A gas detector can respond correctly when tested yet miss a release that does not reach its sensing point. Detection design therefore starts with the gas inventory, credible releases, airflow and required response, followed by sensor selection and accessible testing arrangements.
Fixed and portable do different jobs
These are not alternatives and one does not reduce the number of the other you need. A fixed system watches a place continuously whether or not anyone is there. A portable instrument watches a person, and only while they are carrying it and it is switched on.
Question
Fixed system
Portable instrument
What is it protecting?
A room, a cylinder store, a pit, a plant room, the area around one process tool
The individual wearing it, wherever they walk
When does it work?
Continuously, including nights, holidays and during load-shedding if it is on the essential supply
Only during the shift, only if bump tested and switched on
Can it act?
Configured outputs may drive alarms or engineered interlocks
Warns the wearer; some models also communicate alarms to a wider system
Limitations
Coverage depends on the release reaching a suitable point
Does not replace fixed protection for unattended hazards; pumped sampling may support a qualified pre-entry procedure
What it needs from you
Cause and effect matrix, calibration schedule, an owner for the response
Daily bump test and a docking or test station
Assess the need for both fixed and portable instruments. Confined-space work requires a competent entry and rescue procedure, appropriate atmosphere testing and other controls; carrying a gas detector alone does not make investigation or entry safe.
Choosing the sensing technology
Sensor type is chosen by the gas and by what else is in the room. Each technology has a specific way of going wrong, and specifying around that failure mode is most of the engineering.
Electrochemical cells cover toxic gases at parts per million and oxygen deficiency. They are the mainstay for carbon monoxide, hydrogen sulphide, chlorine, ammonia and oxygen. The cell is consumed as it works, so it has a finite life whether or not it ever sees gas, and heat and humidity shorten that life. In an unconditioned plant room in Dhaka, plan on replacement sooner than a European service interval suggests.
Catalytic bead sensors measure flammables as a percentage of the lower explosive limit, and will see hydrogen. They need oxygen present to work, so they read low in an inert or oxygen-depleted atmosphere. They are poisoned by silicones, sulphur compounds and halogens, and silicone is everywhere in a laboratory: sealants, mould release, some hand creams. A poisoned bead reads low and gives no warning, which is why a bump test is not optional on this technology.
Infrared sensing. A suitable infrared sensor can detect specified hydrocarbons or carbon dioxide, depending on its design. Conventional infrared flammable-gas sensors do not detect hydrogen. Confirm target gas, cross-sensitivity, optical contamination and fault behaviour for the model.
Photoionisation detection. A PID responds to compounds whose ionisation energy is below the lamp energy, including many VOCs. It does not identify every compound or detect every organic gas. Confirm response factors, mixtures, humidity effects and lamp maintenance.
Extractive sampling, as used by the Honeywell Midas and Midas S2, draws a sample down a tube to a detector mounted somewhere accessible: inside a tool enclosure, a duct, or a high ceiling void. It moves the calibration point to somewhere a technician can stand, which is why it is the standard choice on semiconductor and process tools. The trade-off is transport delay down the sample line and a pump to maintain, so confirm the maximum line length and the flow fault alarm at specification stage.
Placement: release conditions and airflow determine coverage
Relative density is one input, alongside release pressure, temperature, momentum, ventilation, obstructions and occupancy. A cold nitrogen release can behave differently from warm nitrogen. Use gas-specific manufacturer guidance and the site risk assessment rather than a height-only rule.
Hazard
Useful physical consideration
Design question
Hydrogen or methane
Buoyant releases may reach high pockets
Where will the credible release travel under actual airflow?
Liquefied ammonia
A cold release may initially form a dense cloud
Are both the release region and subsequent vapour path covered?
CO or mixed gases
Mixing and ventilation can dominate
Where will people be exposed and where will gas reach a sensor?
LPG or dense vapours
May accumulate in low areas
Are pits, drains and low occupied zones considered?
