Detector/Transmițător de Gaz (Fix/Portabil, 4–20mA)

Selecția Tehnologiei Senzorului: Potrivirea Principiului de Transducție cu Gazul și Aplicația

Each of the four detection principles has specific failure modes, maintenance requirements, and cost implications. Getting the selection wrong at design stage means either a safety gap or an overcomplicated system.

Electrochemical (CO, H2S, O2, SO2). A target gas diffuses through a PTFE membrane to an electrochemical cell where it is oxidized or reduced at a working electrode. The resulting current is proportional to gas concentration. Sensitivity is excellent — H2S detection at 1ppm is routine. Response time T90 is typically ≤30s for CO and H2S sensors, meeting EN 45544-3 performance requirements.

The limiting factors are environmental sensitivity and finite sensor life. Electrochemical cells are affected by temperature (the current output shifts by approximately ±3% per °C without compensation — verify the factory implements NTC compensation in firmware), humidity extremes below 15% RH (membrane dehydration causes false zero), and chemical poisons. CO sensors cross-interfere with hydrogen — a CO sensor exposed to H2 will read a positive signal even in a CO-free atmosphere. The cross-sensitivity coefficient for H2 on a standard CO sensor is typically 30–60% (a 100ppm H2 atmosphere reads as 30–60ppm CO). If your application involves hydrogen-rich environments (battery rooms, fuel cell installations), this cross-sensitivity requires explicit management — either specify an H2-compensated CO sensor or accept a conservative alarm threshold.

Sensor life is counted from date of manufacture, not from installation. Electrochemical sensors in stock for 12 months before installation start with a shortened service life. Always request the manufacturing date batch certificate and reject stock older than 6 months from the shipment date.

Catalytic Bead / Pellistor (CH4, propane, H2, general %LEL). A pair of matched resistive elements form a Wheatstone bridge. The active bead is coated with a catalyst; combustible gas burns on the catalyst surface, heating the bead and shifting the resistance. Output is in percent of the lower explosive limit (%LEL), not in absolute ppm concentration.

The critical failure mode is silent failure in oxygen-depleted atmospheres. The combustion reaction requires O2 ≥10% to sustain. In environments where gas inert-inerting or O2 displacement can occur simultaneously with combustible gas accumulation — confined space entry, chemical reactors, CO2 blanketing — a pellistor sensor can read zero (or sub-zero on some designs) in a genuinely hazardous atmosphere. This is a well-documented safety hazard. If your application involves potential O2 depletion, pair the CH4 pellistor with a dedicated O2 electrochemical sensor and interlock the alarm logic.

Poison-resistant beads use alumina substrates with different catalyst formulations to extend life in atmospheres containing silicone vapors (from sealants, lubricants), sulfur compounds, and halogenated hydrocarbons. Specify poison-resistant beads for any application near HVAC systems, industrial cleaning operations, or chemical processing. Standard beads in a silicone-contaminated atmosphere can be permanently poisoned within 24–72 hours of exposure.

Module cost: $5–15 per pellistor sensor element. Replacement is field-serviceable on most 4–20mA transmitter designs (sensor cartridge exchange without recalibration in some designs, though full calibration is recommended).

NDIR — Non-Dispersive Infrared (CO2, CH4, CO at high concentration). An infrared source illuminates a measurement cell. At specific wavelengths — CO2 absorbs at 4.26µm, CH4 at 3.3µm — the target gas attenuates the beam. A reference detector at a non-absorbing wavelength corrects for dust fouling and source aging (dual-beam design). Output is calculated from the ratio of sample to reference beam intensity using the Beer-Lambert relationship.

NDIR has no consumable electrochemical element — sensor life exceeds 10 years in clean applications, making it the correct choice for fixed installations where sensor exchange cost and schedule matter. There is no O2 dependency, and no cross-sensitivity to H2 or silicones.

