Are weight scales truly accurate?

“A spring scale measures weight by the distance a spring deflects under its load. A balance compares the torque on the arm due to the sample weight to the torque on the arm due to a standard reference weight using a horizontal lever. Specialized medical scales and bathroom scales are used to measure the body weight of human beings.

The balance (also balance scale, beam balance and laboratory balance) was the first mass measuring instrument invented. There has been a recent trend to “digital load cells” which are actually strain-gauge cells with dedicated analog converters and networking built into the cell itself.”

“Some of the sources of error in high-precision balances or scales are:

* Buoyancy, because the object being weighed displaces a certain amount of air, which must be accounted for. Some high-precision balances may be operated in a vacuum.
* Error in mass of reference weight
* Air gusts, even small ones, which push the scale up or down
* Friction in the moving components that cause the scale to reach equilibrium at a different configuration than a frictionless equilibrium should occur.
* Settling airborne dust contributing to the weight
* Mis-calibration over time, due to drift in the circuit’s accuracy, or temperature change
* Mis-aligned mechanical components due to thermal expansion/contraction of components
* Magnetic fields acting on ferrous components
* Forces from electrostatic fields, for example, from feet shuffled on carpets on a dry day
* Chemical reactivity between air and the substance being weighed (or the balance itself, in the form of corrosion)
* Condensation of atmospheric water on cold items
* Evaporation of water from wet items
* Convection of air from hot or cold items
* Gravitational anomalies for a scale, but not for a balance. I.e. using the scale near a mountain; failing to level and recalibrate the scale after moving it from one geographical location to another)
* Vibration and seismic disturbances; for example, the rumbling from a passing truck”

Wikipedia

Scales can be as accurate as you like – you just have to be wiling to pay for it.

I have lab scales here that measure down to .001 gram. I can tell you how many shots are left in an inhaler, the number of BBs in my kid’s 700 shot BB gun, or even watch the fuel evaporate out of a closed Zippo lighter. Measure a ream of paper, remove one sheet and it will show 499. Tear off a 1/8 inch square on one page and it knows.
I weigh myself on a 150 lb capacity Mettler balance that has a resolution of 0.5 oz. I can get on the scale, tare it, eat a 6 oz yogurt, get back on the scale, and it will read 5.5 oz! Drink an 8oz glass of water and it reads 8 oz! In my body! I can exercise for 10 minutes and tell you exactly how much I sweated out.

In scales, you get what you pay for. You do have to set them up properly. The units I have need to be set for your location to correct for Earth’s magnetic field.
Except for price, if you go with a Mettler you can’t go wrong.

As far as the origins of our measurements. As I understand it, it used to be based off of the king. Like a foot was the length of the current ruling king’s foot, and so it changed with different rulers. Eventually someone realized that it was a moronic system and they standardized it. I’m not sure about pounds though.

It’s also possible that I could be totally wrong about this.

Define truly accurate.

The pound is defined as the weight of 0.45359237kg @ 1g (on Earth)

The kilogram is defined as the mass of a particular 39.17mm square cylinder of platinum-iridium alloy, which was created in France in 1879. This is roughly (to within 25 millionths) the mass of a litre of cold water.

If your scale and your doctor’s scale differ by the same amount each time, and in the same direction (and your actual weight doesn’t change), then both scales are “accurate” enough, but one (or both) isn’t properly calibrated. I’d bet that your doctor’s scale has some kind of calibration done—or should, at any rate.

Scales used in commerce and industry are regulated (at the state level) by some organization or another who calibrates standard measures traceable to the National Bureau of Standards. That is, they have known measures that they check the scale’s (or fuel pump, regulator or other measuring device, gauge or instrument) against to an acceptable tolerance, plus or minus, and repeatability. That is, can it measure the same quantity in the same way regularly and repeatedly in all of the ways it may be tested: heavy and light weights, high and low temperatures, high / low humidity, etc.

If you look at your neighborhood gas station, you’ll more than likely see some kind of official (hard to counterfeit) certification sticker or decal affixed to it showing the last time it was calibrated by the Tennessee commission responsible for this, and more than likely it’s required on an annual basis. Whether this kind of check extends to physician scales or not, I don’t know. (But you should see the same thing on scales at the deli counter at your grocery, too.)

Scales for home use start out accurate, but I doubt they stay that way long. There is a government agency charged with making sure commercial scales of all sorts stay accurate, and they work based on complaints.