For a gas detector, first decide which gases it must detect and up to what concentration (combustible, toxic, oxygen too low or too high, and the range for each); then look at the sensing principle (each principle has its strengths and limits); next look at the alarms and the type of use (fixed or portable, how it alarms, logging); after that, deal with the environment, maintenance and calibration (protection, functional testing, calibration interval). This article focuses on general gas monitoring in factories, warehouses and maintenance work; household alarms and medical gas monitoring are out of scope. It is divided into 4 stages:
Stage ① | Gases and concentration range: combustible, toxic, oxygen; measuring range; how many gases one unit measures.
Stage ② | Sensing principle: the five common principles, the limits of each, sensor head life.
Stage ③ | Alarms and type of use: fixed or portable, testing before entry, alarms, display and logging.
Stage ④ | Environment, maintenance and calibration: effects of the environment on the sensor, functional testing, calibration interval.
This article starts from the point where you already know the site needs gas detection. Each stage below covers: how to do it, what public sources say, and what Jiahon can help with; the sources and the basis are in the “Notes” at the end. This article is a selection summary, not a hazard assessment, and does not replace regulations; what to detect and how to measure must follow the site assessment and local regulations. This is the general case and still depends on the individual case.
Figure 1: The four stages of gas detector selection (schematic, not to scale). ① Gases and concentration range — combustible, toxic, oxygen: think in three classes first; measuring range: match it to the regulatory thresholds; how many per unit: one to several. Result of this step: the gases on site are clearly listed. ② Sensing principle — catalytic combustion: combustible gases; electrochemical: toxic gases; other principles: depend on the gas and interference. Result: the principle matches the gas. ③ Alarms and type of use — fixed or portable: often both together; test before entry: done by trained people; alarms and logging: sound and light, vibration, logging. Result: type and alarms match. ④ Environment, maintenance and calibration — environment: mud, moisture, extreme temperatures; functional test: confirm it responds; calibration: on schedule, with calibration gas. Result: lasts long and still measures accurately.
Stage ①: Gases and concentration range — first list the gases that can occur on site
How to do it:
First list the gases that may occur on site, in three classes: combustible gases, toxic gases, and oxygen too low or too high. Examples of toxic gases are hydrogen sulfide and carbon monoxide.
For each one, write down the measuring range and compare it with the regulatory thresholds, for example limits such as LEL and PEL; the range has to cover these thresholds.
Oxygen example: normal air contains about 21% oxygen; the Canadian occupational safety agency writes that below 19.5% counts as low; during hot work, oxygen must be kept below 23% and above 19.5%.
Combustible gas example: before hot work, the combustible concentration must be kept below 5% or 10% of the lower explosive limit (LEL); the figure differs slightly from place to place.
How many gases one unit measures: portable units mostly detect 1 to 4 gases, and advanced ones up to 6; a common combination is oxygen, hydrogen sulfide, carbon monoxide and combustible gas.
What public sources say: the confined space explanation of the Canadian occupational safety agency (modified September 2026) says: air contains about 21% oxygen; below 19.5% is low oxygen; the fire prevention explanation for hot work says oxygen must be below 23% and above 19.5%, and the combustible concentration must be below 5% or 10% of the lower explosive limit (it differs slightly by jurisdiction); toxic gases can make people ill or unconscious, examples being hydrogen sulfide and carbon monoxide. An article by a safety products distributor (2024) says: the sensors and their capabilities are the first consideration when choosing a unit; the measuring range has to match regulatory and safety thresholds such as LEL and PEL. An article on a fire equipment website (2026) says: most portable units detect 1 to 4 gases, advanced ones up to 6, and a common combination is oxygen, hydrogen sulfide, carbon monoxide and combustible gas (notes table 1).
What Jiahon Can Help With:
Handheld oxygen detector: the product page says this is a handheld detector that measures oxygen alone; detection principle: membrane galvanic cell, diffusion sampling; measuring range 0–25.0 vol% (it also lists a service range of 25–50 vol%), resolution 0.1 vol%, indication accuracy ±0.5 vol%; alarm set points: first stage 19.5 vol%, second stage 18.0 vol%; response time within 20 seconds (90% of final value). Gases other than oxygen are not measured by the unit on this page.
