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# Units Of Measurement

To express measured values in a scientific correct unit the framework supports units of measurement. By using quantified decimal values in state updates and commands, the framework is able to automatically convert values to a desired unit which may be defined by the system locale or on a per-use-basis.

# QuantityType

Bindings use the QuantityType to post updates of sensor data with a quantifying unit. This way the framework and/or the user is able to convert the quantified value to other matching units:

A weather binding which reads temperature values in °C would use the QuantityType to indicate the unit as °C. The framework is then able to convert the values to either °F or Kelvin according to the configuration of the system. The default conversion the framework will use is locale-based: The framework tries to convert a QuantityType to the default unit of the configured locale. This is the imperial system for the United States (locale US) and Liberia (language tag "en-LR"). The metric system with SI units is used for the rest of the world. This conversion will convert the given QuantityType into a default unit for the specific dimension of the type. This is:

Dimension default unit metric default unit imperial
Acceleration Meter per square second (m/s²) Meter per square second (m/s²)
Amount of Substance Mole (mol) Mole (mol)
Angle Degree (°) Degree (°)
Area Square Meter (m²) Square foot (ft²)
Areal Density Dobson unit (DU) Dobson unit (DU)
Catalytic Activity Katal (kat) Katal (kat)
Data Amount Byte (B) Byte (B)
Data Transfer Rate Megabit per second (Mbit/s) Megabit per second (Mbit/s)
Density Kilogram per cubic meter (kg/m³) Kilogram per cubic meter (kg/m³)
Dimensionless Abstract unit one (one) Abstract unit one (one)
Electric Capacitance Farad (F) Farad (F)
Electric Charge Coulomb (C) Coulomb (C)
Electric Conductance Siemens (S) Siemens (S)
Electric Conductivity Siemens per meter (S/m) Siemens per meter (S/m)
Electric Current Ampere (A) Ampere (A)
Electric Inductance Henry (H) Henry (H)
Electric Potential Volt (V) Volt (V)
Electric Resistance Ohm (Ω) Ohm (Ω)
Energy Kilowatt hours (kWh) Kilowatt hours (kWh)
Force Newton (N) Newton (N)
Frequency Hertz (Hz) Hertz (Hz)
Illuminance Lux (lx) Lux (lx)
Intensity Irradiance (W/m²) Irradiance (W/m²)
Length Meter (m) Inch (in)
Luminous Flux Lumen (lm) Lumen (lm)
Luminous Intensity Candela (cd) Candela (cd)
Magnetic Flux Weber (Wb) Weber (Wb)
Magnetic Flux Density Tesla (T) Tesla (T)
Mass Kilogram (kg) Pound (lb)
Power Watt (W) Watt (W)
Pressure Hectopascal (hPa) Inch of mercury (inHg)
Radiation Absorbed Dose Gray (Gy) Gray (Gy)
Radiation Effective Dose Sievert (Sv) Sievert (Sv)
Radioactivity Becquerel (Bq) Becquerel (Bq)
Solid Angle Steradian (sr) Steradian (sr)
Speed Kilometers per hour (km/h) Miles per hour (mph)
Temperature Celsius (°C) Fahrenheit (°F)
Time Seconds (s) Seconds (s)
Volume Cubic Meter (m³) US Gallon (gal)
Volumetric Flow Rate Liter per minute (l/min) US Gallon per minute (gal/min)

In some cases the system default unit may not be the most useful unit for a given quantity. For measuring precipitation km/h would be a quite uncommon unit, while mm/h would be the expected unit. You can set the unit metadata to a different unit to change the item's unit:

Number:Speed "Rainfall" { unit="mm/h" }

This unit is then also used for persistence. Setting the unit metadata to kW would persist 5.0 for a value of 5.0 kW (while 5000 would be persisted without because the system default is W). Attention: Changing the unit of an item may corrupt your already persisted data, no automatic conversion takes place.

The unit of the item is independent of the state description. The state description is used for display purposes only, it can contain any compatible unit and will not affect what is persisted or used in events.

# Number item with dimension and DecimalType

A DecimalType state update or command to a Number item that contains a dimension is considered to have the item's unit. So updating a Number:Length item with 5.0 will set the item's state to 5 m or 5 in. (depending on your locale). In case you set a different unit (see above, e.g. km) that is taken and the item will set it's state to 5 km.

# Number item without dimension and QuantityType

A QuantityType state update or command to a non-dimensional Number item will result in a state update that consists of the numeric part only. So updating a Number item with 5.0 kW will set the item's state to 5.0, updating the same item with 5.0 W will also set the item's state to 5.0. Linking dimension channels to non-dimensional items is therefore discouraged and will not be permitted in future versions of openHAB.

# Defining ChannelTypes

In order to match NumberItems and Channels and define a default state description with unit placeholder the Channel also has to provide an Item type which includes the dimension information:

<channel-type id="temperature">
    <description>Current temperature</description>
    <state readOnly="true" pattern="%.1f %unit%" />

The state description pattern "%.1f %unit%" describes the value format as floating point with one decimal place and also the special placeholder for the unit.

