Portable Scales: An Overview of Types, Calibration Requirements, and Selection Criteria

According to OIML R 76-1 (2006 edition), the term “mobile instrument” (T.1.2.11) is used for non-automatic scales. In a weighing pallet jack, the weighing mechanism is located in the forks or in the frame, so the measurement is taken directly at the point of handling. Its limitation lies in the fact that movement and load vibrations can interfere with the result. The standard specifies requirements for mobile scales, such as a mechanism to counteract the effects of tilt. Forklift scales use load cells or strain gauges in the fork carriage or in the forks. The load does not need to be set down for this. Limitations arise from the lifting process and tilt, such as on uneven warehouse floors. For hydraulic lifting systems, a warm-up phase must be defined in accordance with Section 4.18. If the tilt limit is exceeded, printing and data transmission must be disabled. According to the manual, infrared transmission does not provide the same level of data security as a cable connection. Wheel loader scales measure the bucket load via the hydraulic system or the lifting mechanism. Calibration capability varies depending on the device; see the chapter on calibration regulations for more information.

Suspended between the crane and the lifting sling, a mobile crane scale weighs the load directly at the hook. A scale for suspended loads measures the force exerted by the load hanging from the lifting sling and displays it as mass. This is useful for loads that are being moved by a crane anyway, since the weighing process is integrated into the lifting operation and does not require a separate workspace. Some devices can be operated wirelessly from the ground. The Weighing Manual specifies a mini handheld transmitter with a range of up to 30 m for this purpose. The limitation lies in the load itself: swaying and swinging initially prevent a stable weight reading. For hook operation, the manual specifies a settling time of up to 20 seconds from the moment the load is picked up until the result is displayed. This time may be shorter during slow lifting. Individual weighings should therefore be performed with the load at rest whenever possible. The chapter on calibration law clarifies whether a device is eligible for calibration in commercial transactions.

A mobile axle load scale consists of load cells or weighing plates positioned beneath wheels or axles. The total weight is calculated as the sum of the individual readings. Such devices measure axle loads and vehicle weights outside of fixed weigh stations, even at varying locations. According to the Weighing Handbook, their primary application is official traffic data collection—that is, determining the load on the road network and identifying overloaded vehicles. There is a reason for this: The stress on a road increases with the fourth power of the axle load. When used as wheel load meters for traffic monitoring, ramps are required (EO 9 No. 14.4.1), and their gradient must not exceed 30%. The devices may only be used in pairs and must rest firmly on the ground when a vehicle drives onto them. For information on the importance of levelness and the subgrade, see the “Installation” section.

If the device is used as a vehicle scale, special requirements apply. The main display and load platform must be visible from the operator’s station, if necessary via camera. It must be possible to zero the scale from the operator’s station before a vehicle drives onto it. The display must not exceed “Max + 9e” (where e is the calibration value). For self-service operation, the zero setting must be technically verified. The calibration interval is governed by the Measurement Ordinance (MessEV); see the section on calibration intervals.

A mobile platform scale is a portable weighing platform whose load cell bridge rests on load cells. In the pallet version, a frame equipped with load cells performs this function. According to the Weighing Manual, compact, portable platforms can be made ready for use in just a few simple steps. For vehicle inspection, the platform area is approximately 0.3 m² and is dimensioned so that even vehicles with dual tires can be inspected. For trucks with dual axles, walkways are provided as extensions. The design typically includes two access ramps. Extremely low overall heights—as low as 20 mm in some cases—even allow vehicles to drive onto the platform without ramps. Collapsible models have load cells mounted at the corners of the platform and can be accessed via ramps on all four sides. A pallet scale shown in the manual with a maximum load of 3 t (Max) can be easily moved to another location using a handle and casters. However, this design requires a level installation surface; see the “Installation” section for details.

There are two options for power supply. A scale with a rechargeable battery operates in different locations without relying on electrical outlets, which the Weighing Handbook explicitly identifies as the obvious solution for highly mobile devices. However, battery operation requires charging planning: The battery must be charged in a timely manner, and in situations where a power failure would halt operations, spare batteries can be useful. An automatic display shut-off feature also saves energy. A battery is therefore particularly suitable where the scale frequently changes locations. According to OIML D 11, the device must respond in a defined manner when the voltage drops—for example, with a warning indicator or by shutting down; the minimum voltage is specified by the manufacturer. Mains operation, on the other hand, provides a continuous power supply but ties the scale to accessible power outlets and cable routes. Hybrid solutions combine both approaches: The scale runs on AC power and switches to the battery when needed, or it charges while in use. In this case, the charging infrastructure must be factored into the planning, such as a fixed charging station or charging points at the devices’ storage location. One key question suffices for selection: How long does the scale need to operate without access to a power outlet?