A site product search (October 13, 2026) for “gas” and “detector” found 4 product pages in the gas detection category: 1 oxygen detector, 1 hydrogen sensor, 1 oxygen sensor head for a fixed controller, and 1 portable gas tester; this article does not link the last one. Detectors for combustible and toxic gases were not seen among the site’s product names; please inquire, and include the gases and concentration ranges on site.
Stage ②: Sensing principle — each principle has its strengths and limits
How to do it:
Choose the principle according to the gas to be detected. The five common ones are in Figure 2: catalytic combustion is used for combustible gases, electrochemical for toxic gases, infrared relies on the gas absorbing light of specific wavelengths, photoionization measures volatile organic compounds, and semiconductor is used for hydrogen, alcohol vapor, carbon monoxide and similar.
Catalytic combustion: it needs at least a few percent of oxygen in the sample to work; siloxanes, inorganic acids, chlorinated organics and sulfur compounds can poison or inhibit it.
Electrochemical: it may be affected by corrosive substances or chemical contamination; some sources give a life of 1 to 2 years, some 2 to 3 years, and some designs reach 5 to 7 years; also watch for cross-interference from other gases.
Photoionization: high sensitivity and easy to use, but not specific to particular compounds.
Semiconductor: affected by humidity and cross-sensitivity to other gases.
Infrared: some sources say it can be used in oxygen-deficient environments and where resistance to sensor poisoning is needed; power consumption is higher, and it is more common in fixed units.
Figure 2: Five common sensing principles and what they detect (schematic, not to scale). Catalytic combustion — target: combustible gases; needs oxygen in the sample; vulnerable to poisoning: siloxanes, sulfur compounds and similar. Electrochemical — target: toxic gases; vulnerable to corrosion and contamination; life counted in years, must be replaced. Infrared — principle: the gas absorbs light; power consumption: higher; common in: fixed units. Photoionization — target: volatile organic compounds; sensitivity: high; limit: not specific to particular substances. Semiconductor — target: hydrogen, alcohol vapor; affected by: humidity and other gases; reading: based on changes in electrical properties.
What public sources say: an online encyclopedia entry says: catalytic combustion needs at least a few percent of oxygen in the sample, and siloxanes, inorganic acids, chlorinated organics and sulfur compounds can poison or inhibit it; electrochemical sensors may be affected by corrosive substances or chemical contamination and may need replacing after just 1 to 2 years; photoionization is highly sensitive but not specific to particular compounds; semiconductor sensors are used for hydrogen, alcohol vapor and carbon monoxide and are affected by humidity and cross-sensitivity to other gases. An article by a safety products distributor (2024) says: catalytic combustion is used for combustible gases, electrochemical for toxic gases, infrared relies on light absorption, photoionization measures volatile organic compounds, and semiconductor sensors read changes in electrical properties. An article on a fire equipment website (2026) says: the typical life of electrochemical sensors is 2 to 3 years, with some designs 5 to 7 years, and cross-interference must be watched; infrared has higher power consumption, is more common in fixed units, and can be used in oxygen-deficient environments and where poisoning resistance is needed; replaceable or smart sensors are one of the selection items for portable units (notes table 2).
What Jiahon Can Help With:
Hydrogen sensor: the product page says this is a dedicated replacement sensor for a certain pump-type detector unit, principle: hot-wire semiconductor; hydrogen measuring range 0–20,000 ppm, overload limit 20,000 ppm, resolution 10 ppm, response time T90 within 110 seconds; expected life (clean air) about 2 years, shelf life in the factory packaging 6 months. The product page also warns: it has cross-sensitivity to high concentrations of organic solvent vapor (such as alcohol), so do not deploy it long-term in environments with heavy solvent evaporation. This matches the statement above that “semiconductor sensors are affected by cross-sensitivity to other gases”. Whether it fits your existing unit, please inquire.
