Calibration of an instrument is the application of accurate upper and lower limits or measurement range. Instruments which might require calibration include some of the following:
- Pressure Gauges
- Temperature Gauges
- pH Meters
- Weighing Scales
Calibration is most commonly achieved by initially setting the Zero-Point. The Zero-Point is the point where there is no load on the instrument. A simple example of this would be in the calibration of a weighing scales. All external loads must be removed from the scales prior to setting the Zero-Point. Only loads which are occurring internally or from the scales itself (i.e. the scales platform or the load cells) should now be acting on the system. At this point, the Zero-Point can be set on the scales.
The next step in accurately calibrating the weighing scales is to apply a known load to the scales and setting the scales to read the applied load. This second weight is generally close to the upper limit of the scales itself. If the scales has a maximum load of 500 Kg, then the weight it should be calibrated at should be either 500 Kg or just under it. The scales must be set to that exact weight in order to obtain the highest accuracy. For example if a calibration weight is 200.005 grams, then the weight entered on the scales should be 200.005 grams, not 200 grams. More points can also be entered between Zero-Point and the set maximum. This would increase the accuracy of the scales even more, and so would be used on scales for lighter and more accurate weights.
The weights are specially manufactured to an exact weight, generally common weights like 100 grams, 250 grams, 500 grams, 1 kilogram and so on. They are manufactured for the sole purpose of calibration, and should be kept in a clean and safe environment to ensure they do not get dirty/damaged which would cause a change in their weight. Calibration of these might be required every months, 3 months, 6 months or, in the case of larger weights where the tolerances are larger, every year. Very small weights like 1 or 2 grams or even lighter are prone to falling outside the tolerance due to dirt or even the presence of a finger print. For this reason, great care must be taken when working with the lighter weights.