Most diverse substances like liquid, gases, steam are transported and distributed in piping systems in every single day. The fluid going through the pipes often have completely different properties, therefore different principles for their measurements are required. A mathematician found that there is a direct relationship between pressure and speed of a fluid flowing in a pipe.
The device which is used to measure the flow of liquid or gas in a pipeline is called a flow meter. There are many types of flowmeters. Most commonly used ones are orifice plate, venturimeter, flow nozzle, rotameter etc. All these measures the flow rate in different ways. Generally, the flow meters can display only the value of flow rate.
• The differential pressure flow meters have an artificial restriction integrated into the measuring tube. For example, an orifice plate illustrated in the figure. Two holes are located in the pipe wall, one before and after the orifice plate.
• Two separate tubes connect these holes to a chamber separated by a diaphragm who measures the differential pressure. The tiniest differential pressure of the flowing fluid can be sensed.
• Of the fluid is not flowing, the pressure before and after the orifice is identical.
• When the fluid is flowing, the velocity of the fluid around the orifice plate increases significantly because of the restriction across the cross-section. According to the fluid mechanics, the static pressure at this point decreases. Thus the diaphragm senses a higher pressure before and a lower pressure after the orifice plate. This pressure is the direct measure of the flow velocity.
The flow transmitter is a flow meter with internal electronic circuits which gives an electrical output in a current (4 to 20 mA) or voltage (1 to 5V). According to the figure given above, the output from the flow transmitter is given to the control valve through a controller. Thereby, flow control and monitoring are made possible.
• The primary element like orifice, venturi, pitot tube flow nozzle etc is designed to create a pressure difference. The secondary element is the differential pressure transmitter is designed to measure the differential pressure caused by the primary element.
• The differential pressure causes deflection in the diaphragm. As a result, the capacitive sensor senses the change in capacitance which is then converted into an electrical signal(4-20mA).