MQ-131 Gas Sensor (Ozone)

Target Gases: MQ-131 is designed for Ozone (O₃) detection. Ozone is a toxic gas often found as a pollutant or used in sterilization. MQ-131 actually comes in two versions: low concentration (10–1000 ppb) and high concentration (0.01–10 ppm), often labelled MQ-131 “L” and “H” for low and high respectively. The one typically referenced is the low concentration because ozone is dangerous at very low levels (threshold limit ~50 ppb). MQ-131 is unique among MQ sensors because it targets an oxidizing gas (ozone), whereas most others target reducing gases. It still works on an SnO₂ base but in reverse behaviour (resistance increases with ozone, I believe, since ozone adds oxygen to surface).

How it Works: In clean air, SnO₂ conductivity is high; when ozone (a strong oxidizer) is present, it grabs electrons from the surface, reducing conductivity. Actually, Winsen notes MQ-131’s conductivity decreases with ozone concentration, opposite of how CO or CH₄ make conductivity increase. So one must be careful: for MQ-131, a higher analogue voltage might mean clean air, and a lower voltage = ozone (depending on how the load resistor is arranged). Usually, these analogue modules output higher voltage when sensor resistance drops.

For MQ-131, ozone raises sensor resistance, so analogue output might fall when ozone goes up. This is an inversion compared to others. The module likely still outputs an analogue that decreases with gas concentration (since typical MQ modules are set up for reducing gases). But here ozone is an oxidizing gas, so yeah, expect reversed behaviour: you might calibrate that downwards analogue means more ozone.

Physical Specs: It’s same shape but using different internals. The MQ-131 sensor head might have a different encapsulation (some have a plastic cap). It should fit same modules. In fact, some sellers sell MQ-131 sensor alone (to be used on some base board). If you get an MQ-131 module, wiring is same.

Wiring: Standard. However, note the output reading in code might need interpretation as mentioned (likely lower reading = more ozone). If using the module’s comparator D0, they probably have it set to trigger when sensor resistance increases beyond threshold, which might mean ozone threshold.

Calibration: A challenge. For low levels, you’d need an ozone calibration source. Possibly could calibrate against a known air quality station readings if you live near one (compare your sensor output on days of known ozone levels). Or, if you have an ozonizer and an ozone meter, you can generate e.g. 200 ppb in a bag and test sensor. Without that, treat it qualitatively: if analog goes significantly below baseline, ozone is present.

Placement: O₃ is slightly heavier than air but in indoor it mixes. It’s also very reactive, so it doesn’t travel far from its source unless there’s airflow. Place MQ-131 near potential ozone sources or in the general breathing zone for air quality measurement.

Average Price: MQ-131 tends to cost a bit more (maybe $10) because it’s specialized. Also, be sure to get the correct version (low concentration vs high). Most sellers list one; if not specified, likely low concentration type, since that’s more useful range. Given ozone’s hazard at ppb levels, low concentration model is the one to use for environment monitoring. The high concentration version is maybe for industrial stacks or something.

MQ-135 Gas Sensor (Air Quality / VOC)

Target Gases: MQ-135 is often called an “Air Quality sensor” or “Gas Sensor for CO₂ and other pollutants”. In truth, MQ-135 isn’t specific to CO₂; it’s sensitive to ammonia (NH₃), benzene/toluene vapors, alcohol, smoke, and a bit of CO. It basically responds to a broad range of volatile organic compounds (VOCs) and some inorganic ones (like NH₃). It’s commonly used to get a single metric of “indoor air pollution.” People often attempt to use MQ-135 to measure CO₂ levels, because CO₂ buildup correlates with exhaled VOCs which MQ-135 can detect to some extent. However, MQ-135 is not directly sensitive to CO₂ (CO₂ is non-reactive to SnO₂). But it will respond to CO or alcohol from human breath, etc., so in a closed room with people, MQ-135 goes up – making it roughly correlate with CO₂ levels from occupancy, which is why some approximate it for CO₂.

