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This VariableType defines the general characteristics of an AnalogItem. All other AnalogItem Types derive from it. The AnalogItemType derives from the DataItemType. It is formally defined in Table 2.

Table 2 – AnalogItemType definition

Attribute	Value
BrowseName	AnalogItemType
IsAbstract	False
ValueRank	−2 (−2 = ‘Any’)
DataType	Number
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the DataItemType defined in 5.3.1 i.e the Properties of that type are inherited.
HasProperty	Variable	InstrumentRange	Range	PropertyType	Optional
HasProperty	Variable	EURange	Range	PropertyType	Mandatory
HasProperty	Variable	EngineeringUnits	EUInformation	PropertyType	Optional

The following paragraphs describe the Properties of this VariableType. If the analog item’s Value contains an array, the Properties shall apply to all elements in the array.

InstrumentRange defines the value range that can be returned by the instrument.

Example: InstrumentRange::= {-9999.9, 9999.9}

Although defined as optional, it is strongly recommended for Servers to support this Property. Without an InstrumentRange being provided, Clients will commonly assume the full range according to the DataType.

The Range Data Type is specified in 5.6.2.

EURange defines the value range likely to be obtained in normal operation. It is intended for such use as automatically scaling a bar graph display.

Sensor or instrument failure or deactivation can result in a returned item value which is actually outside of this range. Client software must be prepared to deal with this possibility. Similarly a Client may attempt to write a value that is outside of this range back to the server. The exact behaviour (accept, reject, clamp, etc.) in this case is Server-dependent. However, in general Servers shall be prepared to handle this.

Example: EURange::= {-200.0,1400.0}

See also 6.2 for a special monitoring filter (PercentDeadband) which is based on the engineering unit range.

EngineeringUnits specifies the units for the DataItem’s value (e.g., DEGC, hertz, seconds). The EUInformation type is specified in 5.6.3.

Important note: Understanding the units of a measurement value is essential for a uniform system. In an open system in particular where servers from different cultures might be used, it is essential to know what the units of measurement are. Based on such knowledge, values can be converted if necessary before being used. Therefore, although defined as optional, support of the EngineeringUnits Property is strongly advised.

OPC UA recommends using the “Codes for Units of Measurement” (see UN/CEFACT: UNECE Recommendation N° 20). The mapping to the EngineeringUnits Property is specified in 5.6.3.

Examples for unit mixup: In 1999, the Mars Climate Orbiter crashed into the surface of Mars. The main reason was a discrepancy over the units used. The navigation software expected data in newton second; the company who built the orbiter provided data in pound-force seconds. Another, less expensive, disappointment occurs when people used to British pints order a pint in the USA, only to be served what they consider a short measure.

The StatusCode SemanticsChanged bit shall be set if any of the EURange (could change the behaviour of a Subscription if a PercentDeadband filter is used) or EngineeringUnits (could create problems if the client uses the value to perform calculations) Properties are changed (see section 5.2 for additional information).