Air Monitor 11000AZ vEL-tROL

Differential pressure flowmeters introduce a constriction in the pipe that creates a pressure drop across the flowmeter. When the flow increases, more pressure drop is created. Impulse piping route the upstream and downstream pressures of the flowmeter to the transmitter that measures the differential pressure to determine the fluid flow.

Bernoulli's equation states that the pressure drop across the constriction is proportional to the square of the flow rate. Using this relationship, 10 percent of full scale flow produces only 1 percent of the full scale differential pressure. At 10 percent of full scale flow, the differential pressure flowmeter accuracy is dependent upon the transmitter being accurate over a 100:1 range of differential pressure. Differential pressure transmitter accuracy is typically degraded at low differential pressures in its range, so flowmeter accuracy can be similarly degraded. Therefore, this non-linear relationship can have a detrimental effect on the accuracy and turndown of differential pressure flowmeters. Remember that of interest is the accuracy of the flow measurement system - not the accuracy of the differential pressure transmitter.

Air Monitor Corporation has engineered airflow measurement systems for primary, secondary and overfired airflow combustion air measurement to assist in NOx reduction and improving plant performance. Developed microprocessor based single loop controllers with ultra-low span differential pressure and flow transmitters with automatic zeroing capability.