The trade-off is cost. An NDIR optical bench module (dual-beam, temperature-compensated, with onboard linearization) runs $80–200 depending on gas species and range, versus $5–15 for a pellistor. For CO2 monitoring in HVAC and building automation — an application with millions of sensors globally — the NDIR cost premium is accepted because CO2 is the primary IAQ metric and no other transduction principle is practical at ppm concentrations.

Ask the factory to document the ABC (Automatic Baseline Correction) algorithm and the correction interval for CO2 sensors. ABC algorithms assume that the sensor will periodically be exposed to outdoor air (~400ppm CO2) and use that minimum reading to correct zero drift. In applications where the sensor is permanently installed in a space that never reaches ambient CO2 (continuous occupied industrial space, cold-storage facilities), ABC will generate incorrect baseline corrections. In these cases, specify a sensor without ABC or with ABC disabled, and establish a scheduled manual calibration program.

PID — Photoionization Detection (VOC, general organic compounds). Ultraviolet light at 10.6eV (standard lamp) ionizes molecules with ionization potential below 10.6eV. The resulting ion current is proportional to total VOC concentration. Detection limit is in the ppb range for many aromatics and halogenated compounds — useful for leak detection and exposure monitoring.

PID has no selectivity. The output is a sum of all ionizable species present, weighted by the ionization potential and response factor of each compound. A PID calibrated to isobutylene (standard reference gas) will give a different numerical reading for toluene, hexane, or styrene at the same actual concentration. A cross-sensitivity / correction factor table for the specific application gases is mandatory before interpreting PID readings as concentrations. Request this table from the factory; it should be based on measured correction factors, not calculated estimates.

For ATEX Zone 1 / Zone 2 applications, confirm whether the UV lamp housing is rated for the zone — some PID designs use a non-rated lamp assembly inside an Ex d flameproof housing and require that the lamp housing itself not be in direct contact with the hazardous atmosphere.

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Certificarea ATEX/IECEx pentru Zone Periculoase: Ce Înseamnă Marcajele

ATEX (Directive 2014/34/EU) is the EU legal requirement for equipment used in explosive atmospheres. IECEx is the international certification scheme — technically equivalent to ATEX but without the EU legal mandate. For European end markets, ATEX marking is required. For Middle East, Australia, and most non-EU markets, IECEx is sufficient and is often accepted in place of ATEX. Verify which scheme your end customer’s safety case or local authority requires before specifying certification.

Equipment Group and Gas Group. Group I covers mining applications (methane in underground mines). Group II covers surface industrial and commercial applications and is subdivided by the maximum experimental safe gap (MESG) of the target gas:

• IIA: gases with MESG ≥0.9mm — propane, methane, butane
• IIB: gases with MESG 0.5–0.9mm — ethylene, town gas
• IIC: gases with MESG <0.5mm — hydrogen, acetylene

A transmitter marked IIC is certified for the highest hazard gas group and is therefore suitable for IIA and IIB applications as well. Specifying IIA when hydrogen is present on site is a certification gap that invalidates the safety case.

Temperature Class. The temperature class (T-class) specifies the maximum allowable surface temperature of the equipment:

• T4: ≤135°C surface temperature
• T5: ≤100°C
• T6: ≤85°C

The T-class must be lower than the auto-ignition temperature (AIT) of the target gas. Hydrogen AIT is 500°C, making T4 acceptable. Carbon disulfide AIT is 90°C — only T6 equipment is suitable. For most common industrial gases (CH4 AIT 537°C, H2S AIT 260°C, propane AIT 470°C), T4 is adequate. Verify the T-class against the AIT of the actual process gases on site.

Protection Concept. The marking Ex d (flameproof) means the enclosure can contain an internal explosion without igniting the surrounding atmosphere. Ex ia (intrinsically safe) limits the electrical energy in the circuit to below the minimum ignition energy of the gas. Ex e (increased safety) applies to terminal boxes and components that are not normally spark-producing.

