Refrigerant sensors are no longer “nice-to-have.” As the market moves from legacy A1 refrigerants toward A2L mildly flammable blends (R32, R454x, R1234yf/ze), leak detection increasingly becomes part of a safety function, not just a maintenance tool. A2L definitions and limits come from refrigerant classification work such as ASHRAE 34 (including the 2L burning-velocity criterion), and many safety discussions focus on activation well below the Lower Flammability Limit (LFL).
This article explains the core sensing principles used for refrigerants, what each does well, where each fails, and how to pick the right approach for your refrigerant and compliance target.
1) The basics: what “refrigerant detection” is trying to measure
Refrigerant sensors typically output one of these:
- ppm (parts per million) or %vol (volume percent)
- %LEL / %LFL (flammability-based thresholds; critical for A2L/A3 systems)
- EN binary alarm (“gas detected above setpoint”)
Why the unit matters: in machinery rooms, ASHRAE 15 requires detector setpoints not exceed the applicable Refrigerant Concentration Limit (RCL) from ASHRAE 34.
For A2L appliances/systems, many widely used guidance documents emphasize activation at < 25% of LFL and response-time expectations.
2) Safety class changes the “why” of detection (A1 vs A2L vs A3)
A1 (nonflammable): detection = exposure/RCL + cost control
A1 leaks are usually managed for safety (exposure/oxygen displacement in confined spaces), equipment reliability, and refrigerant loss. In machinery rooms, the RCL-based setpoint rule is central.
A2L (mildly flammable): detection = prevent flammable mixtures + trigger mitigation
ASHRAE 34 defines Subclass 2L by a maximum burning velocity (≤ 10 cm/s) as part of the classification framework.
In many A2L adoption materials, the detector is part of a “refrigerant detection system (RDS)” that must react early (commonly framed around 25 % LFL) and drive mitigation controls (fan/valve/shutdown strategy).
Related Read: https://refrigerantsensor.com/knowledge/a2l-sensor/
A3 (highly flammable): detection = combustible gas safety practice
A3 refrigerants (like hydrocarbons) often use combustible-gas style thresholds (%LEL), plus strong attention to ignition prevention.
3) The five most common refrigerant sensor principles
Principle A — Er n Infrared (Non-Dispersive Infrared) absorption
Best for: many halocarbon refrigerants (HFC/HFO blends), CO₂, and various IR-active gases.
How it works: gas molecules absorb infrared light at characteristic wavelengths. The sensor measures how much IR is absorbed through a gas path to estimate concentration (often explained using Beer–Lambert concepts).
Typical NDIR block diagram
- IR source → optical path (gas cell) → filter/detector → signal processing
Horiba describes NDIR as using mid-IR wavelengths (2.5–25 µm) to measure gas concentration.
Styrker
- Good selectivity for many refrigerants
- Strong long-term stability vs many surface-chemistry sensors
- Works well for fixed monitors and compliance-style thresholds
Common pitfalls
- Optical contamination (dust/oil aerosols) can reduce signal
- Multi-gas mixtures need careful calibration/compensation (especially blends)
Principle B — Photoacoustic Spectroscopy (PAS)
Best for: high-sensitivity, high-selectivity detection where you can afford more complexity (often in premium instruments).
How it works: modulated light is absorbed by the target gas → turns into heat → periodic pressure waves (“sound”) form in a chamber → microphone/transducer measures the acoustic signal proportional to concentration.
Styrker
- High sensitivity and selectivity potential
- Good for trace detection designs
Tradeoffs
- More complex optics/acoustics
- Cost and integration complexity can be higher than NDIR
Principle C — Katalytisk bead (pellistor) combustion
Best for: hydrocarbons / combustible gases (including propane-based refrigerants like R290) when you want %LEL-style measurement.
How it works: combustible gas oxidizes on a heated catalyst bead, producing heat → the bead’s temperature rises → resistance changes → Wheatstone bridge measures the change.
Styrker
- Proven method for combustible gases
- Direct mapping to %LEL alarm strategies is common
Common pitfalls
- “Poisoning” by silicones, sulfur compounds, or contaminants can reduce sensitivity over time (depends on environment and sensor design)
- Requires oxygen presence for oxidation; performance can degrade in low-O₂ environments
Principle D — Mos / metal-oxide chemiresistive sensing
Best for: cost-sensitive alarms and embedded detection where you can accept more cross-sensitivity and drift management.
