Measuring Force And Load

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Measuring force and load is a simple matter of determining how much force the pull of gravity is exerting on any given load. There are many ways to determine force on a load, depending on the type of scale used, and the mechanics by which the scale reads the weight.

Perhaps the simplest and oldest type of scale to measure this is the equal arm balance. The principle of a balance couldnt be much simpler; the object to be weighed is placed on one platform while, at the opposite end of a beam mounted on a pivot, reference weights are placed. The reference weights are added or subtracted until the two platforms are at dead-level balance with each other.

Larger balance-style scales eliminate the need for heavy reference weights by mounting the beam off-center. In some designs, a sliding weight called a poise can be included, which is movable along a calibrated track parallel with the beam. A poise makes the calibration trickier, since its exact mass has to be adjusted and offset to the beams exact lever ratio. More sophisticated balances can include a platform floating on a cantilever beam system, which transmits force ...
... to a bearing, which then pulls on a stilyard rod and reduces the force down to a more conveniently sized beam.

A spring scale (like the hanging scale in a grocery stores produce department) has a spring that stretches or uncoils in response to how hard gravity is pulling down on the object in the cradle beneath it. These simple scales show the weight on a dial or gauge (sometimes connected by a worm gear or rack-and-pinion setup). Spring scales, simple as they may be, can be fairly accurate, with any loss of accuracy coming from the way the spring responds to temperature. More expensive spring scales have temperature-compensated springs to provide some correction for this.

The most common electronic versions of spring scales or balance scales use an electronic strain gauge, which measures the deflection of a bending beam by means of electrical resistance. These electrical strain gauges are mounted at several locations to measure the deflection. This type of electronic scale can handle a wide range of loads, including very heavy weight capacities, such as railroad cars or loaded trucks. The platform for these industrial scales is referred to as a weighbridge.

Heavy weights are also sometimes measured using hydraulic force. The force of the weight is applied to a piston or diaphragm, and hydraulic fluid flows through lines to a dial indicator or electronic sensor.

Various things can compromise the accurate measurement of force, depending on the circumstances and the type of scale being used:

* displaced air in the object
* error in any reference weight being used
* air gusts pushing the scale platform and load up, down or sideways
* friction in moving parts of the scale
* mis-calibration over time, due to wear, or drift in the electrical components
* magnetic fields
* electrostatic fields
* corrosion
* convection of air
* even gravitational anomalies (like using a scale near a mountain, or failing to recalibrate a scale after being moved)

For extremely accurate measurement jobs, an analytical balance can take many of these variables into account. A lab-quality analytical balance features a transparent enclosure (so that dust does not collect inside and so that air currents dont interfere with the measurement). The sample is also controlled so that convection doesnt form air currents inside the enclosure and affect the reading. Analytical scales can deliver precision all the way to .01 gram.

For higher capacity requirements, the new Ultra Precision Scales use surface acoustic wave sensors to provide accuracy levels that are up to twenty times more accurate then strain gauge scales. These scales are available with maximum capacities ranging from 2 lb all the way to 500 lb and higher.