Today’s precision digital scales, also referred to as digital balances, perform a broad range of weighing-related functions in R&D, quality control, and similar applications where accuracy and precision are key criteria to meeting government, trade association, and industry-promulgated standards. For purposes of discussion this post focuses on weighing equipment used in comparatively protected environments vs. scales found on manufacturing floors, shipping and receiving departments. Let us hasten to add, however, that the need for precision and accuracy is important for any application, and equipment is available to meet required standards regardless of where scales are employed.
Precision and Accuracy Defined
Certain of our posts discussing precision digital scales and analytical balances harp on an important difference between precision and accuracy as selection criteria. Allow us to cite an example: A $5,000 wristwatch may be precise, but if the hands are set wrong it does not show the accurate time. More to the point of this post, repeatedly placing a test weight of known mass on a digital scale may show the same result on the readout, meaning the scale is precise. But if the test weight and the readout differ, there is an accuracy problem.
This leads us to three key selection tips for precision digital scales: Readout, repeatability (AKA reproducibility) and linearity. How important are these to your situation? You decide but keep in mind that not only regulations but also economics can play an important role.
- Linearity is the plus or minus deviation of the readout (see above) from the actual load on a precision digital scale over its weighing range. For example a 10-gram test weight on the balance should show 10 grams. A 20-gram test weight should read 20 grams. Both on the balance should display 30 grams. But nothing is perfect. Precision balances with 0.001 g readout may have a linearity of ±0.002 or ±0.003 g.
- Readout, also termed Readability and Resolution, is the smallest difference in weight that can be read and displayed by the unit. For certain precision balances it can be as low as 0.001 g.
- Repeatability is the ability of a balance to display the same result when an object is repeatedly placed on the weighing pan and removed. In general the difference between the largest and smallest result is used to specify repeatability.
Tradeoffs in Selecting Precision Digital Scales
When selecting a precision digital scale there are, of course, tradeoffs that may be give-and-take between what you would like to have and what meets your particular needs. Since we’ve been on the subject of precision, accuracy and similar criteria let’s start with some refinements. Bear in mind that these are general distinctions, and that you’ll want to discuss your specific needs with professionals such as the scientists at Tovatech.
Weighing capacity, generally indicated as maximum weighing range, is as you would expect, the maximum load that can be placed on the precision digital scale weighing plate. The tradeoffs are in readout, reproducibility and linearity. Scales with higher maximum load capability sacrifice readout, reproducibility and readout. The significance may be small but should be taken into account based on your requirements.
Functionality covers what can be many weight-related functions that can be reported on the scale’s display. Many of today’s precision digital scales are quite versatile. In addition to displaying the weight of a sample other weighing-related functions they perform can include:
- Piece counting. First establish a reference quantity by placing a number of pieces (10 to 20 is generally recommended quantity and all must be the same) on the tared scale and enter the quantity. The scale calculates the weight per piece and enters it into memory. Top of Form
Piece counting functionality is extended when the scale is connected to a bulk scale and data transferred – a useful combination for inventory control. This greatly facilitates stock keeping. For more on this see our post on inventory management systems.
- Check weighing. Also termed tolerance weighing it allows users to set an acceptable plus or minus variation in weight and is achieved by programming the scale with lower and upper acceptable deviations from the target weight. This is because in many weighing applications what is important is the deviation of the target weight from a nominal value. How important is that deviation? For a box of cereal a 1 or 2 ounce deviation may be acceptable to the buyer but to packagers of thousands of boxes it is important. When compounding pharmaceuticals ± tolerances may be expressed in milligrams. Unacceptable deviations can be of major concern.
- Recipe (batch) weighing. This feature is used when compounding products where strict control of ingredients is mandatory and when researchers create and vary formulations while seeking to develop new or improved products. Scales with recipe weighing capability range from milligram laboratory to kilogram production line models that can store multiple recipes in memory.
- Percent determination determines the percentage deviation from the 100% target value.
- Multiple weighing units. Scales with selectable weighing units allow users to select weighing units such as grams, kilograms, ounces, pounds, carats and other units depending on particular models.
- Hold or movement function. Also identified as animal weighing this is an important function when the load does not remain stable. Veterinary scales are examples. As Fido wiggles on the weighing platform the scale calculates a “stable” weight as an average value.
- Record keeping capability. Recording results is a critical part of meeting GLP and GMP requirements. Precision digital scales equipped with a data interface such as an RS 232 or 485 allow users to connect the instrument to a printer or PC. Options depending on models offer USB, Bluetooth and wireless LAN interface connectivity.
Other functions can include battery and rechargeable battery power and suspended weighing capability. Optional accessories can include dust shields, draft shields, density determination units, protective working covers and secondary displays depending on the particular model of precision digital scale.
Calibrating Precision Digital Scales
Not listed in our functionality section above is the super-critical requirement of calibrating a precision digital scale. That’s why we have this special section that applies to any precision weighing instrument.
The importance of calibration and recalibration is covered in user manuals issued with these products and includes early in the instructions how to adjust the instrument to its location. The importance of calibrating the equipment to the location is crucial to achieving the correct readout because the effect of gravity varies across the globe. As insignificant as this may be when thinking of pounds and ounces it does make a difference when weighing in milligrams.
Calibrating and recalibrating a precision digital scale can be done externally (manually) or internally. External calibration involves activating the calibration mode on the control panel and using a calibration weight suitable for the maximum weighing capacity of the scale. Recommended calibration weights and calibration procedures for the model are provided in the user manual. For example, the recommended adjusting weight for the 240 g capacity Kern 572-30 available from Tovatech is 200 g. When the balance is in the calibration mode the weight is put on the weighing pan. The scale adjusts itself to the adjusting weight. No user intervention is required.
Internally calibrated precision balances do not require the purchase of a calibration weight but generally cost more than externally calibrated scales. Activating the calibration mode accomplishes the exercise internally. Certain models have automatic internal adjustment based on temperature changes and time periods.
How to Maintain Precision Digital Scale Accuracy
As noted above the effect of gravity varies throughout the globe. A precision digital scale should be calibrated once it is located in your facility. Where you locate it is extremely important, and once it is located it should not be relocated without being recalibrated at the new location.
Other accuracy-maintaining criteria include (but not limited to):
- Avoiding vibrations from outside or inside traffic transmitted to the scale via its work surface. Vibration insulation is an option if a stable work surface is not readily available.
- Avoiding drafts and temperature extremes. Certain precision balances have standard or optional draft shields. But in all cases extreme fluctuations in ambient temperature should be avoided such as proximity to direct sunlight or HVAC vents.
- Following instructions regarding stabilization time, warm-up time and other criteria before beginning your weighing exercise. Moved or newly installed precision digital scales may require several hours to warm up. Details are provided in manuals. But as an example the Kern 572 model noted above requires a 3-second stabilization time and a 2-hour warm up time.
- Good housekeeping. As we say, a clean precision scale is a happy precision scale. Maintenance procedures are spelled out in user manuals.
Summing it Up
This post covers certain considerations when contemplating the acquisition of a precision digital scale. Other considerations are based on the nature of your operations. For help in selecting a precision digital scale that meets your specific requirements call the Tovatech scientists.