power.
• Watch-outs: the cell is consumable, some cells respond to interfering gases, and extreme heat or dryness can shorten life.

Delphian’s GazTell platform offers electrochemical detector heads for ammonia, carbon monoxide, chlorine, hydrogen cyanide, hydrogen sulfide, nitrogen dioxide, sulfur dioxide and oxygen, plus an infrared methane version. The head recognizes its sensor cell automatically, and optional modules add relays (SLAM), a remote display (EXMO) or Modbus RTU communications (COMO).

Semiconductor (solid-state) sensors

Metal-oxide semiconductor sensors change resistance in the presence of certain gases. They are rugged and have long been used for H2S, but they are less selective than electrochemical cells, so confirm cross-sensitivities for your process.

Where should gas detectors be placed?

Place detectors where a release will reach them quickly, based on gas behavior, leak sources, air movement and where people work. There is no universal spacing rule.

Start with vapor density relative to air (air = 1.0):

Then refine for the real installation:

• Leak sources: compressors, pump seals, flanges, valve packing, analyzer shelters and loading points.
• Airflow: ventilation, doors, wind and thermal currents can carry gas away from a “correct” height.
• People: toxic detectors belong where operators actually stand and walk.
• Access: mount where technicians can calibrate safely.
• Environment: add splash guards, sun shields or duct mounts where needed.

Useful references include IEC 60079-29-2 for flammable gas detector selection, installation and maintenance, and ISA-TR84.00.07 for evaluating gas detection coverage. Hazardous-location equipment must suit the area classification under NEC Articles 500 and 505; the GazTell toxic detector datasheet, for example, lists Class I, Divisions 1 and 2, Groups C and D. Ammonia refrigeration machinery rooms carry their own detection requirements under IIAR 2 and ASHRAE 15.

How often should gas detectors be calibrated?

Calibrate on the manufacturer’s schedule, adjusted for your environment and experience; Delphian recommends every 30 to 90 days. A proper calibration includes:

1. Zero in clean air or with zero gas.
2. Span with certified gas of the target compound at a known concentration.
3. Verification that outputs, alarms and relays respond end to end.
4. Records of as-found and as-left readings, so drift trends become visible.

A bump test with an uncertified gas confirms response but is not a calibration. One-person magnetic calibration, used on Delphian’s SafeCAL and GazTell, avoids opening explosion-proof enclosures in hazardous areas. Delphian also supplies flow and static calibration kits, zero and span gases, and replacement catalytic and filament elements.

What are the most common gas detection mistakes?

• Using infrared detectors where hydrogen is the hazard.
• Installing catalytic sensors in areas with silicone or halocarbon vapors.
• Mounting every detector at one height regardless of gas density.
• Calibrating a hydrocarbon detector with the wrong gas and no correction factor.
• Leaving electrochemical cells in service past their useful life.
• Letting weather guards and sinters clog.
• Never testing the complete chain from sensor to final action.

How do you monitor many points?

For small systems, Delphian’s single-channel Micro Controller and two-channel DC Controller mount in a Flexirack chassis sized for 1 to 16 channels. Larger facilities can use the SAGE-CE computerized system, which networks eight-channel RS-485 sensor and relay satellites with event logging and email or text alarms. Because Delphian’s product pages include legacy content, confirm current model availability before specifying.

Local gas detection support from Vaughn Associates

Vaughn Associates represents Delphian across our territories, with two exceptions: in our Tennessee, Arkansas and North Mississippi territory, coverage is Tennessee and Mississippi only, and in our North Carolina, Virginia, Maryland and D.C. territory, coverage is North Carolina only. Our area managers can help map detector locations, choose sensor technologies, and plan calibration programs for oil and gas sites, chemical plants, wastewater facilities, paper mills and food plants with ammonia refrigeration.

See our gas detection systems overview and the Delphian line page, find your area manager on the territories map, or request application help.