The accurate and stable control of airflow or static
pressure is only possible when Specifying Engineers understand the importance of accuracy,
application and product selection when specifying each of the control devices to be
utilized for a specific control loop. Airflow measurement and control is a three step
process, requiring the selection and application of:
The focus of this section is on the need for quality airflow transducers in order to help assure that accurate control is achieved. |
Transducer Reference Accuracy
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The expected overall accuracy of the control loop can be determined utilizing the standard "square root of the sum of the squares" equation. When applying this equation for airflow control it is important to note that the stated accuracy for the differential pressure transducer is a value based on its' full scale output range. Therefore, due to the square root conversion required to make this signal linear to flow, even greater error will occur as the airflow system turns down. Add to this the issue of "signal to noise error" and the resulting control system performance is typically unacceptable. |
Linearization Reference Accuracy
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The importance of quality transducer performance specifications for successful airflow control is commonly overlooked by Specifying Engineers. The following graphs are intended to illustrate the importance of accuracy, span selection and signal conditioning when specifying airflow transducers, and hopefully, encourage the engineering community to utilize performance specifications which will assure airflow measurement accuracy. The equation used for calculating velocity and velocity pressure for each of the various graphs shown is: |
Control Reference Accuracy
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"Reference Accuracy" meaning
zero |
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Graphic illustration of why a DDC system performance is greatly enhanced when utilizing an input from a quality airflow transducer. This groph was created using data logged output values from a difterential pressure transducer, the mathematical conversion of the differential pressure transducer output to airflow as interpreted by a DDC system, as well as the accurate and stable output of PO's airflow transducer. |
Transducer Error |
Transducer Error |
Linearization Error |
Linearization Error |
Control Error |
Control Error |
Expected
accuracy from a properly |
Expected
accuracy from a typically |
| The scenario utilized for the above two (2) sets of graphs is for a variable air volume control system in which the maximum design air velocity through the airflow measuring station is 2,000 feet per minute. The first set of graphs show excellent control accuracy when utilizing a transducer meeting the accuracy, span and signal con-ditioning performance specification required for airflow control. The second set of graphs exemplifies the cause | of many control system failures due to poor performance criteria being specified for the airflow transducer. The control contractor is only responsible to select and furnish a transducer which meets the specification. Therefore, the Specifying Engineer must issue a transducer performance specification that will meet the operational expectations of the airflow control system. |