# Implementing UoM

When creating QuantityType states the framework offers some useful packages and classes: The org.openhab.core.library.unit package contains the classes SIUnits, ImperialUnits and Units which provide units unique to either of the measurement systems and common units used in both systems.

The MetricPrefix class provides prefixes like MILLI, CENTI, HECTO, etc. and the BinaryPrefix class provides prefixes like KIBI, MEBI, etc. both of which are wrappers to create derived units.

The org.openhab.core.library.dimension and javax.measure.quantity packages provide interfaces which are used to type the generic QuantityType and units.

# List of Units

All units which are currently supported by default are listed in the tables below.

Imperial (base unit symbols):

Type Unit Symbol
Length Inch in
Length Foot ft
Length Yard yd
Length Chain ch
Length Furlong fur
Length Mile mi
Length League lea
Pressure Inch of Mercury inHg
Pressure Pound per square inch psi
Speed Miles per Hour mph
Temperature Fahrenheit °F
Volume Gallon (US) gal
VolumetricFlowRate Gallon (US) per minute gal/min

SI (base unit symbols):

Type Unit Symbol
Acceleration Metre per Second squared m/s²
Acceleration Standard Gravity ɡₙ
AmountOfSubstance Mole mol
AmountOfSubstance Deutscher Härtegrad °dH
Angle Radian rad
Angle Degree °
Angle Minute Angle '
Angle Second Angle ''
Area Square Metre
ArealDensity Dobson Unit DU
CatalyticActivity Katal kat
DataAmount Bit bit
DataAmount Byte B
DataAmount Octet o
DataTransferRate Bit per Second bit/s
Density Gram per cubic Metre g/m³
Dimensionless Percent %
Dimensionless Parts per Million ppm
Dimensionless Decibel dB
ElectricPotential Volt V
ElectricCapacitance Farad F
ElectricCharge Coulomb C
ElectricCharge Ampere Hour Ah
ElectricConductance Siemens S
ElectricConductivity Siemens per Metre S/m
ElectricCurrent Ampere A
ElectricInductance Henry H
ElectricResistance Ohm Ω
Energy Joule J
Energy Watt Second Ws
Energy Watt Hour Wh
Energy Volt-Ampere Hour VAh
Energy Volt-Ampere Reactive Hour varh
Energy Calorie cal
Force Newton N
Frequency Hertz Hz
Frequency Revolutions per minute rpm
Illuminance Lux lx
Intensity Irradiance W/m²
Intensity Microwatt per square Centimeter µW/cm²
Length Metre m
LuminousFlux Lumen lm
LuminousIntensity Candela cd
MagneticFlux Weber Wb
MagneticFluxDensity Tesla T
Mass Gram g
Power Watt W
Power Volt-Ampere VA
Power Volt-Ampere Reactive var
Power Decibel-Milliwatts dBm
Pressure Pascal Pa
Pressure Hectopascal hPa
Pressure Millimetre of Mercury mmHg
Pressure Bar bar
Radioactivity Becquerel Bq
RadiationDoseAbsorbed Gray Gy
RadiationDoseEffective Sievert Sv
RadiationSpecificActivity Curie Ci
SolidAngle Steradian sr
Speed Metre per Second m/s
Speed Knot kn
Temperature Kelvin K
Temperature Celsius °C
Temperature^ Mired mired
Time Second s
Time Minute min
Time Hour h
Time Day d
Time Week week
Time Year y
Volume Litre l
Volume Cubic Metre
VolumetricFlowRate Litre per Minute l/min
VolumetricFlowRate Cubic Metre per Second m³/s
VolumetricFlowRate Cubic Metre per Minute m³/min
VolumetricFlowRate Cubic Metre per Hour m³/h
VolumetricFlowRate Cubic Metre per Day m³/d

^: Technically, mireds are the reciprocal of Temperature, but QuantityType and NumberItem will transparently convert between mireds and Kelvin. This technicality might be a problem if you are using mireds in rules. Be sure to use the toInvertibleUnit method on QuantityType, rather than toUnit.

Metric Prefixes:

Name Symbol Factor
Yotta Y 10²⁴
Zetta Z 10²¹
Exa E 10¹⁸
Peta P 10¹⁵
Tera T 10¹²
Giga G 10⁹
Mega M 10⁶
Kilo k 10³
Hecto h 10²
Deca da 10
Deci d 10⁻¹
Centi c 10⁻²
Milli m 10⁻³
Micro µ 10⁻⁶
Nano n 10⁻⁹
Pico p 10⁻¹²
Femto f 10⁻¹⁵
Atto a 10⁻¹⁸
Zepto z 10⁻²¹
Yocto y 10⁻²⁴

Binary Prefixes:

Name Symbol Factor
Kibi Ki 2¹⁰
Mebi Mi 2²⁰
Gibi Gi 2³⁰
Tebi Ti 2⁴⁰
Pebi Pi 2⁵⁰
Exbi Ei 2⁶⁰
Zebi Zi 2⁷⁰
Yobi Yi 2⁸⁰

To use the prefixes simply add the prefix to the unit symbol - for example:

  • milliAmpere - mA
  • centiMetre - cm
  • kiloWatt - kW
  • KibiByte - KiB