ISO 7278-3:1998

Overview

ISO 7278-3:1998 - Liquid hydrocarbons - Dynamic measurement - Proving systems for volumetric meters - Part 3: Pulse interpolation techniques - provides guidance on using pulse interpolation to increase the effective resolution of volumetric meters (turbine and displacement meters) when proving with pipe or small-volume provers. The standard explains three common interpolation methods (double‑timing, quadruple‑timing, and phase‑locked‑loop), defines performance conditions, and sets out tests and meter requirements to achieve reliable proving for liquid hydrocarbons.

Key topics and technical requirements

• Pulse interpolation methods
  • Double‑timing: measures two time intervals (T1, T2) and computes fractional pulses; requires long test periods for high resolution.
  • Quadruple‑timing: uses four timing intervals (t1…t4) for improved interpolation accuracy.
  • Phase‑locked‑loop (PLL): multiplies meter pulses with a VCO and divider R; interpolated count n′ = n*/R.
• Resolution and rounding
  • Indication device resolution must be better than 1 in 10 000.
  • Interpolated results are limited to five significant digits.
  • For PLL, the multiplied count n* should be ≥ 10 000 for ±0.01% resolution.
• Flow and stability
  • Pulse interpolation assumes stable pulse frequency; flow fluctuations during a proving pass should be within ±2% of mean flowrate.
  • Intra‑rotational linearity (regularity of pulse spacing) impacts accuracy and may necessitate collecting more pulses.
• Instrumentation and signal integrity
  • Detector switching edge must be well defined and repeatable.
  • Clocks used for timing require stability consistent with required resolution.
  • Systems must demonstrate immunity to electrical interference; signal‑to‑noise must be adequate.
  • PLL divisors should be preset and protected (sealed).
• Testing and verification
  • Clause 7 provides test procedures, circuits and schedules (including immunity tests).
  • Annex A covers measurement techniques for determining pulse intervals.

Applications and users

ISO 7278-3 is intended for:

• Petroleum and fuel industry metrology teams performing meter proving.
• Manufacturers of turbine and displacement meters and pulse interpolation hardware.
• Operators of pipe provers and small-volume provers seeking improved discrimination when pulse counts are limited.
• Calibration laboratories and quality assurance groups ensuring proving uncertainty meets industry requirements.

Practical benefits include increased effective meter resolution during proving runs, improved accuracy of custody transfer measurements, and clear test/acceptance criteria for pulse‑based proving systems.

Related standards

• ISO 7278 (Parts 1, 2, 4, 5) - General principles, pipe provers, operator guides, small-volume provers.
• ISO 6551:1982 - Cabled transmission of pulse data (normative reference).
• Annex B (bibliography) in ISO 7278-3 lists additional references for further reading.