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A metering pump precisely controls the flow rate of a liquid by pumping it in a continuous succession of small doses. They can only be used for liquids because liquids are incompressible; they occupy the same volume independently of their pressure. For that reason, metering pumps are positive displacement pumps — the pumping mechanism moves only the exact amount of liquid that it is set to deliver in one revolution or one stroke. When working correctly, a metering pump will have no leakage or slippage from input to output or vice versa.
Metering pumps are widely used in laboratories, chemical plants, medicine, and commercial vending. When used in medicine, they can mete out exact amounts of medications to patients over a period of time. For manufacturing, they assure the right amounts of ingredients are measured into production processes. In commerce, metering pumps make sure customers receive all of the liquid that they have purchased. Metering pumps are often used to dispense oils, syrups, and food-related items, such as a steaming cup of hot chocolate from a vending machine.
There are two basic types of metering pump, and a number of different technologies that can be used to facilitate either type. The first type is the variable displacement constant speed pump. Its motor turns at constant speed, but the amount of liquid pumped per revolution can be adjusted. The other type, fixed displacement variable speed, varies the motor speed to change the pumping flow rate.
The variable displacement metering pump works by the same hydraulic principle used in hydrostatic transmissions. Four to eight small hydraulic cylinders are arranged in a circle, with pistons with rolling balls protruding from the open end of the cylinders. An adjustable wobble or swash plate spins on a shaft turning at a constant speed. As the shaft goes around, the lowest point on the wobble plate depresses each piston in sequence by a precise amount, causing it to push out a precise amount of liquid with each turn. Increasing the wobble angle increases the amount each piston moves, and with it the amount of liquid each piston and cylinder dispenses to the output per motor revolution.
The variable speed type pump merely increases or decreases a non-adjustable wobble plate shaft speed. It may have a radial arrangement of cylinders and pistons around a center crankshaft. This type is usually less expensive to produce, and is by far the more popular of the two.
Another type of metering pump, the peristaltic metering pump, is widely used in medicine and laboratory work in part because of its ease of use. A peristaltic pump retains a length of flexible plastic tubing in a circular track while it is alternately squeezed and relaxed by a number of rollers on a turning disk at the center of the track. This rolling action divides the length of tubing into equally-sized moving segments of fluid that are drawn in at one end of the tubing and discharged at the other.
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