Sensor head for gas detection alarm: the product page says this is an oxygen sensor head for a certain series of fixed gas detectors, sensing principle electrochemical, with intrinsically safe explosion-protection certification; the suggested replacement interval is about 1–2 years (depending on the actual operating environment). This article does not link the controller it is used with; please inquire.
Stage ③: Alarms and type of use — fixed, portable, or both
How to do it:
Fixed or portable: fixed units are installed in fixed locations such as process areas and control rooms for continuous monitoring, can be wired by cable to a monitoring system and can trigger interlocks in an emergency; portable units are handheld or worn, mostly battery-powered, and measure the air around the person.
It is usually not an either-or choice: one source says fitting fixed units in every confined space is hard to justify, and sending someone with a portable unit to every location is often impractical, so the answer is usually somewhere in between; portable units supplement fixed units rather than replace them.
Test before entry: a portable unit with a sampling pump and tubing can take samples from outside before entry; testing must be done before entry, during entry and continuously, depending on the hazard, by trained people.
Example of time intervals: in British Columbia, Canada, the pre-entry test must be completed within 20 minutes, and if everyone has been out for more than 20 minutes it must be repeated. Regulations differ from place to place.
Alarms and logging: besides sound and flashing light, some add vibration; whether the thresholds can be adjusted; some have maintenance reminders and man-down alarms; some can log concentration over time.
What public sources say: an online encyclopedia entry says: portable units monitor the air around the person, are handheld or worn, are usually battery-powered, and give sound and light alarms when a dangerous concentration is detected; fixed units are installed in process areas, control rooms and similar, industrial ones are wired by cable to a monitoring system for continuous monitoring, and they can trigger interlocks in an emergency. An article on a fire equipment website (2026) says: fitting fixed units in every confined space is hard to justify, and sending someone with a portable unit to every location is often impractical, so the answer is usually in between; portable units with a pump and tubing can do pre-entry testing, and they supplement rather than replace. The Canadian occupational safety agency’s explanations of atmospheric testing and confined spaces say: testing must be done before entry, during entry and continuously, by trained people; British Columbia says the pre-entry test must be no more than 20 minutes before entry, Yukon says less than 20 minutes, and British Columbia also says that if everyone has been out for more than 20 minutes the test must be repeated (notes table 3). An article by a safety products distributor (2024) says: alarms can be visual and vibrating, and vibration is useful in noisy places or for people with poor hearing; there are maintenance reminders and man-down alarms; the display should be easy to read in the dark, for example two-button operation that can be used with gloves; logged concentrations can be used for regulatory reports, trend analysis and incident investigation.
What Jiahon Can Help With:
Handheld oxygen detector (the page from Stage ①): the product page says this is a personal handheld unit, size W65 × D22 × H64 mm, about 75 g (with battery); alarms by buzzer, flashing red LED and vibration (auto reset); LCD display (manual backlight); peak hold and peak memory, battery level display. Whether it can log concentration over time is not stated on the product page; please inquire.
Sensor head for gas detection alarm (the page from Stage ②): this is a part for a fixed controller, and the controller itself was not seen among the site’s product names; the alarms, outputs and interlocks of fixed units are not stated on the product page; please inquire, and include the type of work it will be used for.
Stage ④: Environment, maintenance and calibration — what begins after you buy
How to do it:
Look at the environment: detectors are often dropped and exposed to extreme temperatures, humidity, moisture, dust and mud; enough mud can block the sensor completely, and filters can clog. For selection, check whether the housing is durable and whether the protection rating suits the site, and inspect the filters regularly.
Functional test (bump test): briefly expose the sensor to gas at the expected concentration to confirm that the sensor and the alarm respond; it does not check accuracy. A vendor article says that makers recommend doing it every day before use.
Calibration: accuracy depends on calibration, which is a different procedure from the functional test. Examples of intervals: fixed units usually quarterly, every six months or yearly; portable units usually a daily functional test plus monthly calibration; follow the maker’s instructions. Most portable units need a specific calibration gas.