There are several ways to transfer data from the scale: Wireless and Wi-Fi connect the scale to a tablet, PC, or network without cables; a printer produces the receipt; and the weight value is transferred to the inventory management system via an ERP connection. According to EU Directive 2014/31/EU, the design and manufacture of the device must ensure that its metrological characteristics are maintained when used properly. A printer that only outputs measurement data and cannot modify it is treated as a simple printer in WELMEC guidelines. With regard to display and storage, there is a clear distinction between certified and non-certified components for truck scales. The display of an auxiliary device is not always considered a primary display. A primary display on a PC is permitted only as a certified software module with a display and control unit capable of being calibrated. The requirements for additional display devices and the primary display also apply when measurement values are transmitted to calibrated data storage devices. One simple question suffices for selection: Where is the weight value supposed to go, and is a verified display required there?

From the weight placed on the scale to the number on the display, every electromechanical scale follows the same process. The load cell senses the force of the load and converts it into a measurement signal that is proportional to the load. The evaluation and display unit amplifies and digitizes this signal, converts it into units of mass, and uses it to calculate the weight value. In compact scales, the load cell, electronics, and display are housed in a single enclosure. In weighing systems, the functional blocks may be separate and connected via cable or radio. The zero setting is crucial for the interaction between the load cell and the display. The zero-setting device sets the unloaded scale to zero before a weighing operation. It thereby sets the zero point from which the display indicates the applied load. Tare, on the other hand, is a separate tare mechanism that sets the display to zero when the scale is loaded and (with the exception of tare input devices) can also be used for zeroing operations. The relevant legal regulations are outlined in the “Shipping” section. Maximum load (Max), verification value (e), and scale division are explained in the “Accuracy Classes” section.

A change in location alters the installation conditions and can therefore affect the weighing result. The Weighing Encyclopedia lists environmental factors such as ambient temperature and fluctuations in air pressure that affect the scale. To level the scale, check the spirit level: The air bubble should be centered; if not, correct the position by turning the leveling feet (Mettler-Toledo Weighing Guide). The scale must be aligned to its reference position (usually horizontal). For drive-on scales, the subfloor is particularly important: According to the Weighing Handbook (pp. 361–362), the measurement accuracy of drive-on scales depends on several factors. Up to about 5 km/h, an accuracy of ±1% of the measurement range is statistically achievable; up to about 100 km/h, values of ≤ ±5% are achievable. Therefore, even a mobile industrial scale requires a stable base. After each change of location, follow this routine: 1. level the scale, 2. zero the scale (see the previous section for instructions), 3. perform a test weighing, using calibration weights if necessary. For the use of mobile scales subject to legal metrology requirements—such as a calibrated parcel scale—the scales must comply with legal metrology regulations. If you have questions about the use of scales subject to legal metrology requirements, the responsible metrology office can assist you; see also “Having a Scale Calibrated.”

Section 34 of the Measurement Ordinance (MessEV) governs the calibration period for a scale: It is generally two years, unless Annex 7 of the MessEV specifies otherwise. According to the overview provided by the calibration supervisory authorities, there is a specified calibration period for industrial scales above a certain maximum load. The calibration period for a scale begins in the year it is placed on the market and ends at the close of the year in which the calibration period is scheduled to expire. For periods of one year or longer, it ends at the close of the calendar year in which it is scheduled to expire. For example, if a scale bears the marking “M 26,” its calibration period is calculated based on the year indicated on the marking. Since the underlying overview dates from 2022, the current legal status should be verified before applying these guidelines.

Data sheets can only be understood if you know the terminology. Non-automatic scales are subject to Directive 2014/31/EU. Each class specifies how many verification values (n = Max/e) are permitted. The maximum load (Max) is the largest load within the weighing range. The verification value (e) is the value used for classification and verification. The division value (d) is the display resolution. Without an auxiliary display device, d = e; with it, d is less than e, and e is at most 10 d. Class III with e ≥ 5 g allows between 500 and 10,000 verification values according to Table 3 of OIML R 76-1. For example, Max = 15 kg and e = 5 g result in exactly 3,000 calibration values. Since e remains constant, the relative resolution decreases for small loads: For a load of, say, 100 g, a 5 g increment corresponds to 5%; at maximum load, it corresponds to only about 0.03%. Caution regarding “mobile instruments”: OIML R 76 refers specifically to vehicle-mounted scales, not every scale commonly referred to as “mobile.”

Whether a portable scale is subject to calibration does not depend on its design, but on the purpose of the weighing. A scale is subject to calibration if it is used in commercial or official transactions and its measurement is decisive for billing. Typical examples include supplier billing upon receipt of goods, the sale of agricultural products by weight, and waste disposal billing by weight. A scale is subject to calibration if it determines the measurement value that is decisive for billing in commercial transactions. This also includes checkweighing, on which the acceptance of a delivery depends. Weighing operations not related to billing—such as those for internal process control—do not, however, trigger a calibration requirement. Anyone wishing to select a portable scale that is eligible for calibration should distinguish between two terms: “Eligible for calibration” means that the type or model has been approved for calibration, which is a prerequisite for calibrating a measuring instrument. “Calibrated” means that the individual device has been tested and marked. In cases where calibration is required, only a calibrated scale may be used; otherwise, a fine may be imposed. The user submits the application; the article “Having a Scale Calibrated” explains the process.