For a fixed-point transmitter with a 4–20mA output, Ex d is the most common protection concept in Chinese OEM production — the entire transmitter head is housed in a cast aluminum or stainless steel flameproof enclosure. Ex ia requires the loop circuit to be intrinsically safe (IS) rated, which imposes constraints on the associated apparatus (barriers or galvanic isolators in the control room) and the total cable capacitance and inductance — verify these parameters if you are designing an Ex ia loop.

Chinese ATEX Certification Path. Chinese factories can obtain ATEX certification through a notified body accredited under the ATEX directive. CESI (China Electric Power Research Institute) and CQST (China Quality & Safety Testing) hold ATEX notified body status. The certification document structure mirrors EU practice: Ex Certificate of Conformity (CoC) + Quality Assurance Notification from the manufacturing site. IECEx certificates are issued through IECEx ExCB (Certified Body) — CESI and CQST also hold IECEx accreditation.

Request the actual certificate numbers and verify them on the ATEX Equipment Certification (Notified Body) database (ec.europa.eu) or the IECEx Equipment Certificate database (iecex.com) before accepting the first production batch. Certificate numbers should be visible on the product nameplate and in the Ex marking string.

A complete ATEX marking example: II 2G Ex d IIC T4 Gb. Parse it as: Group II Surface, Category 2 (Zone 1), Gas atmosphere, flameproof, Gas Group IIC, Temperature Class T4, Equipment Protection Level Gb.

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Calibrarea și Gestionarea Derivei: Menținerea Preciziei Măsurării pe Durata Vieții Senzorului

A gas transmitter that was accurate on day 1 may be reading 30% low by year 2 if calibration is not maintained. For safety-critical applications, this matters. Calibration interval requirements are often specified by the applicable standard (EN 45544, IEC 60079-29-1) and should be reflected in the product’s installation and maintenance manual.

Electrochemical Sensor Drift. Zero drift (output in clean air) typically tracks within ±2% FS per year if the sensor is stored and operated within the specified temperature range. Span drift (sensitivity change over time) is typically ±5% FS per year — larger than zero drift and not self-correcting. The implication: a transmitter that passes a zero check in fresh air can still have a significant span error at mid-range concentrations. Both zero and span calibration are required for a valid calibration event.

Calibration gas must be NIST-traceable (or equivalent national metrology standard) certified gas in a certified cylinder, with a certificate of analysis specifying the gas concentration ±1% accuracy and the cylinder shelf life. Most electrochemical calibration gases have a shelf life of 12–24 months. Cross-sensitivity interference gases must be absent during calibration — a CO calibration performed in an atmosphere with background H2 will absorb the H2 cross-sensitivity into the span setting, creating a systematic error.

A bump test (functional check) verifies that the sensor responds to the target gas and triggers the alarm output — it does not measure accuracy. A bump test using a concentration above the alarm setpoint is sufficient for a daily or weekly functional check but does not substitute for a calibration event. Regulatory requirements (e.g., EN 60079-29-1 Annex E) distinguish between functional tests and full calibrations. Specify in the product documentation which tests satisfy each requirement.

Catalytic Bead Drift and Poison Detection. Pellistor sensitivity decreases as the catalyst surface is deactivated. The recommended approach is to track the sensor’s span response over time — if the calibration gas response requires progressively larger span adjustments, the bead is aging. A bead that requires more than 30% upward span correction relative to its original factory setting should be replaced. Some transmitter designs include a poison detection algorithm that monitors span deviation between calibrations and triggers a fault output if the deviation exceeds a threshold.

NDIR Dual-Beam Baseline Correction. The dual-beam configuration measures sample and reference simultaneously, cancelling lamp aging and dust effects. However, the linearization algorithm and the reference wavelength selection must be matched to the specific gas being measured. For CH4 NDIR modules, cross-interference from CO2 (which also absorbs weakly at 3.3µm) must be quantified — request the interference table from the factory.

