SenML Units¶

SenML (Sensor Measurement Lists, RFC 8428) defines a data format for sensor values. The u field (unit) specifies the physical unit of a measurement. The official values are listed in the IANA SenML Unit Registry.

Used, among others, in: taskit BLE Manufacturer Advertisement (byte 11), CoAP/LwM2M sensor payloads, MQTT IoT applications.

1. SI Base Units¶

Symbol	Unit	Quantity
`m`	Meter	Length
`kg`	Kilogram	Mass
`g`	Gram	Mass (permitted)
`s`	Second	Time
`A`	Ampere	Electric current
`K`	Kelvin	Thermodynamic temperature
`cd`	Candela	Luminous intensity
`mol`	Mole	Amount of substance

2. Derived SI Units¶

Symbol	Unit	Quantity
`Hz`	Hertz	Frequency
`rad`	Radian	Angle
`sr`	Steradian	Solid angle
`N`	Newton	Force
`Pa`	Pascal	Pressure, mechanical stress
`J`	Joule	Energy, work, heat
`W`	Watt	Power, radiant flux
`C`	Coulomb	Electric charge
`V`	Volt	Electric voltage
`F`	Farad	Electric capacitance
`Ohm`	Ohm	Electric resistance
`S`	Siemens	Electric conductance
`Wb`	Weber	Magnetic flux
`T`	Tesla	Magnetic flux density
`H`	Henry	Inductance
`Cel`	Degree Celsius	Temperature
`lm`	Lumen	Luminous flux
`lx`	Lux	Illuminance
`Bq`	Becquerel	Radioactivity
`Gy`	Gray	Absorbed dose
`Sv`	Sievert	Equivalent dose
`kat`	Katal	Catalytic activity

3. Geometry, Kinematics & Flow¶

Symbol	Unit	Quantity
`m2`	Square meter	Area
`m3`	Cubic meter	Volume
`l`	Liter	Volume
`m/s`	Meter per second	Velocity
`m/s2`	Meter per second²	Acceleration
`m3/s`	Cubic meter per second	Volumetric flow rate
`l/s`	Liter per second	Volumetric flow rate

4. Optical & Radiation Units¶

Symbol	Unit	Quantity
`W/m2`	Watt per square meter	Irradiance
`cd/m2`	Candela per square meter	Luminance

5. Information & Counting Units¶

Symbol	Unit	Quantity
`bit`	Bit	Amount of data
`bit/s`	Bit per second	Data rate
`count`	Counter	Dimensionless count
`/`	Ratio	Dimensionless ratio
`1`	Dimensionless	Pure number

6. Geo-Coordinates¶

Symbol	Unit	Quantity
`lat`	Degree north latitude	Latitude (WGS84)
`lon`	Degree east longitude	Longitude (WGS84)

7. Specialized & Sensor Units¶

Symbol	Unit	Quantity
`pH`	pH value	Acidity (logarithmic)
`dB`	Decibel	Logarithmic ratio
`dBW`	Decibel-watt	Power level
`Bspl`	Bel SPL	Sound pressure level
`NTU`	Nephelometric Turbidity Unit	Turbidity

8. Percentage & Ratio Units¶

Symbol	Unit	Quantity
`%`	Percent	Ratio × 100
`%RH`	Percent relative humidity	Humidity
`%EL`	Percent remaining	Battery charge level
`EL`	Seconds remaining	Remaining battery life

9. Frequency & Rotational Speed Units¶

Symbol	Unit	Quantity
`1/s`	Per second	Frequency, rate
`1/min`	Per minute	Rotational speed, rate
`beat/min`	Beats per minute	Heart rate
`beats`	Number of beats	Pulse events

10. Material Properties¶

Symbol	Unit	Quantity
`S/m`	Siemens per meter	Electric conductivity
`kg/m3`	Kilogram per cubic meter	Density
`J/m`	Joule per meter	Force / energy per distance

11. Secondary Units (with Conversion)¶

Secondary units are not recommended as the primary value in the SenML u field, but are listed in the IANA registry and commonly used in practice:

11.1 Time¶

Symbol	Unit	SI Factor
`ms`	Millisecond	× 10⁻³ s
`min`	Minute	× 60 s
`h`	Hour	× 3600 s

11.2 Electrical Energy & Power¶

Symbol	Unit	Meaning
`MHz`	Megahertz	× 10⁶ Hz
`kW`	Kilowatt	× 10³ W
`kVA`	Kilovolt-ampere	Apparent power
`kvar`	Kilovar	Reactive power
`Wh`	Watt-hour	× 3600 J
`kWh`	Kilowatt-hour	× 3.6 × 10⁶ J
`VAh`	Volt-ampere-hour	Apparent energy
`varh`	Var-hour	Reactive energy
`kVAh`	Kilovolt-ampere-hour	Apparent energy
`kvarh`	Kilovar-hour	Reactive energy

11.3 Angle¶

Symbol	Unit	SI Factor
`deg`	Degree	× π/180 rad

12. Usage in SenML Records¶

Example SenML record (CBOR/JSON):

[
  { "n": "temperature", "u": "Cel", "v": 24.7 },
  { "n": "humidity",    "u": "%RH", "v": 58.3 },
  { "n": "pressure",    "u": "Pa",  "v": 101325 },
  { "n": "battery",     "u": "%EL", "v": 87 }
]

The u field always references a registered SenML unit from the IANA table.

13. Usage in taskit Products¶

In the taskit BLE Manufacturer Advertisement byte 11 (SenML Unit ID), the following numeric IDs are used (position within the IANA registry):

ID	SenML Symbol	Product
`55`	`Cel`	Measure2Go Blue RTD
`75`	`pH`	Measure2Go Blue pH-Meter
`143`	`mA`	Measure2Go Blue Loop RX / TX

⚠️ The numeric IDs are based on the order in the IANA registry. If the registry changes, these IDs may shift – the taskit firmware therefore defines them as fixed constants.