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deformation of the spool body . With proper calibration, the uncertainty of these various effects can be
minimized so that the individual meter is made to operate within specifications dictated by various
standards bodies worldwide.
Ultrasonic TOF Flow-Measurement Principle
A typical ultrasonic TOF water-flow meter spool body is depicted below in Figure 1.
Keywords: TDC, ultrasonic, metering, MAXREFDES70#, flow meter, heat meter, analog front end, analog
front-end, AFE, time-of-flight
APPLICATION NOTE 6104
CALIBRATION OF WATER FLOW RATE IN AN
ULTRASONIC FLOW METER USING THE
MAX35101
Abstract: The MAX35101 is a time-to-digital converter with built-in amplifier and comparator targeted as a
complete analog front-end (AFE) solution for the ultrasonic heat meter and flow meter markets. With a time
measurement accuracy of 20ps and automatic differential time-of-flight (TOF) measurement, this device
makes for simplified computation of liquid or gas flow. Because of 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 precise accuracy with averaging, flow compensation and multipoint calibration. This
application note focuses on the actual procedure for performing the calibration of the flow meter.
Introduction
The flow rate measurement that occurs in an ultrasonic water meter is critical to the accuracy of the meter.
All water meters built upon the transit-time ultrasonic principle require some kind of flow calibration. With
the deployment of a reference standard flow measurement device, the meter can be calibrated across a
specific known flow range. The calibration technique used for the set of measurements taken in this
application note to obtain the multipoint compensation curve is based upon an electromagnetic reference
flow meter that is traceable to the NIST gravimetric reference system. Calibration of each meter is unique,
and usually performed by the meter manufacturer before shipment to the end customer. In the example
within, a 22-point compensation curve is created for the meter and then the linear interpolation method is
deployed to yield actual flow rates from the data measured by the meter. The accuracy of the newly
calibrated meter is then measured by comparing the reference meter flow rate to the meter flow rate in a
system where a single flow is measured by both the meter and the reference flow meter.
Why is Calibration Needed?
It has been shown that effects of nonidentical piezoelectric transducers on reciprocity and zero-flow
calibration will always be present in a flow meter that uses the transit-time ultrasonic principle for flow
measurement . The dynamic properties of fluids including viscosity influence the distribution of the
velocity in the pipe create nonlinear behavior . Process temperature and pressure lead to mechanical
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