A hanging scale is one of the lowest cost weigh scale you can use for general purposes. It tends to have low parts count and potential high reliability: fewer moving parts if designed well, at least no tray, springs, or a large chassis. In the old days, they were just heavy duty springs with a pointer glued to one twist, and the degree of stretch was proportionate to weight.
Besides being small and portable, it can also weigh items far above it size, just fashion a sling for it to hang the object to the hook.
For general electronics use, it good for weighing devices for mailing or for air transport restrictions.
Its very rare that scales be sensitive enough to use as a postal scale, or food scale, if its made to weigh heavier objects such as packages.
This cheapo Chinese design is such a rare scale, and well designed for function based on the tear down analysis, and ingenious in its use of cheap parts. Its sensitive enough to weigh 5 grams. Its > 99% accurate to 110 lbs or 50 kg, support SI and US units, and has a built in celsius thermometer. It can tare. Electronics scales have advantages over mechanical ones: smaller, lighter, greater dynamic range and sensitivity. Cons are durability, longevity and need for batteries.
As a travel scale, the cheapo construction insures its very lightweight. The heaviest item are the metal strain gauge assembly and the batteries to power it. See tear down photos.
Its widely found, chances are a local eBay seller has it anywhere in the world, so you needn't buy it from China. You'll find them under many brand names, often at higher prices, but the hidden value are the brand-less package-less bulk sellers selling them for $4-6 shipped [ in local USA] as a fishing scale. If you find it listed as a 'luggage scale' the price for the same item easily >>2x .
http://www.ebay.com/itm/20g-40Kg-Digital-Hanging-Fishing-Luggage-Weight-Scale-/250779372627?pt=LH_DefaultDomain_0&hash=item3a639d8c53To use the scale, you hold or tie the nylon handle while dangling the item to be weighed on the bottom hook. If set to auto lock, it will lock on to a value in 3 seconds. It will lock on items > ~ a pound or 500 gms, and below it depends on the rate of change of weight.
The strain gauge, d, is made of a tied nylon handle and bolted hook. The [resistive, likely] white-blob sensor detects the degree of flex on the central metal plate. The whole weighing assembly floats in the chassis, so the chassis need not be as strong as the sensor assembly.
The PCB has an SMT LED, e, blinks during measuring refracted by red diffuser on the chassis. The LCD [ white rectangle] is connected by elastomers to the PCB, f, running the unlabeled microcontroller, g. The controls a,b,c are all SMT types and the switches are unattached plastic knobs on the PCB.
To check the calibration, I compared its readings to a known good scale and weight of water, i.e. 1kg ~ 1L ~ 1000ml. For gram weights I use US coins.
Cons
Cheap thin plastic, so durability is low. Treat it delicately.
Longevity? Unknown, so I have 2.
Comments on other variants of this design you can find in many places:
Avoid those that use watch batteries, the quiescent drain current is high enough to kill batteries left in here for months. Its also more a pain to remove and replace batteries, compared to the AAA design. With NiMH LSD cells, battery usage and replacement is inconsequential.
Avoid those than have integrated plastic handles into the chassis; the worse are those where the electronic assembly doesn't float relative to the strain gauge. This increases the strain on the plastic, which will later fail and break. It also more likely to cause weigh calibration problems as the plastic ages and wears or deforms.
Avoid a variant were the nylon cord is replaced with a straight through metal rod connected to the strain gauge, and a metal ring or handle at the top. This rod can act as a lever on the plastic shell, and pulled the wrong way can break the plastic shell. It also adds more weight. The nylon version is cheap yet ingenious as it will not mar soft plastic chassis, and nylon is very easy to service or replace when aged.
Enjoy.