Key detectable: Ammonia (most sensitive), so cleaning products or pet urine ammonia vapors will spike it. Smoke and organic solvent vapors also trigger it strongly. The datasheet calibrates it around 100 ppm NH₃, 10 ppm Benzene, 1~2 ppm NOx, etc. (just an idea, not exact).

How it Works: It’s a typical MOS sensor for gas that reduce or oxidize on surface. Most of the MQ-135 target gases are reducing (except maybe NOx). So sensor resistance drops with increasing pollution. Range: datasheet might say 10–1000 ppm for “pollution” broadly. In practical use, people measure an “MQ-135 ppm” that doesn’t truly map to a single gas but can be used as an air quality index. It’s good for noticing if something smelly or smoky is in air.

Physical Specs: Standard. The MQ-135 module is in many Arduino kits as the “CO2 sensor” incorrectly. Pin-out same.

Wiring: Standard 4 pins.

Calibration: This calibration is very speculative. To truly calibrate:

Leave sensor in fresh outside air for baseline voltage (assume 400 ppm CO₂ eq, since outside CO₂ ~ 400 nowadays).

Then perhaps calibrate by subjecting it to a known higher CO₂ environment or known VOC. Some calibrate with baking soda vinegar CO₂ (but MQ-135 doesn’t respond to pure CO₂, so that won’t work directly). Perhaps calibrate using exhaled breath in a bag: exhaled breath has ~40,000 ppm CO₂ and lots of VOCs; but that might saturate sensor. Instead, calibrate with say an indoor baseline vs outdoor.

Average Price: Very cheap, often < $2, because it’s in many starter kits. Widely available.

Note: The MQ-135 is sometimes replaced by newer digital sensors (like CCS811, BME680, SGP30) which directly output IAQ or CO₂eq. Those are more precise for indoor air quality. But MQ-135 still sees hobby use for simplicity and cost, albeit with a trade-off in calibration and power usage.

MQ-136 Gas Sensor (Hydrogen Sulfide)

Target Gases: MQ-136 is a sensor for Hydrogen Sulfide (H₂S) – the rotten egg gas. H₂S is extremely toxic and flammable at higher concentrations, but has a strong odor even at low ppm. MQ-136 detects H₂S in range of about 1 to 200 ppm. It’s intended for places like sewers, swamps, oil/gas industry, or anywhere H₂S might leak or accumulate.

How it Works: SnO₂ sensor sensitive to H₂S. H₂S is a reducing gas, so it will lower the sensor’s resistance (increase conductivity). MQ-136 is highly sensitive (1 ppm is already within detection). That means it can pick up even small H₂S like from a sewer gas leak in a bathroom or a cesspit. It may also respond to other sulfur-containing volatiles to some degree, but mainly H₂S.

One challenge: H₂S can actually poison some metal-oxide sensors over time (it can react with metal in the sensor). But presumably MQ-136 is built to handle some of it, though its life might shorten if constantly exposed to high H₂S.

Physical Specs: Standard MQ form, maybe slightly bigger (spec said φ19×? mm, similar to MQ-6 physically)

Wiring: Standard 4 pins.

Calibration: H₂S calibration is ideally done with a test gas (e.g., 20 ppm H₂S in air cylinder). If not, one can try using something like sodium sulfide solution which releases some H₂S, but that’s tricky and not recommended for amateurs (H₂S is toxic!). Perhaps calibrate by assumption: in a normal environment, reading should be zero; if you can smell rotten egg faintly, maybe that’s a few ppm (just guessing).

Average Price: Not super common, might be around $10 or more each. Possibly only specialized retailers. If you absolutely can’t find one, note MQ-135 has some sensitivity to H₂S (but less) – it’s listed that MQ-135 can detect “smoke and CO and NH₃ etc.” Not sure about H₂S. Likely minimal. Best to get MQ-136 for H₂S specifically.

