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A19E_EET_2_37x10_87_A19E.qxd 8/28/14 12:41 PM of time-of-flight, exceeds the specification SU R FA CE MO UNT ( and thru -h o le ) Tr a n sf or me r s & I n d u c t o r s Size does matter! from lowprofile .18"ht. • Audio Transformers • Pulse Transformers • DC-DC Converter Transformers • MultiPlex Data Bus Transformers • Power & EMI Inductors See Pico’s fu ll Cata log im me diat ely w w w. p i c o e l e c t r o n i c s . c o m PICO Electronics,Inc. 143 Sparks Ave. Pelham, N.Y. 10803 E Mail: info@picoelectronics.com Pico Representatives Germany ELBV/Electronische Bauelemente Vertrieb E-mail: info@elbv.de Phone: 0049 89 4602852 Fax: 0049 89 46205442 England Ginsbury Electronics Ltd. E-mail: rbennett@ginsbury.co.uk Phone: 0044 1634 298900 Fax: 0044 1634 290904 error bars by a significant margin. This simulation depicts the entitled flowrate accuracy without the effects of flow turbulence and mechanical design limitations in the spool body. Any additional error in the system would add to the error shown in this plot. The main sources of that additional error include the effects of flow turbulence and mechanical design limitations in the spool body. The actual flow-rate measurement accuracy is limited and can only approach that of the simulation model with sample averaging, totalizing, and multipoint flow calibration. The typical ultrasonic time-of-flight heat meter incorporating the MAX35101 is then connected to a water-flow system so that the IC can actually measure flow rate through the meter spool body. The block diagram for MAX35101 is shown in figure 3. The meter itself is composed of a MAX35101 mated to a low-power microcontroller, a battery, the spool body, and the transducers. The MAX35101 can take automatic differential time-of-flight measurements. Its Early Edge Detect feature allows the MAX35101 to measure zero-crossing data consistently between measurements. With its ability to measure the temperature of the water, the time-offlight data can compensate for the differences in the speed of sound in water at different temperatures. The MAX35101 offers an event timing mode that is configurable and runs cyclic algorithms to minimize microprocessor interactivity and increase battery life. The real-time clock (RTC) provides one programmable alarm and watchdog functionality. A simple opcode-based 4-Wire SPI interface allows any microcontroller to effectively configure the device for its intended measurement. Onboard user flash allows the MAX35101 to be nonvolatile configurable, and provides nonvolatile energy use data to be logged. Fig. 4: Complete heat meter design using the MAX35101. Fig. 5: Uncompensated flow-rate accuracy for the typical ultrasonic timeof flight heat-meter spool body. Fig. 6: Compensated flow-rate accuracy for the typical ultrasonic time-of-flight heat-meter spool body. A full system diagram of a reference Heat Meter using the MAX35101 is shown in figure 4. Raw flow-rate data can be taken with the system of Figure 4. This raw data is depicted in figure 5. The plot represents a data set that comprises multiple samples taken at fixed flow-rate intervals. Each flow-rate interval is sampled 50 times to obtain a statistical distribution of the measurement error in the spool body. Notice that this plot is focused on the low-rate region from 0 to 10 liters per minute. The error that is revealed by this plot is sourced from the effects of flow turbulence and mechanical design limitations in the spool body. The accuracy of the MAX35101 is not a contributing factor to this error. The accuracy of the plot of figure 5 can be increased by applying a typical multipoint compensation curve to the raw data. The data needed to produce the compensation curve is gathered from a highly accurate flow-rate reference. The National Institute of Standards and Technology (NIST) in the United States uses a gravimetric reference system. This is a weigh system with collection tank and a flow-diverting device. www.electronics-eetimes.com Electronic Engineering Times Europe October 2014 45


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