Nitrogen/argon or cryogenic release
Oxygen can be displaced; temperature changes dispersion
Which sensing locations detect oxygen deficiency before unsafe exposure?
Select sensing points from the release and airflow assessment, then check sensor access and environmental limits. A remote-sampling inlet can cover a location that is difficult to access, but line material, length, adsorption and response delay matter. Duct sampling is appropriate only where deliberately designed for that duty.
Alarm levels and what happens next
Define alarm levels, automatic actions, evacuation and re-entry in an approved cause-and-effect matrix. The number and settings depend on the gas, exposure limits, flammability risk, sensor response and applicable requirements. A low-level alarm may already require evacuation.
Initial alarm. Define the protective action for this gas and site. Do not tell staff to investigate a possible hazardous release without a trained response procedure, and do not assume occupants may remain until a second alarm.
Protective action. Confirm which alarm initiates evacuation, gas isolation or process shutdown, and which ventilation state is safe. The control logic must not draw the release through occupied areas or create another hazard.
Fault. A separate, distinguishable signal for sensor failure, flow fault on a sampling detector, or loss of supply. A detection system that cannot tell you it has stopped working is the failure mode described at the top of this article.
Escalation out of hours. Most laboratories here are unoccupied from evening to morning. Route the panel to the security desk and to a duty engineer's phone through the building management system, and rehearse who calls whom.
Post-alarm procedure. Write down who may re-enter, with what instrument, and what reading permits normal work to resume. It is the step that is almost always missing, and the one people need at two in the morning.
Two Bangladesh-specific points. The gas panel needs its own battery backup and the essential supply, or a load-shedding event silences detection at exactly the moment the extract fans stop and gas can accumulate. And the sounder must be audible over the fume hoods and the air handling plant, measured with everything running rather than in a quiet building on a Friday.
Bump testing and calibration are not the same thing
A bump test applies gas and confirms the sensor responds and the alarm activates. It answers one question: is this thing alive? A calibration applies a certified concentration and adjusts the reading so the number means something. You need both, on different schedules. Use the Midas gas-specific cartridge and sampling documentation.
Portable testing. Follow the manufacturer’s pre-use bump-test, calibration and inspection instructions. A compatible docking system may automate records, but properly documented manual testing can also be valid.
Fixed heads follow the manufacturer's stated interval, and additionally after any alarm event, after a sensor change, and whenever poisoning is suspected on a catalytic bead.
Shorten the interval for the environment. Heat, humidity, dust and corrosive atmosphere age sensors faster than the datasheet assumes. Use the as-found readings from previous calibrations to justify the interval you set.
Calibration gas is a lead-time item. Certified cylinders carry an expiry date, they are hazardous goods, and they are imported. Order the replacement while the current cylinder is still in date, and keep the certificate with the calibration record.
Replaceable cartridges. Check the exact gas cartridge, expiry, calibration and installation procedure. Pre-calibration does not remove installation verification, and no enclosure should be opened live unless the instructions and area classification explicitly permit it.
Keep a register per detection point: location, gas, range, setpoints, sensor serial number, last bump, last calibration, next due, and who signed. Auditors ask for this by name and they check the dates.
Commissioning: the tests to insist on
Apply gas at every head, not a sample of them. The point is to find the head wired to the wrong channel, and that head is never in the sample.
Sign off a cause and effect matrix with the laboratory before commissioning, listing every input and every output. Test each row against the matrix and mark it off.
Verify each point at the building management system and at the security desk, with the correct location text. A panel that says nothing more than 'Zone 3' at three in the morning has not been commissioned.
Prove audibility and visibility in the occupied space with the ventilation running, and from behind a closed fume hood sash.
Confirm the automatic actions physically happen: watch the solenoid close and the tool stop, and time it.
Take handover of as-built drawings showing every head with its height above floor level, the sample line routes and the setpoints as commissioned. Without heights on the drawing, the next engineer cannot tell whether a head is in the right place.
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