TDC7200_15 Datasheet, PDF (36/50 Pages) Texas Instruments – TDC7200 Time-to-Digital Converter for Time-of-Flight Applications in LIDAR,Magnetostrictive and Flow Meters

TDC7200
SNAS647C – FEBRUARY 2015 – REVISED AUGUST 2015
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The design of flow-meters requires a thorough technical assessment of the system where the device will be
used. The following is a list of areas to consider:
• Minimum and maximum flow rate at maximum allowable error in the system
• Transitional flow rate
• Instantaneous and total quantity pumped over time
• Accuracy of the meter within prescribed limits per applicable standards
• Pressure in the system
• Operating temperature range
The appropriate ultrasonic sensor and the proper electronics for interfacing to the sensor are determined
based on the system requirements. The following is a list of specifications applicable to the senor/assembly
used in the system:
• Excitation frequency
• Excitation source voltage
• Pipe diameter
• Distance between the transducers (or reflectors)
9.2.2 Detailed Design Procedure
The following subsections describe the detailed design procedure for a flow meter application.
9.2.2.1 Flow Meter Regulations and Accuracy
If the flow meter is intended for residential applications, it must be designed to meet the required standards. For
example, per the INTERNATIONAL ORGANIZATION OF LEGAL METROLOGY (OIML), the metrological
requirements of water meters are defined by the values of Q1, Q2, Q3 and Q4, which are described in Table 26.
Table 26. Flow-rate Zones per OIML
FLOW-RATE ZONE
Q1
Q2
Q3
Q4
DESCRIPTION
Lowest flow rate at which the meter is to operate within the maximum permissible errors.
Flow rate between the permanent flow rate and the minimum flow rate that divides the flow
rate range into two zones, the upper flow rate zone and the lower flow rate zone, each
characterized by its own maximum permissible errors.
Highest flow rate within the rated operating condition at which the meter is to operate within
the maximum permissible errors.
Highest flow rate at which the meter is to operate for a short period of time within the
maximum permissible errors, while maintaining its metrological performance when it is
subsequently operating within the rated operating conditions.
A water meter is designated by the numerical value of Q3 in m³/h and the ratio Q3/Q1. The value of Q3 and the
ratio of Q3/Q1 are selected from the lists provided in the OIML standards.
Water meters have to be designed and manufactured such that their errors do not exceed the maximum
permissible errors (MPE) defined in the standards. For example, in OIML standards, water meters need to be
designated as either accuracy class 1 or accuracy class 2, according to the requirements.
For class 1 water meters, the maximum permissible error in the upper flow rate zone (Q2 ≤ Q ≤ Q4) is ±1%, for
temperatures from 0.1°C to 30°C, and ±2% for temperatures greater than 30°C. The maximum permissible error
for the lower flow-rate zone (Q1 ≤ Q < Q2) is ±3%, regardless of the temperature range.
For class 2 water meters, the maximum permissible error for the upper flow rate zone (Q2 ≤ Q ≤ Q4) is ±2%, for
temperatures from 0.1°C to 30°C, and ±3% for temperatures greater than 30°C. The maximum permissible error
for the lower flow rate zone (Q1 ≤ Q < Q2) is ±5% regardless of the temperature range.
The flow meter accuracy specified in the standards dictates the required accuracy in the electronics used for
driving the ultrasonic transducers, circuits in the receiver path, and time measurement sub circuits. The stringent
accuracy required at lower flow rates would require a very low noise signal chain in the transmitter and receiver
circuits used in ultrasonic flow meters, as well as the ability to measure picosecond time intervals.
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