MQ-137 Gas Sensor (Ammonia)

Target Gases: MQ-137 is for Ammonia (NH₃) gas. Ammonia is a common gas in agriculture (animal waste, fertilizer), refrigeration systems (older systems use NH₃), and certain industrial processes. MQ-137 detects NH₃ roughly in the 5 to 500 ppm range. It might also be slightly sensitive to other amines or maybe hydrogen to some extent, but primarily ammonia.

How it Works: Ammonia is a reducing gas (it gives electrons to SnO₂, increasing conductivity), so MQ-137’s resistance falls when ammonia rises. It’s calibrated typically at 50 ppm or 100 ppm NH₃ in datasheet. It’s good for detecting ammonia leaks (from a refrigeration coolant or an industrial process) or high buildup (like in poultry farms where manure releases NH₃).

Physical Specs: Standard MQ package, presumably similar to others (Winsen says range 5-500 ppm and likely similar dimension to MQ-136)

Wiring: Standard.

Calibration: If possible calibrate with a known concentration – maybe a dilution of household ammonia cleaner in air. For example, if you vaporize some ammonia cleaner in a bag and measure, you could approximate ppm by the concentration of the cleaner. There are calculators for ammonia evaporation but it’s tricky. Alternatively, calibrate at the smell threshold: if you start to smell ammonia faintly, that might be ~5-10 ppm – note the sensor reading at that point. Use that as one calibration point.

Average Price: A bit specialized, likely $10 or so. Similar to MQ-136. If unavailable, note MQ-135 is also quite sensitive to ammonia (as listed in its target gases. Note Ammonia can also poison MOS sensors over time by reacting with them, but presumably MQ-137 is built for it or you may need to recalibrate occasionally.

MQ-138 Gas Sensor (VOC/Organic Vapor)

Target Gases: MQ-138 is a general Volatile Organic Compound (VOC) sensor, with high sensitivity to Benzene, Toluene, Alcohol, Acetone, Propane, Formaldehyde, Hydrogen. In short, it covers a wide range of organic vapors (like an air quality sensor focusing on VOC). It overlaps with MQ-135 but seems to skew towards solvents like toluene/acetone. Possibly used for indoor air quality or solvent leak detection in factories. Detection range ~5–500 ppm VOC mix (according to Winsen).

How it Works: Similar to MQ-135, it’s a broad-spectrum MOS sensor. Likely SnO₂ with some doping to be more sensitive to aromatic compounds (like benzene ring molecules) and formaldehyde. It will respond to many things: paint fumes, glue fumes, gasoline vapor, alcohol, etc. It might be a bit less sensitive to ammonia than MQ-135, focusing more on organics.

Physical Specs: Standard MQ can. Possibly same sizing as others. If you see SainSmart or others, they often put MQ-138 on a module just like others.

Wiring: Standard.

Calibration: Hard without references. You could calibrate relative to a known safe limit. For formaldehyde, safe limit is around 0.1 ppm for long exposure. But MQ-138 likely needs at least a few ppm to see a big change (it’s rated 5-500 ppm for VOC).

Average Price: in line with MQ-136, MQ-137. Possibly $10 or more. Not in beginner kits, but available through specialized sources. If not found, MQ-135 covers many VOC use cases, though MQ-138 is explicitly said to be good for toluene/acetone whereas MQ-135 is more for ammonia/benzene. If you needed formaldehyde specifically, there’s MQ-139 (exists, not asked, but FYI there’s an MQ-139 for formaldehyde).

Other “Extended” MQ Sensors (Less Common)

• MQ-214: High-temperature methane detector (already included earlier).
• MQ-216: Sometimes listed as LPG/natural gas sensor, similar to MQ-6.
• MQ-303A: Ultra-sensitive alcohol sensor, miniaturized.
• MQ-309A: Combustible gas, compact format.
• MQ-309C / MQ-319 / MQ-390: VOCs, formaldehyde, smoke – niche or OEM-only modules.

These “extended MQ” sensors often share the same SnO₂ principle but differ in heater power, coating material, and mechanical design. Many aren’t well-documented in English datasheets, so they’re better treated as “experimental”.

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