The principle of differential pressure transmitter for flow measurement is based on Bernoulli equation and differential pressure-flow relationship in fluid mechanics.
Principle statement
Differential pressure transmitters calculate flow by measuring the difference in pressure produced by a fluid before and after a special device in the pipe, such as a throttling device. According to Bernoulli's equation, when a fluid passes through a throttling device in a pipe (e.g., orifice plate, venturi tube, nozzle), there is a pressure difference between the front and back of the throttling device due to the change in flow rate. The pressure difference is related to the flow rate of the fluid.
Calculation formula
The relationship between the pressure difference measured by the differential pressure transmitter and the volume flow rate can be expressed by the following formula
Compatible hardware
Differential pressure transmitters need to be used with the following hardware when measuring flow:
1. Throttle device: used to produce pressure difference in the pipeline. Common throttling devices include:
• Orifice plate: A simple sheet with a small hole in the center, suitable for most fluid media.
• Venturi pipe: shrinkage-expansion pipe, low pressure loss, suitable for high-precision measurement.
• Nozzle: Suitable for high flow rate fluid, pressure loss is smaller than the orifice plate.
2. Flow calculation device: used to convert the electrical signal output by the differential pressure transmitter into a flow signal. This can be achieved by a flow integrator or PLC in an industrial control system.
3. Pipes and fittings: pipes and fittings for installing and fixing differential pressure transmitters and throttling devices.
The selection basis of throttling device
The selection of the appropriate throttling device requires consideration of the following factors:
1. Fluid characteristics: Different devices are suitable for different fluids (such as liquid, gas, steam).
2. Measurement accuracy: For high-precision measurement, venturi tube or nozzle is more suitable.
3. Pressure loss requirements: If low pressure loss is required, venturi or balance flowmeter is a better choice.
4. Cost and maintenance: hole plate cost is low, but maintenance is more frequent; Venturi tubes and nozzles are expensive but easy to maintain.
