A positive displacement flow meter is used to measure and control the rate at which a fluid or gas is moving. For example, displacement flow meters are used in medical devices that control the output of a chemical or medication or in food-processing devices that need systematic precision. A flow meter divides the gas or fluid being measured into individual packets of equal volume, called parcels, that are counted as they pass through the meter. A displacement meter has two main parts: a chamber that stops the flow of the gas or liquid and a mechanism that allows only a specific amount to pass through at a time. The flow rate of the gas or fluid through the displacement meter depends on how many of the parcels are allowed to pass through at one time.
There are two main types of positive displacement flow meters. The first is called a sensor-only displacement flow meter. This type of displacement meter is a type of switch that releases a certain amount of volume of the liquid or gas without any additional information. The second type of meter is considered to be a complete sensor system., and it includes a user interface or a display that gives the user more information about the operating pressure, the density of the fluid or gas and the percent accuracy of the device. Some displacement flow meters might also include alarms or flow history information.
The metering portion of the displacement meter can operate with a variety of technologies. Gear meters use rotating, interlocking gears to turn the valve at equal intervals so that the correct parcel size and volume flows through. Vortex meters actually calculate the velocity of the gas or liquid by determining how frequently pieces of a test material shed into the flow. The most common type of displacement meter uses a device called a nutating disc meter. In these displacement flow meters, the pressure of the parcel rotates a disc inside the meter, and a counter keeps track of how many rotations the disc has made as a means of controlling the flow.
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Positive displacement flow meters are available in both analog and digital models. Analog models can have output signals in sine wave, pulse wave or AM or FM frequencies. Digital models are commonly made with Recommended Standard 232 (RS232), RS422 or RS485 protocols. A parallel protocol model with a general-purpose interface bus (GPIB) also is available.