Detectors often display ppm while occupational limits use mg/m³. They are not 1:1—conversion depends on molecular weight, temperature, and pressure. Correct conversion is essential for compliant alarm setpoints.
What do ppm and mg/m³ mean?
ppm is a volume fraction (parts per million by volume), common in analyzers and process control. mg/m³ is mass concentration per cubic meter, widely used in occupational health and emission limits.
Different numbers can describe the same exposure—e.g., 30 mg/m³ CO ≈ 26 ppm at standard conditions. Many Senseiot transmitters allow unit selection—match your SCADA/HMI before commissioning.
Standard conversion formula
For ideal gases: mg/m³ = ppm × M × P / (R × T × 1000), with M in g/mol, P in Pa, T in K. At 25°C and 101.325 kPa: mg/m³ ≈ ppm × M × 0.041.
Examples: CO 1 ppm ≈ 1.14 mg/m³; H₂S 10 ppm ≈ 14 mg/m³; NH₃ 25 ppm ≈ 17 mg/m³. Higher temperature or altitude changes the ratio—document T/P for audits.
Alarm mistakes when units mismatch
A frequent error is entering a mg/m³ limit directly as ppm without conversion—e.g., 30 mg/m³ CO entered as 30 ppm ≈ 34 mg/m³, exceeding MAC. Light molecules like H₂ need careful math, not rules of thumb for heavy gases.
Build a gas/limit/conversion/range matrix and sign off with EHS. Senseiot engineering can review setpoints—contact us.
Default output units by sensor type
Electrochemical and NDIR modules are usually calibrated in ppm or %vol; mg/m³ may be computed in the transmitter or host. Check Modbus scaling and decimal places.
Combustible catalytic sensors use %LEL—a third unit system not interchangeable with ppm/mg/m³. See product catalog per gas.
Field verification tips
Use certified calculators with M, T, P; cross-check span gas ppm against portable mg/m³ readings; audit HMIs after unit changes so alarm logic stays consistent.
Senseiot transmitters support ppm, mg/m³, and %vol. Request a quote for bulk orders and register maps.