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">
    <item-type>Number:Temperature</item-type>
    <label>Temperature</label>
    <description>Current temperature</description>
    <state readOnly="true" pattern="%.1f %unit%" />
</channel-type>

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	m²
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	m³
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

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