The conformity assessment of a scale is solely concerned with the question of how a device is placed on the market in compliance with the law. According to Section 6 of the German Measurement Act (MessEG), a conformity assessment has generally been required for new devices since January 1, 2015. Placing a device on the market refers to the first-time making available of a device on the European Union market. Type approvals issued up to December 31, 2014, may result in different deadlines. For used devices, this means: A scale initially calibrated on this basis may continue to be used as long as it remains validly calibrated. When purchasing a used scale, it is therefore worth checking the markings. Newer scales bear the CE marking, an “M” with the year, and the identification number of the conformity assessment body. Older devices display approval marks, verification marks, and the year. The verification period begins when the measuring instrument is placed on the market. The WELMEC Guide (WELMEC Guide 2-4, pp. 5–6) is not intended to apply to scales installed in trucks or mobile pallet scales. It therefore does not provide guidance for such designs. The duration of the validity periods is specified in the next section.

Where will the weighing take place: indoors or outdoors, in a barn, on a construction site, or at a waste disposal facility? This question must be considered before making any selection, because dirt, moisture, dust, impacts, and temporary surfaces affect each design differently. Axle load scales used outdoors are designed to withstand a wide range of temperatures. There’s a practical reason why a mobile scale must be robustly built: In scales with a lever mechanism, dirt and impacts can affect the cutting edges and pans; in scales with load cells, moisture requires special protective measures for the electrical and electronic components. Key questions: Will the scale be exposed to rain or dust? Will it be hosed down or cleaned? Will material fall on it, or will a vehicle drive over it? Portability is also a factor. The scale’s weight determines whether one person can carry it or if additional equipment is needed. The more frequently the scale is moved, the more important handles, casters, and a shock-resistant design become, since every transport puts additional strain on the device. Those who move the scale infrequently are more likely to accept a heavier device than those who move it daily.

When evaluating the costs of a mobile scale, one should not focus solely on the purchase price but rather consider the total costs over the scale’s useful life. The following items serve as a framework for assessment: First, the method of acquisition—that is, whether to buy or rent. Added to this are the recurring costs of calibration and inspection; re-calibration intervals are regulated by law, while the frequency of operational inspections depends on the weighing risk. Calibration or verification may incur transportation or travel costs depending on the location, and the scale is unavailable during this time. This also includes maintenance, repairs, and replacement parts. The Weighing Handbook points out that certain failures of electronic components—which can be detected through measurement—typically go undetected without metrological testing and lead to systematic measurement errors. Following a repair, re-calibration may also be necessary for applications subject to legal metrological requirements. For battery-powered devices, battery replacement is an additional cost, as is user training. Finally, the risk of failure and downtime must be factored in: What is the cost of the scale being unavailable? The section “Mobile or Stationary” describes the benefits of mobile weighing in the workflow.

There is no one-size-fits-all answer to whether a mobile or stationary scale is the better choice. The decision has two sides and primarily concerns process and organization. A mobile solution is preferable in situations where weighing takes place during picking, lifting, transporting, counting, or packaging. If the scale moves with the goods or the workstation, intermediate transport and wait times at central weighing points can be eliminated. Whether this advantage applies, however, depends on the specific application and must be evaluated individually for each process. On the other hand, fixed, high throughput rates at a consistent location favor a stationary solution. If there is a permanently designated weighing point in the material flow that the goods pass through anyway, that is the natural location for the scale. Added to this is centralized control: a single location is responsible for the weighings, and the processes are organized around this point. Therefore, when deciding on a weighing system, the first question to ask is whether the goods come to the scale or the scale goes to the goods. Distributed, changing weighing locations tend to favor the mobile option, while a fixed location with high throughput tends to favor the stationary option.

Start by recording the loads, not the catalog. First, note the smallest load that actually needs to be weighed over the course of a few typical workdays. Next, record the typical load that occurs most frequently, and finally, the largest load, including rare exceptions. This load range determines the maximum load: it should be higher than the largest load, with an appropriate margin for outliers, but not significantly higher. The trade-off is easy to explain. A scale that is too large has poorer resolution for small loads because the weighing range and scale division are coarser relative to the small load. If the scale is too small, there is insufficient margin, and even occasional peaks will push the device to its limit. If the typical load is well below the maximum, it is worth considering whether this exceptional case warrants a separate solution. For axle load scales, the Weighing Handbook describes different designs depending on the application, such as scales for stationary vehicles and dynamic scales for moving traffic. The overload capacity is very high in these cases and can reach up to 200% in individual instances. It serves as a safeguard, not as a design margin. The relationship between Max, e, and scale interval is explained in the section on accuracy classes.