9.1. Principle
9.1.1. Tare Adjustment
9.2 Measurement Methods
9.2.1. Mechanical
9.2.2. Hydraulic and Pneumatic
9.2.3. Load Cells (Strain Gauges)
9.3. Related A.P.C.S. Products
9.1. Principle
Weight (W) is the force generated by a mass when an acceleration is acted on it. This acceleration is typically gravity but it can be any acceleration acting on the mass.
W = F = m x a Where:
- F = the force (N)
- m = the mass of the object
- a = the acceleration acting on the mass, typically gravity
9.1.1. Tare Adjustment
Tare adjustment is the cancelling out of the load effect of the material holding container or vessel. An example would be the common metal tray used in grocery shopping, its influence (weight) is removed before the weight of the groceries is taken.
9.2. Measurement Methods
Industry use many different types of methods to measure weight. Each has its own advantages and disadvantages and applications to suit. Such methods include traditional mechanical and the more popular electronic methods.
Selection Guide
|
Type |
Advantage |
Disadvantage |
|
Mechanical |
|
|
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Hydraulics and Pneumatic |
|
|
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Load cell (Strain gauge) |
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9.2.1. Mechanical
In mechanical weighing machines, the load is reduced by levers so that a smaller proportional force is created. This reduction is typically achieved by levers and a combination of springs and counter balances. The smaller proportional force is then mechanical coupled to a indicator with appropriate scaling.
9.2.2. Hydraulic and Pneumatic
Hydraulic pressure can be used to measure weight. A properly designed capsule containing oil can be connected to a Bourdon tube or other pressure gauge. The hydraulic system serves the same purpose as the level system of a traditional scale, reducing the force due of the load by the use of a large capsule so that the weight is reduced to a measurable hydraulic pressure. In some scales of very large capacity, levers may be used to initially reduce the large force prior to final balancing by the hydraulic counter force. Pneumatic systems in which air is the force-transfer medium operate on similar principles.
9.2.3. Load Cells (Strain Gauges)
A load cell is a device for converting mechanical force into a proportional electrical signal. The applied force usually being gravity. Modern load cells use several techniques to convert the mechanical force into an electrical signal but typically connections are in the form of a Wheatstone bridge configuration.
Typical Strain Gauge Transmitter Connection Diagram
There are many traditional applications for load cells, such as weigh bridges, but they have a wide application in almost every factory as a measuring and controlling device.
Selection Guide
Example 1
Load cell of 1000kg capacity, with 2mV/V sensitivity. Set up using 10Vdc excitation.
Then if the "actual load" is 500kg max.,
then
Range
Example 2
If 1000kg load cell has a 200kg tare (tare = empty tank for example), with
a 800kg live load, then tare off the 200kg and Range