The equipment itself: the equipment used for testing must also be kept calibrated and maintained; check the detector housing, screws and filters for damage.
What public sources say: an article in an industrial safety and health magazine (2018, the author holds a marketing position at an instrument maker, vendor material) says: detectors are often dropped and exposed to extreme temperatures, humidity, moisture, dust and mud; enough mud or sludge can block the sensor completely; filters can be clogged by moisture or dust; the functional test briefly exposes the sensor to gas at the expected concentration and checks the sensor and the alarm, not accuracy; accuracy depends on calibration, which is a different procedure; makers recommend doing it every day before use. An online encyclopedia entry says: all detectors must be calibrated on a schedule, portable ones more often; fixed units usually quarterly, every six months or yearly, portable units a daily functional test plus monthly calibration; portable units almost always need a specific calibration gas; check the housing, screws and filters for damage; the entry cites a statement that 1 in every 2,500 untested instruments does not respond to dangerous concentrations, which is why many large organizations use automated test stations. The Canadian occupational safety agency writes: testing equipment must be kept calibrated and maintained. An article by a safety products distributor and an article on a fire equipment website both list a rugged housing and the protection rating as selection items (notes table 4).
What Jiahon Can Help With:
Handheld oxygen detector: the product page lists operating temperature −10 to +50 °C, operating humidity 30–90% RH (non-condensing); power: 1 AAA alkaline battery, battery life about 5000 hours (20 °C, no alarm). The calibration interval, calibration gas and protection rating are not stated on the product page; please inquire.
Hydrogen sensor: the product page lists operating temperature −20 to 50 °C, humidity RH 15–90% (non-condensing), long-term drift less than 2% of signal per month, and says “calibrate at least once every six months”; housing material ABS, weight about 8 g.
Sensor head for gas detection alarm: the product page notes that the sensor head is a consumable to be replaced periodically; past its service life it may give false alarms or fail to detect dangerous gas, so replace and calibrate it as the maker specifies.
Calibration certificates and supply of calibration gas were not seen among the site’s product names; please inquire, and include the site environment and frequency of use.
When you need to match a type, please prepare six items first: the gases to be detected and their ranges; the type of work (fixed monitoring or confined space entry); fixed or portable; alarm and logging needs; the site environment; and the calibration and maintenance arrangements. You are welcome to contact Jiahon.
Further reading: “Gas Detector vs Flame Detector“
Notes: sources and exceptions
The following is for readers who need to check; skipping it does not affect the approach above. Where your system specifications, the detector maker’s written specifications or local regulations differ from this article, follow them.
Where each stage comes from
| Source | Where it is used, and the source |
|---|---|
| Source 1 | Stages ②, ③, ④: each principle, fixed and portable, calibration. Source: an online encyclopedia entry (edited by many people, subject to change) |
| Source 2 | Stages ①, ③, ④: gas hazards, oxygen and combustible concentrations, testing personnel. Source: the Canadian occupational safety agency’s confined space explanation (modified September 17, 2026) |
| Source 3 | Stages ①, ③, ④: the three classes to test for, when to test, examples of pre-entry timing. Source: the Canadian occupational safety agency’s atmospheric testing explanation (modified September 17, 2026; the page marks a last revision on November 8, 2022) |
| Source 4 | Stage ④: environment, functional test and calibration. Source: a bump-testing article in an industrial safety and health magazine (May 3, 2018; the author holds a marketing position at an instrument maker; vendor material) |
| Source 5 | Stages ①, ③, ④: sensors and measuring range, alarms, display, logging, housing. Source: a selection article by a safety products distributor (February 26, 2024; vendor material) |
| Source 6 | Stages ① to ④: how many gases per unit, principle and life, combined fixed and portable use, protection. Source: a fixed vs portable comparison article on a fire equipment website (published August 27, 2026, modified September 1; the page contains vendor promotion) |
| Jiahon data | “What Jiahon Can Help With”. Source: Jiahon product pages |
Table 1 | Gases and concentration range
| Item | Content |
|---|---|
| The three classes to test for | Toxic gases, combustible or explosive gases, oxygen too high or too low (source 3) |
