Precision Balances

To assist you in your precision balance purchasing decision, the PCE Instruments (PCE) technical sales and support team has put together the below criteria for selecting a precision balance. However, should you require additional assistance with finding the right precision balance for your application, please don't hesitate to contact PCE at or +44 (0) 2380 987 035 for PCE Instruments UK , or at or +1 (561) 320-9162 for PCE Americas Inc.

Clarification of the term precision balances
The term "precision balances" in the verification law does not refer to the balances with the finest gradation of the calibration class I, which are referred to there as analytical balances and precision scales, but the balances of the calibration class II. The German Verification Law determines certain technical specifications that must be met in order to be allowed to call the applied weighing instrument precision balances. However, only a few users are aware of this, so the first thing to clarify is whether the calibration law must be taken into account when looking for the precision balances. The verified precision balances are required by law, for example, for certain applications in pharmacies or when buying gold and other precious metals. The verified precision balances do not necessarily weigh more accurately than non-verified precision balances, but verification officially confirms compliance with the legal requirements for accuracy.
If verification is not important, precision balances are generally understood to be the balances that achieve a very high measuring accuracy. Here, it is subjective what someone understands by an accurate measured value. For one user, precision balances with a measuring accuracy of 1 gram are sufficient, while another user requires a measuring accuracy of 0.1 milligram. It is therefore important to clarify what measurement accuracy is required for the measurements with the new precision scales. Some users assume that the measuring accuracy of the precision balances corresponds to the resolution of the balances. However, this is WRONG. If the technical data of a precision balance do not contain any separate information on the accuracy of the measurement, the user should be prepared for a possible measurement error of at least 3 to 4 times the measurement resolution. To be on the safe side, a factor of 10 should be considered. If the precision balances with a resolution of 0.1 grams are selected, a possible measurement error of 0.3 grams to 0.4 grams must therefore be taken into account. With the factor 10, the deviations of up to 1 gram should be taken into account. If an application, such as formulation weighing, requires a measuring accuracy of 1 gram, the precision balances with a resolution of 0.1 gram should be selected.

Maximum load: After clarification of the verification requirement and the required measuring accuracy, the maximum load is the next important criterion for the precision scales. Here, the user's wishes can be limited by the technical possibilities, since the resolution and accuracy of the scales are always related to the weighing range. High measuring resolution and accuracy require more graduation steps when the maximum weight is larger and thus a more complex design and higher sensitivity of the precision scales. This is then reflected in the price and in the requirements for the place of installation. If there are very large differences in the quantities to be weighed, it often makes sense to use several scales with different maximum weights instead of just one precision scales. Depending on which materials are weighed, it may also be necessary to consider whether an additional container is required for weighing. This must also be taken into account for the maximum weight.
Models: If you compare the available scales with suitable accuracy and maximum load, in some cases, you can choose between different designs. In addition to the most common compact scales, precision scales can also be built as suspended scales or as platform scales with a separate display. Some compact scales also offer the option of underfloor weighing using a hook attached to the bottom of the housing of the precision scales. In the case of the suspended scales, not only the load is suspended but also the precision scales itself. In the areas where electronic components are not required or should not be used, mechanical spring scales can be used as precision scales. Platform scales can be used as precision scales in the areas where the display should not be installed in the same housing as the load cells, but should be placed or suspended slightly away from them for protection or for better operability.

Interfaces: In many application areas of the precision scales, interfaces for data transmission are required for outputting, storing and evaluating the weighing data. For this purpose, an RS-232 or USB interface is often already built into the precision scales as standard. For some scales, an additional interface can be selected as an option. If the data of the precision balances are to be used directly for the process control, the measured values can be transmitted as a signal, for example via an analog output with 4 to 20 milliamperes or 0 to 10 V to an evaluation or control device.

Platform size: The weighing platform of the precision scales built as compact or platform scales should fit the contact surface of the materials to be weighed. The weighing platform should always be loaded centrally or as evenly as possible. This ensures that the load cells installed underneath it are not loaded contrary to their design and can record and transmit the correct value. For liquids, powders and small parts, suitable containers must be selected that match the size of the weighing platform.

