A digital scale uses computerized pressure gauges to weigh objects. Calibration of those instruments is usually done in the factory and shouldn’t be required unless they lose accuracy. That said, if your readings start to seem suspect, there are some steps you can take.
Scale Calibration Overview
Normally you need special weights to calibrate a scale and the best way to accomplish this is using the proper scale weights. Sometimes they come with the scale, but if not, such weights can also be purchased (Figure 2). If weights aren't available and the scale needs to be checked in a hurry, you can improvise makeshift weights using common items available around the house.
Some people use coins, but beware! During all the years that coins have been produced, their weights haven’t remained consistent. Different metal alloys have become available, and the ratio of their use in coins have evolved over time. This is particularly true of Canadian coins, which have thus proven to be unreliable throughout the years for calibration. US coins, however, have been more consistent for periods of many years, as shown in the table of Figure 3.
If the calibration isn't urgent, it's probably worth purchasing the proper weights. They’re actually available at a great variation in price. As people say, you get what you pay for—pharmacists might need a set of weights priced at a few hundred dollars for their accuracy, where the common household baker would probably be satisfied with a set under $10.
Step 1 - Clean the Scale
The scale must be completely clean so nothing interferes with the calibration process. Many things can make it difficult for the scale to zero, and often you may not even be able to see what's affecting the scale. Cleaning will require special tools and products that will not damage or degrade the scale and its surfaces.
Use a soft bristle brush and a mixture of warm water with a mild soap, to remove residue from the creases around and underneath the pan, then finish with a soft piece of cloth moistened with the same soapy solution. It’s usually not a good idea to wash electronics with a fully soaked cloth, since that creates the risk of water seeping into the internal components, causing short outs or corrosion. Once this is done, use a different cloth to thoroughly dry out every part and surface of the scale.
Step 2 - Prepare the Weight Samples needed
The amount of weight required to perform the test should be at least half of the capacity of the scale, with two-thirds being ideal to obtain the most accurate reading. So if a scale is rated at 500g, samples weighing between 250g and 350g should be used to do the testing. Whether using proper weights or coins or anything else, the type of weight samples and the quantity of each required should be selected beforehand.
Step 3 - Prepare the Scale for Verification
The scale has to be reset at zero, but first it should be turned on and allowed to warm-up for 10 minutes before calibration. Before starting to calibrate, press the power button to turn it on for one minute, then the off button to turn the power off. The next step is to set the instrument to “Calibration” by pressing some buttons on the front in a specific sequence that varies between sale models.
Consult the owners manual for this information, and for any other details pertaining to calibration. If you don't have it, try looking up your model online for detailed instructions. The calibration has to be done with nothing on the scale.
First, turn the power on, then press the “Zero” or “Tare” button if there is one present. Wait patiently while the scale clears any remaining data from previous uses. It may take a second, but your scale should show a “0.00” weight once it is zeroed.
Step 4 - Proceed to Calibrate
Once the scale is properly zeroed, you can go about calibrating it properly. If the scale is not zeroing properly, make sure it's on a completely level surface. It could take up to a minute for the scale to zero, so be patient. The process could vary slightly with different models, so consulting the manual is strongly recommended to ensure proper preparation for the calibration.
Place the calibration weight on the plate, then press the "Unit" key to show the value on the readout screen. If the right value is displayed within the scale’s tolerance, the calibration is completed. Then remove the samples from the plate, and return the scale to weighing mode by turning it off and then back on again. Then add the weight to the plate again to see if the same reading appears. If it doesn’t, start the calibration again.