| Examples of toxic gases | Hydrogen sulfide, carbon monoxide; can make people ill or unconscious (source 2) |
| First consideration in choosing a unit | The sensors and their capabilities (source 5) |
| Measuring range | Has to match regulatory and safety thresholds such as LEL and PEL (source 5) |
| Oxygen (example) | Normal air about 21%; below 19.5% is low; fire prevention explanation: below 23% but above 19.5% (source 2) |
| Combustible concentration (example) | Before hot work, keep below 5% or 10% of the lower explosive limit; differs slightly by jurisdiction (source 2) |
| How many gases per unit | Most portable units 1 to 4, advanced ones up to 6; common combination: oxygen, hydrogen sulfide, carbon monoxide, combustible gas (source 6) |
Sources: 2, 3, 5, 6
Table 2 | Sensing principle
| Principle | Content |
|---|---|
| Targets | Catalytic combustion: combustible gases; electrochemical: toxic gases; infrared: light absorption; photoionization: volatile organic compounds; semiconductor: changes in electrical properties (source 5) |
| Catalytic combustion | Detects combustible gases between the lower and upper explosive limits; needs at least a few percent of oxygen in the sample; siloxanes, inorganic acids, chlorinated organics and sulfur compounds can poison or inhibit it (source 1) |
| Electrochemical | May be affected by corrosive substances or chemical contamination, may last just 1 to 2 years (source 1); typically 2 to 3 years, some 5 to 7 years; watch for cross-interference (source 6) |
| Photoionization | High sensitivity, easy to use; not specific to particular compounds (source 1) |
| Semiconductor | Used for hydrogen, alcohol vapor, carbon monoxide; affected by humidity and cross-sensitivity to other gases (source 1) |
| Infrared | Relies on the gas absorbing light of specific wavelengths (sources 1, 5); higher power consumption, more common in fixed units; can be used in oxygen-deficient settings and where poisoning resistance is needed (source 6) |
| Sensors | Replaceable or smart sensors are one of the selection items for portable units (source 6) |
Sources: 1, 5, 6
Table 3 | Alarms and type of use
| Item | Content |
|---|---|
| Portable | Monitors the air around the person; handheld or worn; usually battery-powered; sound and light alarms (source 1) |
| Fixed | Installed in process areas, control rooms and similar; industrial units wired by cable to a monitoring system; can trigger interlocks in an emergency (source 1) |
| Combined use | Fitting fixed units in every confined space is hard to justify, and sending someone with a portable unit to every location is often impractical, so the answer is usually in between; portable units supplement rather than replace (source 6) |
| Pre-entry sampling | Portable units with a pump and tubing can do pre-entry testing (source 6) |
| When to test, and who | Before entry, during entry and continuously, by trained people (sources 3, 2) |
| 20 minutes (example) | British Columbia: no more than 20 minutes before entry, and retest if everyone has been out for more than 20 minutes; Yukon: less than 20 minutes (source 3) |
| Alarms and display | Visual and vibrating alarms; maintenance reminders; man-down alarms; easy to read in the dark, two-button operation usable with gloves (example); logged concentrations used for regulatory reports, trend analysis and incident investigation (source 5) |
Sources: 1, 2, 3, 5, 6
Table 4 | Environment, maintenance and calibration
| Item | Content |
|---|---|
| Environment | Often dropped, exposed to extreme temperatures, humidity, moisture, dust and mud; enough mud or sludge can block the sensor completely; filters can be clogged by moisture or dust (source 4) |
| Housing and protection | Ruggedness and protection rating listed as selection items (sources 5, 6) |
| Functional test | Briefly expose the sensor to gas at the expected concentration, checking the sensor and the alarm; does not check accuracy; makers recommend doing it every day before use (source 4) |
| Calibration | Accuracy depends on calibration, which is a different procedure (source 4); all detectors must be calibrated on a schedule, portable ones more often (source 1) |
| Interval (example) | Fixed units usually quarterly, every six months or yearly; portable units a daily functional test plus monthly calibration; portable units almost always need a specific calibration gas (source 1) |
| Inspection | Housing, screws and filters checked for damage (source 1) |
| Statistical statement | 1 in every 2,500 untested instruments does not respond to dangerous concentrations; many large organizations use automated test stations (source 1) |
| Equipment calibration | Testing equipment must be kept calibrated and maintained (source 2); calibrate according to the maker’s instructions (source 3) |
Sources: 1, 2, 3, 4, 5, 6
This is the general case and still depends on the individual case.