Additional functions: Precision scales can also be equipped with additional functions. A frequently found additional function is the piece counting function, which makes it easier to record the quantity of similar small parts and pack them in fixed quantities. More rarely, a percentage function is stored in the precision scales, which can be used to calculate and display the percentage deviation of a sample to be weighed from the reference weight. Very special extra functions are density determination and moisture analysis.
Below is a brief list of the points for selecting a suitable model of the precision balances that should be clarified in advance

1. Is there a verification requirement for the intended use? (YES / NO)
2. What accuracy of the measurement is necessary? (0.001 g / 0.01 g / 0.1 g / 1 g etc.)
3. What is the maximum load? (50 g / 300 g / 2,000 g / 6,000 g ...)
4. Which design is suitable for the application (compact scales, hanging scales, platform scales)?
5. Are interfaces required for the data transfer? (RS232 / USB / LAN / analogue output 4-20 mA - 0-10V)
6. How large must the weighing platform be?
7. Are special functions required? (piece counting, percentage function, density determination, moisture determination ...)

In compliance with the EU Directive 90/384/EEC on the approximation of laws relating to non-automatic weighing instruments, precision balances must be verified within the European Union if they are used, for example, as follows

- In commercial transactions, when the price of a product is determined by weighing,
- In the production of pharmaceuticals in pharmacies, as well as in analyses in medical/pharmaceutical laboratories.
- For official purposes such as the determination of fees and penalties.
- For court opinions.
- In the manufacture of prepackages.

This list is not final. If you would like more information on the verification of the precision balances or if you do not know whether you actually need the verifiable balances, you can contact your local weights and measures office. If it is necessary to have the precision balances verified, this must be indicated when you order the balances. You can only select from the models that are verifiable, and you must also specify the place of installation – i.e., the name of the town or city and the postal code – for setting the gravity value. Whether, after what period of time and where the precision balances must be re-verified is regulated country-specific within Europe. For more information, please refer to WELMEC, the international organization for European cooperation in state-regulated metrology. On their website, you will find particular information for each country concerning the measuring instruments subject to the legal metrological control, type approvals, the offices responsible for monitoring, inspection intervals and possible penalties for violations.

Calibration certificate: To meet special quality assurance requirements, our precision scales can be supplied with a current calibration certificate and can be recalibrated annually or at other intervals specified by the user. During calibration, the precision scales are tested with traceable reference weights. This includes several measurements to evaluate the accuracy, linearity and repeatability of the weighing results. The values in the case of the edge loading of the weighing platform are also recorded and registered in the calibration certificate. If calibration determines that the precision balances do not meet the specified accuracy, it can be adjusted – i.e. readjusted – if necessary and then recalibrated. There are two calibration options that vary in complexity, one according to ISO 9000 and the other according to DIN EN ISO / IEC 17025, which is known as DAkkS calibration.

User calibration and adjustment: You can also test the precision balances by using suitable test weights yourself at regular intervals or before each new series of measurements to ensure that the results are correct. If unacceptable deviations are found, the balances must be reset to the correct value. In some balances, you can perform this adjustment yourself using suitable weights via a menu function. In the operating instructions of the precision balances with adjustment options via the operating menu, you will find a description of the adjustment procedure, possibly under the heading Calibration. The verified precision balances must be re-verified after adjustment. For this reason, the verified balances usually cannot be adjusted, or the verification seal must be destroyed for adjustment.

Internal automatic adjustment: The illustration shows the precision balances with internal adjustment weight without housing cover and weighing platform. The internal weight is required for automatic adjustment. This adjustment runs in the turned-on balances automatically after a preset time, for example, every two hours, or after a change in temperature of 1 °C. Due to this automatic feature, only a few incorrect measurements can occur within one working day, and errors will quickly become apparent within a series of measurements. Even small temperature fluctuations that affect the sensitive electronics are corrected immediately. Also, every time the precision balances with internal automatic adjustment are turned on, a function test runs first. This self-test includes checking the electronics of the balances and adjusting the balances using the internal adjustment weight.

