Tips on Choosing Check Weighing Scales

The PES lab balance with check weighing capability

The PES lab balance with check weighing capability

Check weighing scales help deliver an honest measure. Check weighing benefits both buyer and seller because when properly programmed these scales help ensure buyers get what they pay for and sellers do not over or under package their products.

The importance of accurate check weighing – also called tolerance weighing – extends across virtually every market where weight (as opposed to quantity*) determines price and in most instances compliance with regulations such as promulgated by government, trade and professional associations.

Applications for check weighing scales run from the simple to the critical. Your box of breakfast cereal may say net wt 18.2 oz (why they don’t pack 19 oz is a mystery). It does not necessarily relate to volume, as many packages say “contents may settle due to handing.” At the other end, compounding pharmaceuticals calls for precise ingredient compilations within tightly defined ± tolerances of the target weight.

Repetitive weighing a.k.a. target weighing is a related function where researchers are preparing multiple samples and want to insure the same amount is being used repetitively.

Check Weighing Defined

Simply stated, check weighing is achieved by programming the scale with lower and upper acceptable deviations from the target weight. This is because in many weighing applications it is not the absolute weight of an item that is of interest but the deviation of this weight from a nominal value. In our cereal box example, 19.2 or 20 ounces may not seem much but multiplying that small deviation over hundreds of thousands of boxes does add up for the manufacturer. Maybe that’s good for the customer. Most customers probably would not know or care if the box contains 16 or 17.2 ounces. But government inspectors would – in both cases.

In our pharma example, ± tolerances may be expressed in milligrams. Unacceptable deviations can be of major concern.

Two Examples of Check Weigher Scales

Check weigher scales are manufactured for multiple applications. Examples are the Kern scales available from Tovatech that carry the [± TOL] pictogram.

Check weighing in the research lab can be achieved using the four Kern ABS-N analytical balances with maximum weighing ranges of 82 to 320 grams depending on model. Readout of all four is 0.1 mg.

When programmed for check weighing the ABS-N balances allow researchers to set target weight and the upper and lower deviation limits that take into account taring for the sample pan.

Indicators HI, OK or LO in the display show where the samples fall within the set tolerance range. These indicators only function when the balance is operating in the check weighing or target mode, otherwise they cannot be seen.

Scales with higher load capacities are exemplified by the Kern IFB series of industrial platform scales. These available in maximum weighing ranges of 6 to 600 kg and readout from 0.2 to 20 grams based on the maximum weighing capacity.

The Kern IFB industrial platform scale with check weighing capability

The Kern IFB industrial platform scale with check weighing capability

As with the analytical balances described above, scales are programmed with upper and lower weight limit values. The control panel emits an audible beep and optical signal indicating red if the weight is above or below the tolerance margin and green if it is within the tolerance. An optional remote signal lamp can be used as an accessory, indicating green if the weight is on target and red or yellow if off target.

As with all operations where weighing accuracy is paramount, balances and scales used for tolerance or check weighing should be calibrated according to procedures spelled out by relevant regulatory authorities and in the company’s Quality Management manual. Set-up, operating and calibration procedures are also provided in user manuals.

The scientists at Tovatech are ready to help you select a scale to perform accurate check weighing functions at your organization. Please contact us for an in-depth discussion on the topic.

* For more information on weighting for exact quantities see our post on how counting scales count.

About Rachel Kohn

So how did an MIT Ph.D. end up selling refrigerators? When I figured out that a lot more scientists buy lab refrigerators than innovative leading-edge instruments. I hope that my many years of lab experience will help you find the right equipment for your work. Before co-founding Tovatech I worked in business development and project management at Smiths Detection, Photon-X, Cardinal Health, and Hoechst Celanese. And before that I spent 12 years as an R&D chemist at Hoechst Celanese and Aventis working on advanced drug delivery systems, polymer films and membranes, optical disks, and polysaccharides. Some day, eventually, I’ll make enough money to develop an innovative technology that will change the world. Read More