ABC (Automatic Baseline Correction) for CO2 transmitters continuously adjusts the zero based on the lowest reading seen over a rolling window (typically 7 days). This corrects upward zero drift automatically in spaces that reliably reach ambient CO2 levels. For applications where this assumption does not hold — permanently occupied spaces, agricultural environments, confined process areas — ABC must be disabled. Request firmware documentation specifying the ABC algorithm, correction interval, and disable procedure.

Ask the factory to provide a sample calibration record from the factory calibration station — the raw sensor output at zero gas and at span gas before and after calibration adjustment, the calibration gas lot number and certificate number, and the date. This record should accompany each unit as a factory calibration certificate. For ATEX-certified units, the calibration certificate is referenced in the quality system documentation and must be traceable to the NB quality assurance notification.

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Peisajul Furnizorilor Chinezi: Reference Points and Red Flags

The gas detection market has a clear tier structure. Tier-1 global players — MSA Safety (Pittsburgh), Dräger (Lübeck), Honeywell Analytics (formerly Manning/Vulcain/GMI) — define the performance benchmark against which OEM products from China are evaluated. These brands do not manufacture in China for export; their products are produced in their own certified facilities in the US, Germany, and UK. They are the reference, not the competition for OEM sourcing.

The credible domestic Chinese producers include Shenzhen Hanwei Electronics (subsidiary of Siemens China for some product lines), Zhengzhou Winsen Electronics (electrochemical and NDIR sensor modules, widely used as OEM components by other manufacturers), and RKI Instruments (California-based, with OEM production relationships with Chinese factories). Smaller Shenzhen-based enclosure manufacturers buy sensor modules from Winsen and Hanwei and integrate them into ATEX-certified housings — this is the typical OEM structure you will encounter.

Quality verification indicators to request before production:

Cross-sensitivity data for common interferents. A CO sensor datasheet that shows only “CO: 0–300ppm” without a cross-sensitivity table is incomplete. The minimum acceptable cross-sensitivity disclosure for a CO sensor includes: H2 cross-sensitivity coefficient (%), ethanol cross-sensitivity (%), H2S cross-sensitivity (%). Request this as a tabulated data sheet, not a verbal assurance. Values should be based on measured testing with the specific sensor batch.

T90 response time measurement methodology. Factory-stated T90 figures are sometimes derived from sensor element specifications rather than from complete transmitter testing with the actual diffusion path. Request the T90 test protocol — the gas should be applied as a step change using a certified gas cylinder injected through a calibration adapter that replaces the diffusion head. T90 measured with a bag-applied gas flow is not representative of fixed-installation performance.

IP66 dust and water jet test certificate. An IP66 marking on a nameplate requires that the transmitter was tested per IEC 60529 with a 100-liter/minute water jet from any direction for 3 minutes. Request the IP test certificate (test date, test standard, pass/fail) — not just the declaration of conformity. This is particularly important for wastewater treatment plant and offshore applications.

Electrochemical sensor batch certificate with date of manufacture. Request the certificate of conformity for the sensor batch installed in your production lot, specifying the manufacturing date, batch number, and initial calibration gas response. Electrochemical sensors deteriorate from the date of manufacture. For a sensor with a 2-year service life, stock manufactured 12 months before delivery has an effective field life of 12 months — this should be reflected in the pricing and in the maintenance documentation delivered to the end customer.

Our sourcing service maintains a qualified supplier list for ATEX-certified fixed gas detectors, including NDIR CO2/CH4 and electrochemical multi-gas transmitters. For a new product line, our factory audit service covers the NB quality assurance notification review, production process audit, and sensor batch traceability verification. Pre-shipment quality inspection includes T90 response time verification with certified calibration gas, alarm relay function test, and IP rating spot-check — conducted before the shipment leaves the factory.

For industrial IoT applications where the gas transmitter is integrated into a Modbus RTU or HART instrument network, we can coordinate factory-level Modbus register map documentation review and protocol conformance testing as part of the pre-production sample evaluation.