An old saying goes that a man with one watch knows what time it is, but a man with two watches is never sure. Scales must contend with a similar conundrum: If you weigh the same object twice, do you get the same result? How about on different days, under changing weather conditions? If so, can the scale remain consistent despite wear and tear?
These factors -- respectively known as repeatability, reproducibility and adjustment -- are all important, but a prospective buyer must also consider more basic questions, such as the scale's weight range, including its minimum and maximum capacity [source: AWS].
Then again, those statistics matter little if a device lacks the necessary accuracy and precision for the job. Although the two terms are often misused as synonyms, they're not interchangeable: Accuracy describes how closely an instrument measures an actual or expected value, whereas precision refers to how granular or exact that measurement is. A speed gun that clocks a 90.4213 mph fastball at 90 mph is accurate but not precise; a speed gun that clocks the same fastball at 88.3246 mph is precise but not accurate.
Scales usually express accuracy as a percentage of full scale (FS) -- the total calibrated range a device can measure. For example, a 1 percent FS accuracy can mean +/- 5 pounds for a 500-pound scale, but +/- 1 pound on a 100-pound scale. Alternatively, error can be expressed as percentage of actual reading (AR): For an object weighing 100 pounds on such a scale, a 2 percent AR would mean an error of 2 pounds, whether the scale had a range of 10 pounds or 10,000 [sources: AWS; Busch; Plint].
The precision of a scale is similarly variable. By industrial standard, regardless of capacity, scales are split into a maximum of 10,000 divisions: Hence a 10,000-pound scale will divvy into one-pound increments, whereas a 500,000-pound scale will divide into 50-pound increments [sources: AWS; Crowley; Mashaney]. Weighing devices are grouped into categories based on these divisions (see sidebar).
With this in mind, always try to select a scale that is has a high enough capacity to handle what you are measuring, but not much higher. This will ensure you get the most precision possible, pound for pound.
Author's Note: How Digital Scales Work
One of the joys of writing for HowStuffWorks.com springs from encountering the surprising delicacy, beauty and complexity to be found in everyday objects. The deft engineering and intricately milled parts that go into these items are, alone, enough to inspire any technophile. Look beyond the appurtenances and appliances to the heart of any device -- particularly one used for measurement -- and you find something even more wondrous: a physical law, ingeniously harnessed to an array of specific and useful tools.
Nowhere is this truer than in scales. In this article alone, I encountered Hooke's law for springs; Pascal's law for fluid pressure; Boyle's law, Charles' law and Gay-Lussac's law describing the behavior of gases; and Ohm's law for electrical resistance -- and that's to say nothing of the various unnamed laws governing stress and strain.
So the next time you encounter some politician who wants to cut science education in the name of balancing the budget, it might be worth reflecting that our most enduring fiscal facilitators, scales, are made possible by a centuries-old march of scientific discovery.
- American Weigh Scales, Inc. "Weighing Scale Terminology." 2011. http://www.awscales.com/support/terminology
- Busch Electronics. "Understanding Melt Pressure Sensor Accuracy." http://www.buschelectronics.com/Pressure_Sensor_Accuracy.html
- Camarda, Jennifer. Sales specialist, Sartorius Lab Products & Services. Personal correspondence. Feb. 5, 2013.
- Cappella, B. and G. Dietler. "Force-distance Curves by Atomic Force Microscopy." Surface Science Reports. Vol. 34. Page 1. 1999. http://www.see.ed.ac.uk/~vkoutsos/Force-distance%20curves%20by%20atomic%20force%20microscopy.pdf
- Cox, Fred. Vice president of sales, Cardinal Scale Manufacturing Co. Personal correspondence. Feb. 5, 2013.
- Craig, James I. "Electrical Resistance Strain Gage Circuits." Resistance Strain Gage Circuits. http://soliton.ae.gatech.edu/people/jcraig/classes/ae3145/Lab2/strain-gages.pdf
- Crowley, Ann. Product manager, Rice Lake Weighing Systems. Personal correspondence. Feb. 4, 2013.
- Cumpson, Peter J., Charles A. Clifford and John Hedley. "Quantitative Analytical Atomic Force Microscopy: A Cantilever Reference Device for Easy and Accurate AFM Spring-Constant Calibration." Measurement Science and Technology. Vol. 15. 2004.
- Eilersen Industrial Sensors. "Different Types of Load Cells and their Uses." 2010. http://www.eilersen.com/Different_Types_of_Load_Cells_and_their_Uses.html
- Encyclopaedia Britannica. "Cupronickel (Constantan)."
- Encyclopaedia Britannica. "Ancient Egypt."
- Fairbanks Scales. "'Type S' Pit-Type Truck Scale." Brochure. http://www.fairbanks.com/documents/literature/100208.pdf
- Mashaney, Derrick. Director, product development, Fairbanks Scales Inc. Personal interview and correspondence. Feb. 4, 2013.
- National Institute of Standards and Technology. "Specifications, Tolerances and Other Technical Requirements for Weighing and Measuring Devices as adopted by the 96th National Conference on Weights and Measures 2011 (Handbook 44)." 2012 Edition. http://www.nist.gov/pml/wmd/pubs/upload/2012-hb44-final.pdf
- National Instruments. "Strain Gauge Measurement – a Tutorial." Application Note 078. August 1998. http://www.ing.unp.edu.ar/electronica/asignaturas/ee016/anexo/r-an078.pdf
- Omega Engineering. "Introduction to Load Cells." http://www.omega.com/prodinfo/loadcells.html
- Petruso, Karl. "Early Weights and Weighing in Egypt and the Indus Valley." Museum of Fine Arts Bulletin (Museum of Fine Arts, Boston). Vol. 79. Page 44. 1981.
- Plint, George. "System Accuracy." Phoenix Tribology. 2005. http://www.phoenix-tribology.com/cat/at2/index/system%20accuracy.pdf
- Pratt, William F. "Strain Gage Measurement System – Fundamentals." http://www.suu.edu/faculty/pratt/spring04/engr2050/StainGageMeasurementSys.pdf
- Sartorius AG. "The Fundamentals of Weighing Technology Terms, Methods of Measurement, Errors in Weighing." Brochure.
- Sartorius Weighing Technology GmbH. "Correct Use and Handling of Analytical and Microbalances." Brochure.
- Stefanescu, Dan Mihai. "Handbook of Force Transducers." Springer. 2011.
- Takhirov, Shakhzod M., Dick Parsons and Don Clyde. "The 4 Million Pound Southwark-Emery Universal Testing Machine." Earthquake Engineering Research Center, University of California, Berkeley. August 2004. http://nees.berkeley.edu/Facilities/pdf/4MlbsUTM/4Mlb_Southwark_Emery_UTM.pdf