This incorporated a contra-propagating pulse phase method with time of flight tracking similar to a method used by the same author in a previous study for an ultrasonic tank level meter. In a study by Olmos an ultrasonic airflow measurement device for measuring airflow and temperature within solar chimneys was developed. This is because of the linear response to flow velocity change that these devices have, so their sensitivity does not degrade with low airflow velocity as opposed to what happens with pressure differential airflow measurement devices. The measurement of low airflow velocities in natural ventilation systems has created an interest in developing an ultrasonic airflow velocity measurement device. , which described a solution for measuring HVAC air velocity by using a contra-propagating pulsed phase method. In 1994 a patent was filed by Strauss et al. In 2002 a similar but more developed device was described by Rabalais and Sims but was only available on a case-by-case basis as they were still classed as experimental devices. A reflective path was used and it was suggested that optional sensors could be combined with this unit such as pressure, temperature and relative humidity. (Waltham, MA, USA) at a conference in 1994 which used a single port solution consisting of two transducers mounted using O-rings to reduce crosstalk on a single circular flange. There are commercially available systems for use in monitoring industrial processes such as exhaust gases and automotive test bed air intakes but the historically high cost has restricted their application uses in HVAC systems.Ī duct airflow measurement device was proposed by Bragg and Lynnworth from Panametrics Inc. Ultrasonic flow sensors are commonly used to measure pipeline liquid flow in industrial applications but they are not as common for gas flow. One of the possible alternatives, listed on pages 36.15 and 36.21 of, to using pressure differential type devices such as Pitot tube/arrays, orifice plates and Venturis is to use an ultrasonic flow rate sensor which, according to Lynnworth and Liu, have been around for at least 60 years. The monitoring of airflow rates can indicate problems in the design, installation and operation of a HVAC system. This will necessitate the need for monitoring of the operating performance of these systems so that air quality or building energy efficiency is not detrimentally affected. As buildings evolve, they will require higher levels of insulation and air tightness which will require ventilation systems to provide the minimum number of air changes and reduced energy usage by recovering heat from the air before it is expelled.
they are fragile, expensive and create an additional pressure loss, and their sensitivity is also reduced with the reduction in airflow velocity. A typical duct airflow monitoring device uses a pressure differential method to determine the airflow. The device is also designed so that it does not have to rely on high-speed analogue to digital converters (ADC) and intensive digital signal processing, so could be implemented using voltage comparators and low-cost microcontrollers.Ĭurrently heating, ventilation and air conditioning (HVAC) systems are difficult and costly to monitor for energy efficiency performance and reliability. The design choices that were found to work well are the single sided transducer arrangement for a “V” shaped reflective path and the use of square wave transmitter pulses ending with the necessary 180° phase changed pulse train to suppress transducer ringing. The design uses a number of techniques and design choices to provide solutions to lower the implementation cost of the device compared to traditional airflow meters. This proof of concept device has provided evidence that with further development it could be a low-cost alternative to pressure differential devices such as the orifice plate airflow meter for monitoring energy efficiency performance and reliability of ventilation systems. The airflow measurement results for a small range of airflow velocities and temperatures show that the accuracy was better than 3.5% root mean square (RMS) when it was tested within a round or square duct compared to the in-line Venturi tube airflow meter used for reference. In this study, an in-duct ultrasonic airflow measurement device has been designed, developed and tested.
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