First, it is better to see the difference between flow transmitter and flowmeter :
Many different materials such as liquids, gases, vapors are transported and distributed in piping systems every day. The liquid that passes through the pipes often has completely different properties, so different principles are needed to measure them. A mathematician found that there was a direct relationship between the pressure and velocity of a fluid flowing in a tube.
What is a flowmeter ?
A device used to measure the flow of liquid or gas in a pipeline is called a flowmeter. There are several types of flowmeters. The most common of these are Oriflate Plate, Venturi, Flow Nozzle, Rotameter and so on. All of these measure the velocity of the flow in different ways. In general, flowmeters can only show the amount of flow velocity.
Here’s how the measurement method works:
- Differential pressure meters have an artificial limitation that is placed inside the measuring tube. For example, a narrow page shown in the figure. There are two holes in the pipe wall, one before and after the orifice plate.
- Two separate tubes separate these holes into a chamber that is measured through the diaphragm and measures the differential pressure. The lowest differential pressure of the flowing fluid can be felt.
- From the fluid flow, the pressure before and after the cavity is the same.
- When the fluid is flowing, the velocity of the fluid around the cavity plate increases significantly due to the cross-sectional limitation. According to fluid mechanics, static pressure decreases at this stage. Therefore, the diaphragm senses higher pressure before and lower pressure after the orifice plate. This pressure is a direct measure of flow velocity.
What is a Flow Transmitter ?
A current transmitter is a flowmeter with internal electronic circuits that output electricity at current (4 to 20 mA) or voltage (1 to 5 volts). According to the figure above, the output of the flow transmitter is given to the control valve through a controller. This makes it possible to control and monitor the flow.
- The main element such as orifice, venturi, pitot pipe flow nozzle etc. are designed to create pressure difference. The secondary element is that the differential pressure transmitter is designed to measure the differential pressure caused by the primary element.
- Differential pressure causes deflection in the diaphragm. As a result, the capacitive sensor senses a change in capacitance which is then converted into an electrical signal (4-20mA).
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