Exceptions and limits
What was seen and what was not: this article looked at an online encyclopedia entry (edited by many people), two confined space explanation pages of the Canadian occupational safety agency (modified September 2026), a 2018 bump-testing article in an industrial safety and health magazine (the author holds a marketing position at an instrument maker, vendor material), a 2024 selection article by a safety products distributor (vendor material), and a 2026 comparison article on a fire equipment website (the page contains vendor promotion). No full text of any gas detector standard or Taiwanese regulation was read; no specification sheet of any Jiahon product was read; no explanation of which principle oxygen sensors use, and no content requirements for calibration certificates, were found in the sources, so this article does not write about them.
The figures are examples: oxygen 19.5% and 23%, 5% or 10% of the lower explosive limit, 1 to 4 gases and 6 gases, 1 to 2 years, 2 to 3 years and 5 to 7 years, 20 minutes, quarterly, six-monthly and yearly, daily and monthly, and 1 in every 2,500 are all examples from the sources, not selection thresholds; the first two come from Canadian explanations, regulations differ by province, and in Taiwan local regulations apply.
Sources disagree: on the life of electrochemical sensors, source 1 says they may last just 1 to 2 years, and source 6 says typically 2 to 3 years, with some designs 5 to 7 years; the text gives both side by side without picking one.
Single source: the 20-minute pre-entry test comes from source 3 alone, with examples from just two jurisdictions; the definition of the functional test and doing it daily come from source 4 alone; calibration intervals and calibration gas come from source 1 alone; 1 in every 2,500 not responding is a statement cited by source 1, and this article did not check its primary source; alarm types, man-down alarms and display come from source 5 alone; combined use of fixed and portable units comes from source 6 alone.
Sources with a position: sources 4, 5 and 6 are vendor material or contain vendor promotion, sources 2 and 3 are explanations by a Canadian government agency, and source 1 is an online encyclopedia edited by many people; none is a standard itself or a third-party test report.
Wording of the sources: the sources are in English, and the statements above are this article’s paraphrase; no conversions of its own were added.
This article’s own summary: the text does not use a “this article’s own summary” marker; each statement can be matched to the sources above.
Scope: this article is a selection summary, not a hazard assessment, and does not replace regulations. It does not discuss the classification of models for hazardous locations (intrinsic safety, flameproof enclosures and so on): classifications and certifications differ by country and were not checked against standards; for this, please inquire and follow the hazardous area classification of your site. It also does not discuss model cross-references, prices or lead times.
Equipment and products: the figures and statements in “What Jiahon Can Help With” were read from the text of the product pages on October 13, 2026; the product images were not looked at and no specification sheets were read, so they were not verified by this article; the specification is as on the product page, and for what the product page does not state, please inquire. The model numbers, series codes and brands on the product pages are not written here; the product pages’ “also see” sections, selection suggestions and marketing wording were not quoted. Which gas detection products the site has was found with the site’s product search and product names; this is not a stock inquiry.
This article is a summary for the general case, not a restatement of standard clauses, and not an endorsement of any maker, equipment supplier or service provider, nor an assurance for your equipment and process. Please take your system specifications, the detector maker’s written specifications and local regulations, and your own validation as the reference; this is the general case and still depends on the individual case.