Scales are used everywhere from grocery stores to jewellery stores to the doctor's office, and we rely on them to accurately tell us the weight of things. But we can only be sure that a scale is working properly if it has been calibrated and has been shown to report weights accurately.
Start with nothing on the scale, and make sure that the scale says there is no weight on it. Then place an object of known weight on the scale and record what weight the scale actually reports. Repeat this with objects that have different, known weights that cover the scale's weight range. The scale will probably not report the exact value of the known weights, but as long as the error is smaller than the needed accuracy, the scale is usable. The needed accuracy will depend on a scale's intended use; a scale that will be used to measure gold must be much more accurate than one used to sell vegetables.
Different Methods for Different Scales
The calibration process differs somewhat depending on the type of scale being used. When zeroing the scale, a triple beam balance has a counterweight that can be adjusted. Mechanical weights often have a screw that can be turned to adjust the zero point. Electronic scales have a tare button. Simply pressing the button sets the scale's reading to zero regardless of how much weight is currently on it. Electronic scales are useful for measuring items without their containers. For example, you can tare the scale with an empty basket on it and then weigh a basket full of fruit, giving the weight of the fruit minus the weight of the basket.
Scales can lose accuracy over time and should be recalibrated periodically. Checking that the scale reads zero when there is no weight can be done more or less all the time because the process is straightforward. How often you should recalibrate the entire weight range depends on how accurate you need the scale to be.
It is assumed that you have objects of known weight before you start the calibration process, but in practice this can be difficult. Many companies sell calibration weights whose weight has been verified with high-quality scales. You can also use a known volume of water to calibrate the scales, since water's weight is directly related to its volume. If high accuracy is desired, use distilled water since impurities will change the water's weight somewhat. It is also possible to check two scales against each other. If two scales report the same weight for a given object, it is very unlikely that they are both off by exactly the same amount.