When evaluating different precision balances, ask yourself the following questions:

- What is the anticipated weight of the lightest sample?
This helps you determine the minimum capacity needed for your precision balance.

- What is the anticipated weight of the heaviest sample?
This helps you determine the maximum capacity needed for your precision balance. Please note that typically the higher the maximum load that a balance will accommodate, the less accurate the weighing capability of the balance will become. There are high-capacity high-resolution precision balances; however, the cost may be prohibitive.

- What are the anticipated dimensions of the sample?
This helps you determine the size of the platform or weighing pan needed for your precision balance. The platform or pan size should relate to the maximum load and measuring accuracy of the balance, as the platform or pan should be constructed to ensure the ideal placement of the sample in the center of the surface. If the load is placed outside the center of the pan, leverage can occur. While the load cells of most precision balances can compensate for the effects of leverage up to a certain angle, samples larger than the pan size must be positioned more carefully to ensure the center of gravity falls within the center of the pan. Traditionally, the more accurate a balance is, the smaller the platform or pan size is.

In some instances, a precision balance will feature an underfloor weighing function for weighing objects unable to be placed on the scale due to the object's size or shape. A precision balance with an underfloor weighing function will feature a hook on the bottom for suspending loads.

- What level of accuracy is required for your weighing purposes?
This helps you determine the resolution needed for your precision balance. If the measuring accuracy of the precision balance is not indicated separately, the basic formula to assess the accuracy of a precision balance is that the measurement deviation is the resolution value multiplied by 3 or 4. If better accuracy is desired, consider a buffer factor of 10 for the precision balance. For example: If a balance has a resolution 0.1 g, then a measurement uncertainty of approx. ± 0.3 g ... ± 0.4 g has to be assumed (unless otherwise described). However, if a measuring accuracy of 0.1 g is desired, then you should look for a precision balance with a resolution of 0.01 g (factor 10).

Depending on the application, a calibration weight set may come in handy for verifying the accuracy of the precision balance and, if necessary, to adjust the balance accordingly. Like any piece of weighing equipment, a precision balance can drift over time and increase the measuring error.

- What is the goal of weighing the sample?
This helps you determine if advanced functionality beyond weighing is needed for your precision balance. This question can only be answered by the balance's end user. Some examples of advanced functions include: sum, tare, recipe, piece counting, hold (animal weighing), density measuring, suspended weighing and drying (moisture content measuring). For some users, the piece counting function is essential for performing an inventory quickly. At the same time, other users need the piece counting function for checking product completeness. For the product completeness application, a light or indicator is required to confirm the desired result. While both kinds of users would like to use the piece counting function, each type of user has a different purpose. Not every precision balance with a piece counting function will include a light or indicator. That is why the purpose of weighing should be clearly defined by the end user and only then, according to the user's specific criteria, a precision balance should be selected.

- What do you plan to do with the weight data after the weighing process is complete?
This helps you determine if an interface is needed to connect your precision balanced to a PC or printer. An interface allows the weighing data to be transferred. Examples enabling data transfer include USB, RS-485 and RS-232 ports, Bluetooth and WLAN interfaces, analog outputs and removable memories (like SD cards or USB sticks).

- What documentation requirements must the precision balance meet for your organization's quality assurance program?
An optional ISO calibration certificate is available as an accessory to most PCE-branded precision scales for an additional fee. An ISO calibration certificate is a document with a PRESET / ACTUAL comparison of your device. An ISO calibration certificate documents the measurement deviation, linearity error, repeatability and measurement error of the excessive load.

- What is your price limit?
This helps you determine if a precision balance falls within your budget. After answering the above questions, you may have your purchasing decision narrowed down to a handful of different models of precision balances. It should be taken into account that the appropriate precision balance should simplify, expedite and/or make the weighing process safer or better. Therefore, it is not always worthwhile to choose the most affordable precision balance.