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Scale calibration is a process used to ensure that a weight scale is providing accurate information. For scales used commercially, calibration may be necessary every few weeks or months, to confirm that the scales are still weighing accurately, while home scales may be left uncalibrated for longer periods of time
Scales begin to show inaccurate weights for a variety of reasons. Sometimes components of the scale become worn out, changing the reading, and environmental factors can play a role as well. For example, many scales do not perform well in the cold, and a scale calibrated to work in an environment like a warm, humid bakery might be off if it is taken into a cool store room. The more a scale is used, the more likely it is that inaccuracies will arise.
The best way to perform scale calibration is to use a known weight to see if the scale returns the correct measurement. Several companies manufacture weights specifically for use for calibration, but it is also possible to use something like a sack of flour, a set of barbells, or another object with a weight that is definitively known. To calibrate the scale, the weight is set on it and the reading is noted. Next, the scale can be adjusted until it yields a correct weight measurement.
Some scales are adjusted simply with screws or levers which can be moved. Digital scales may have buttons used to calibrate them. The owner's manual for a scale usually provides specific information on how this process is performed.
Most people use several different weights during scale calibration to confirm that the scale is correct in multiple ranges. Some scales may be less accurate at lower weights, for example, in which case adjustments performed with a low weight on the scale may throw off the scale at higher weights.
A special type of calibration is performed when people want to weigh goods in containers without including the weight of the container in the measurement. First, the empty container is set on the scale, and then the scale is zeroed out to set the “tare,” or unladen weight. The goods are added to the container, and the container is weighed again. The measurement on the scale will reflect the weight of the goods only, not the weight of the container. Some scales have preset tare settings that people can program for containers they use all the time, so that they do not have to reset the scale every time they weigh something.
@irontoenail - Another thing that might throw scales off is using them on carpet rather than a hard, flat surface. And scales aren't just used for people who are obsessing over losing weight.
There are people who need to gain weight and use scales to track their process. There are people who are relatively small and even half a kilo either way is important, so they need a really well calibrated set of scales.
I personally prefer to use the scales at the doctor's though, since they must be relatively accurate. They use one of those old fashioned scales with weights which I don't think need to be calibrated all that often.
I like to kid myself that it's just that my scales need calibration, but I think electronic bathroom scales are generally not going to need it all that much.
They are more likely to need the battery changed. However, if you're worried about it, the way I would check is to use something like a bottle of milk to check. Liquids just seem like they will be more accurate a measure, as a sealed bottle isn't likely to change weight at all, while I feel like a sack of flour could gain or lose moisture over time and not necessarily be completely accurate.
Generally though I think that people should pay more attention to waist measurements than to the scales as they aren't really a great way of telling you what's going on with your body.