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. A variety of spring based hanging scales are available to capacities of 400 pounds. Generally speaking, the scale should be
selected based on accuracy rather than capacity (obviously overloading scales is a poor and unsafe practice). Accuracy is generally reflected as a
percent of capacity, so there are more actual pounds of error associated with a 400 pound scale as opposed to a 100 pound scale. Considering the
average item to be weighed and selecting a spring scale of twice that capacity is generally satisfactory. A 50 pound capacity scale with 1/2 pound
graduations costs around $50 while a 400 pound unit with 1 pound graduations would be around $200. The top loading spring scale is still commonly used in
the kitchen. These typically have a separate top pan and capacity of around 10 pounds. The typical
bathroom scale used to be primarily a spring type scale, but they are gradually being replaced with new electronic module units.
Crane scales are useful for determining in-process weights of materials. They are mounted in line with the hook. Mechanical units with a 20,000 pound
capacity cost in the $1500 range with an accuracy of +/- 0.5%. Electronic versions are typically accurate to +/- 0.1% with the cost of about $3500 for a 10,000 pound capacity unit
 Beam Balance Scales rely on a comparison between a known standard and the material to be weighed to produce a reading. The known
standard is generally a sliding or series of sliding weights that are moved to balance out the weight of the material in question.  These are much more precise and accurate that a
spring based scale. They are used in schools, laboratories and at home for hobbies. Capacities are typically much smaller than spring scales with
around 600 grams being typical and 2600 grams being maximum. The costs range from $100 to $200. There are still many high precision balances in use
in laboratories. While they are mechanical, provided they can be calibrated they are quite satisfactory. They will typically have a visible pan behind sliding
doors to keep drafts and particulate away from the system. The pan is linked to the comparison assembly which is commonly out of site. Scales can
be selected depending on the quantity of material to be weighed.
Electronic scales are typically fitted with a load cell or strain gage.  As weight is
applied, the tiny resistance of the load cell changes and weight is determined by electronic modules. There are a wide variety available from the scale around the bathroom to the
balance in the laboratory. Scales that are used in trade (selling things by weight) need to be certified and the electronic units are far easier to certify.
The costs for electronic scales are a function of their capacity and accuracy. A
bathroom scale typically costs less than $150.  A 20 pound commercial “Legal for Trade” unit with graduations of 0.01 pounds costs
in the $300 to $400. Counting Scales (determining number of pieces in a container based on the weight) in the 100 pound range with .01 pound graduations
cost approximately $1000 range. While precision laboratory balances with 200 gram capacities and resolutions of 0.01 milligrams cost about $4000.
Then there’s the mass comparator for ultra-precision. These go for around $20,000 with a capacity of around 5 grams but precision in the microgram range.
Of course you need to make your own judgments regarding the calibration of
scales and balances. If you are working in high precision at least look into calibration by an A2LA certified laboratory. Consider all of you costs to do it
internally using traceable standards and prescribed methods and compare that to the outside cost. Calibration Masses are used to calibrate or set scales to a
true reference value. They are available in several classes. Class 1 is for high precision balances, Class 6 for scales and dead weight testers, Class 7 for
rough weighing applications, Class F for checking “legal for trade” applications and Class M2 for educational applications and low accuracy scales. The % of
error varies by class as you might imagine with the Class 1 being the most precise. Costs for the masses varies based on class and the number of units in
the set. A 21 piece Class 1 set with masses from 1 milligram to 100 grams runs around $600. For cost comparison a 50 gram mass in Class 1 is around
$40, in Class F around $25 and Class M2 around $5.
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