Scales are used in a wide range of applications. Therefore, this section describes the schematic structure of a scale and includes a simple classification of scale types.

In addition to the definition of a scale of OIML R76 the following definition applies:

A scale is a computer system, device or measuring instrument used to determine the mass or mass flow of a quantity of material and consisting of one or more weighing modules (including load receptor, load-transmitting device, load cell and data processing device) and additional modules or peripheral devices (see Figure 1). Each weighing module determines a mass. The weighing result of the scale may be the result of one weighing module or the addition of several weighing modules. Not all partial results need to be included in the result.

In addition, a scale can also have subdevices such as printers or feeder systems. Each scale needs a data processing device and at least one interface like a display or field bus. Thus, an interaction other than OPC UA is always possible.


Figure 1 – Schematic view of a scale system

Each scale provides a specific set of functions. These functions can be divided into three categories. The basic set of functions is available for all scales and behaves the same way for all scales. The scale type functions depend on the scale types (see Section 5.1.2) and behave the same within this type. An example is the administration of zones at the checkweigher or the administration of recipes at the recipe scale. In addition, further functions or applications can be specifically defined by a vendor.

A scale system is the combination of several scales that are addressed by a common interface and can have additional functions, such as a state machine or a production preset. However, a scale must be included in any case.

The schematic described here is a significant simplification of scales and focuses on the external representation of entire scale systems. It does not include any metrological details. In addition, real systems can deviate significantly from this schematic.

This classification of the scales is based on the aspect of communication via OPC UA. If possible, existing classifications (e.g. from OIML) were used. However, further conflicting classifications may exist. Since the boundaries are partly fluid, some devices can be assigned to several scale types. The resulting classification is shown in Figure 2.


Figure 2 – Scale Types Classification

The definitions of the individual types can be found in Table 10.

Table 10 – Overview of scale types



Automatic Filling Scale

Instrument which fills containers with predetermined and virtually constant mass of product from bulk by automatic weighing, and which comprises essentially automatic feeding device(s) associated with weighing unit(s) and the appropriate control and discharge devices. (Definition is based on OIML R61)

Catchweighing Scale (catchweigher)

Automatic weighing instrument that weighs pre-assembled discrete loads or single loads of loose material. (Definition is based on OIML OIML R51)

Automatic Weight Labeling Scale

catchweigher that labels individual pre-assembled discrete loads (e.g. prepackages) with the weight value. (Definition is based on OIML OIML R51)

Automatic Price Labeling Scale

catchweigher that calculates the price to pay on the basis of the indicated mass and the unit price and labels individual pre-assembled discrete loads (e.g. prepackages with the weight value, unit price and price to pay). (Definition is based on OIML OIML R51)


catchweigher that sub-divides prepackages of different mass into two or more sub-groups according to the value of the difference between their mass and the nominal set point. (Definition is based on OIML OIML R51)

Continuous Scale

An automatic weighing instrument for continuously weighing a bulk product on a conveyor belt, without systematic subdivision of the mass and without interrupting the movement of the conveyor belt. (Definition is based on OIML OIML R50)

Loss In Weight Scale

A special type of continuous scale is the loss in weight scale. It works according to the principle of controlled material increase or removal. The weight of the material is weighed by load cells and the change in weight per time unit is determined. The change in weight corresponds to the actual feed rate of the feeder when filling or emptying is stopped.

A schematic overview is shown in Figure 3.

Piece Counting Scale

A scale that can determine the number of parts in the load. A reference weight must first be determined or stored.

Recipe Weighing Scale

Scale with the capacity to manage and process recipes. Single recipe steps in a recipe can be the weighing of ingredients, the display of user instructions, the monitoring of switching values or the activation of aggregates.

Simple Scale

Simple scales do not have any major functional extensions and provide only basic weighing functionality, i.e. acting as a sensor.

Hopper Scale

A scale for weighing a bulk product with a tank, box or hopper mounted on one or more weighing bridges. The primary use case is tank level monitoring.

Laboratory Scale

The laboratory scales have a particularly high resolution and represent highly sensitive measuring insturments. For this reason, additional processes and measures are necessary in order to carry out an accurate measurement. For this purpose, e.g. the shielding of the environment with additional signs may be necessary.

Totalizing Hopper Scale

An automatic weighing instrument that weighs a bulk product by dividing it into discrete loads, determining the mass of each discrete load in sequence, summing the weighing results and delivering the discrete loads to bulk. (Definition is based on OIML OIML R107)

Vehicle Scale

Automatic or non-automatic scale having one or more weight bridges that determines the mass of a vehicle. The vehicle is typically a truck with or without a trailer or a rail wagon.

A special variant is the automatic rail-weighbridge (see OIML R106), which has one or more load receptors, including rails for the transport of rail vehicles, which determine the mass of the wagons and/or of the entire train by weighing during travel.


Figure 3 – Schematic Overview of a Loss In Weight scale

In addition to measuring, scales may also perform other tasks such as managing product data (production presetting and statistics/production output). To ensure that product-related information does not become inconsistent during the weighing process, it must be protected accordingly. For this purpose, a corresponding product information handle model shown in Figure 4is described in this Companion Specification. It defines two different modes (ProductHandleMode/AccessRestrictionMode).


Figure 4 – Overview of a Product handle

The ProductHandleMode indicates if a product can currently be processed by the scale or not. It is possible that the product-related information can be writeable or read only in both modes. The transition between the two states of the mode can be triggered internally in the scale or via the OPC UA methods selectProduct, DeselectProduct, SwitchProduct (see sections 8.7 - 8.8). In some cases it is possible that several products are in the "Processing" mode. For example, a catchweigher can process different products in parallel, while a continuous scale can only process one product at a time.

The AccessRestritionMode can be used to define the access restriction of the product. A client can make a set of changes (e.g. multiple write operations and method calls) only when the mode is in "WritableMode". This mode applies to the entire product Object , including all components such as zones. The access level of all child nodes of the product Object should at least have written access and the method should be executable. The AccessRestritionMode is used via the LockingService descripted in OPC 10000-100.

NOTE 1: When a physical product is to be processed but is not in the correct mode or has no OPC UA representation, the behaviour of the scale is application-specific and must be specified separately.

NOTE 2: The combination of the "Processing" mode and the "Writeable" mode should be used with caution, as the writeable element may be in an inconsistent state.