MAKING ACCURATE WEIGHT MEASUREMENTS
Weighing The World
Scales are used every day throughout the world in a wide range of industries. From the raw material used in the products around us to the food that we buy at the grocery store - almost everything around us was weighed using a scale at some point.
Scales and other measuring devices come in a wide range of accuracy classes. For instance, most deli scales are accurate to only a few grams while a diamond or jewellery scale is typically accurate to a few milligrams (0.003g). For food portion use, a deli scale measures weight accurately enough to make your sandwiches, but if you had to measure something like gold or platinum, you would want to use a jewellery scale.
But how do you know if the scale you are using is accurate? If you check the various clocks around you, you might notice they don’t all agree with each other, with some more accurate than others. The process of checking the accuracy of a measuring device is called calibration. Calibration is done by comparing your measurements with a reference standard. For your clocks, you would compare the time with one of the atomic clocks using a service such as the NIST’s time and date server time.gov. For scales, you have to use accurate calibration weights that match your scale’s specific requirements.
All of our scales come calibrated from the factory and most users will not need to calibrate their scale for some time. However, if you require the most accuracy from your scale, you should purchase a calibration weight along with your scale to check the accuracy and adjust as necessary. Calibrating your scale is permanent so you should only do so using the specific mass listed in your user manual, or you risk miss-calibrating your scale.
Sources of Error
Calibration of your scale is only one part of maintaining accuracy in your measurements. There are also a number of environmental influences that can affect measurement accuracy which you should try to account for and minimize. These include:
- Gravity - The slight differences in local gravity around the world make a difference in how much things weigh. These differences are small but can be detected by very accurate scales. Calibration eliminates the differences in weight, making sure the same mass weighs the same everywhere.
- Wind - Drafts of wind from nearby vents or fans can be detected by precision scales, even if you don’t necessarily feel them. You should use your scale in a location free from drafts or use a draft shield.
- Temperature - The metal components in scales such as the load cell and strain gauges expand and contract slightly as temperatures change. These changes are corrected for using temperature compensation sensors built into the circuit, but large fluctuations in temperature will cause scales to drift. If temperatures change drastically, you may need to let your scale acclimatize to its surrounding temperature, then calibrate your scale.
- Vibration - Sensitive scales can pick up slight vibrations from nearby machinery or foot traffic. You should find a stable and level surface to use your scale. A heavy countertop or sturdy table should work well.
- Static - The sensitive circuits inside digital scales can be disturbed by static electricity. Static may cause weight readings to fluctuate randomly. Try to eliminate static by keeping humidity levels in a comfortable range or use anti-static wipes around your work-space.
- Radio Frequency Interference (RFI) - Strong radio wave emitting devices can also disturb digital scale readings. You should keep cell phones and routers away from your scale to minimize interference.
Last but not least, you should practice good measurement technique when taking weight measurements with your scale. Consistency is key. Be sure to place your items on the centre of the weighing platform and do not leave items on the scale for prolonged periods of time. You should try not to breathe over the scale or lean against the table or countertop as this will cause the scale to tilt slightly. When measurements are critical, multiple measurements are sometimes made.