How it works: gas interactions with a heated metal-oxide surface change the sensor’s electrical resistance (a surface chemistry process influenced by adsorption/desorption and oxygen species).
Styrker
- Low cost, compact, simple electronics
- Useful for “gross leak” warnings in controlled environments
Common pitfalls
- Cross-sensitivity to VOCs/cleaners, humidity effects, temperature dependence
- Drift and baseline shifts often require calibration strategy and compensation
Principle E — Thermal Conductivity (TCD / katharometer-style)
Best for: specific industrial setups where the target gas strongly changes thermal conductivity relative to the background gas, or as part of analytical systems.
How it works: a heated wire’s temperature (and thus resistance) changes depending on how well the surrounding gas conducts heat; that change is measured to infer concentration.
Styrker
- Simple physical principle
- Useful in some gas analysis contexts
Tradeoffs
- Less selective than spectroscopic methods unless the gas/background are well controlled
- More common in analytical instruments than mass-market HVAC leak detectors
4) Which principle should you use for which refrigerant?
| Refrigerant type | Eksempler | Recommended principles | Why |
|---|---|---|---|
| Halocarbons (HFC/HFO blends) | R134a, R410A, R32/R454 blends | Er n, sometimes PAS | Strong IR absorption signatures; stable thresholds |
| Hydrocarbons (A3) | R290, R600a | Catalytic bead, NDIR hydrocarbon | Combustible safety (%LEL) or IR stability depending on design |
| CO₂ (R744) | Co₂ | Er n, sometimes TCD | CO₂ is a classic NDIR target gas |
| “Harsh” industrial environments | machine rooms, oil mist | NDIR (with protection), PAS | Better stability; design enclosure/filtration carefully |
5) “Principle” is only half the story: system requirements that make sensors pass (or fail)
Setpoint logic must match the code goal
- Machinery room (A1): setpoint typically anchored to RCL (ASHRAE 15 → ASHRAE 34).
- A2L systems: many adoption references emphasize activation < 25% LFL and timely output response at that exposure.
Response time + mitigation outputs
Some industry/standard-aligned discussions specify mitigation actions (like energizing fans) quickly after exceeding the setpoint.Placement matters (more than people think)
Even the “best” sensor fails if it’s mounted in a dilution zone or away from leak points. Good practice is to place detectors near likely leak sources and consider airflow patterns.Fault handling is a safety feature
If the sensor is part of a safety loop (A2L/A3), define what “fault” means (open/short, out-of-range, self-test fail) and what the equipment must do in that state.6) Buyer/OEM checklist
When you specify a refrigerant sensor, ask for:
- Target refrigerant(s) + calibration method (single gas vs blend handling)
- Output units (ppm, %vol, %LFL) and how thresholds are enforced
- Response time at relevant threshold (e.g., 25% LFL exposure for A2L discussions)
- Drift expectations + maintenance plan (test interval / calibration interval)
- Cross-sensitivity and environmental robustness (humidity, cleaners, oil mist)
- Fault outputs and fail-safe behavior
FAQ
What is the most common principle for refrigerant leak detection in HVAC?
For many modern HVAC refrigerants and blends, NDIR infrarød is widely used because it measures gas absorption directly and can be stable long-term.
Why do A2L refrigerants change sensor requirements?
A2L is mildly flammable (2L has a defined burning-velocity criterion), so detection often needs to trigger mitigation godt under LFL, commonly framed as < 25% LFL.
What’s the difference between catalytic bead and NDIR for R290 (propane)?
Catalytic bead measures combustion heat (great for %LEL alarms) but can be poisoned and needs oxygen; NDIR measures IR absorption and can be more stable if optics are protected.
Why do MOS sensors drift more?
MOS sensing depends on surface chemistry and is affected by humidity, contaminants, and baseline shifts, so compensation and calibration strategy matter.
Are photoacoustic sensors “better” than NDIR?
PAS can be extremely sensitive and selective, but it’s typically more complex and costlier; many HVAC fixed detectors prefer NDIR for robustness and cost.
