About Weighing Scales
Scales are practically used everywhere. Everyday consumer transactions are based on weight measurements. When we buy a quantity of apples we have to know their exact weight in order to pay the correct amount of money. Weighing scales are used also in the medical and pharmaceutical industries, in the building sector and in the food industry. There are many reasons (health, safety, invoicing, etc.) that require a proper calibration of weighing scales. We have to know if our scale is measuring accurately. Scales used for commercial purposes may need calibration every few months or even weeks.
Scales may lose their accuracy for a variety of reasons. Sometimes their components (mechanical or electronic) may become worn out and cause a change in the reading. Environmental factors play an important role as well. A scale, calibrated to work accurately in a cold environment like the one inside a refrigerator, will not have the same performance in the warm and humid environmental conditions of a bakery.
Difference between automatic and non-automatic scales
Automatic weighing instruments are the ones capable of performing consecutive weighing cycles without any intervention of an operator. These instruments are a special category and are handled according to the Measuring Instruments Directive (MID – 2004/22/EC).
Non-automatic weighing instruments are the ones that require an operator to determine or verify the weighing result. Determining the weighing result includes any intelligent action of the operator that affects the result, such as deciding when an indication is stable or adjusting the weight of the weighed product. Verifying the weighing result means making a decision regarding the acceptance of each weighing result on observing the indication.
Terms and procedures described below are referring to non-automatic weighing instruments.
The SI measurement unit of weight is the kilogram (kg). Kilogram, milligram (mg), gram (g) and tone (t) are the most common units used by scales.
A scale may be equipped with:
- Load-measuring device
- Display
- Printer
- Preset tare function
Weighing scales are classified according to:
- the verification scale interval (e), representing absolute accuracy and
- the number of verification scale intervals (n), representing relative accuracy
A minimum capacity (Min) is also specified to indicate that any use of the instrument below this value is likely going to produce considerable relative errors.
Non-automatic weighing instruments are characterized by their accuracy class which is defined in OIML R76-1, as shown in the following table:
|
Name |
Symbol |
|
Special Accuracy |
I |
|
High Accuracy |
II |
|
Medium Accuracy |
III |
|
Ordinary Accuracy |
IIII |
OIML R76-1 also provides the following table for the classification of weighing scales according to their verification scale interval (e), number of verification scale intervals (n) and the minimum capacity (Min), in relation to the accuracy class:
| Accuracy Class | Verification Scale Interval e | Number of verification scale intervals n = Max/e | Minimum capacity Min | |
| minimum | maximum | |||
| Special (I) | 0.001g £ e | 50 000 | – | 100 e |
| High (II) | 0.001g £ e £ 0.05g 0.1g £ e | 100 5 000 | 100 000 100 000 | 20 e 50 e |
| Medium (III) | 0.1g £ e £ 2g 5g £ e | 100 500 | 10 000 10 000 | 20 e 20 e |
| Ordinary (IIII) | 5g £ e | 100 | 1 000 | 10 e |
Maximum permissible errors for initial verification and for re-calibration are also defined in OIML-R76-1.