# Understanding Beta Ratio in Orifice Plates > An engineering guide to selecting orifice beta ratio for signal strength, pressure loss, uncertainty, Reynolds number and operating range. - Category: Technical Fundamentals - Published: 2026-06-28 - Reading time: 10 min - Canonical article: https://www.shizhongflow.com/blog/understanding-beta-ratio-in-dp-flow-measurement For a concentric orifice plate, beta ratio is the orifice bore diameter divided by the upstream pipe internal diameter at operating conditions. This simple ratio drives differential pressure, permanent loss, Reynolds-number limits, installation sensitivity and the practical measurement range. ## Definition and dimensional discipline Beta is written β = d/D, where d is the orifice bore and D is the upstream pipe internal diameter as defined by ISO 5167-2:2022. Both dimensions must use a consistent reference condition. Thermal expansion can matter in high-temperature service, so calculations may need bore and pipe dimensions corrected from inspection temperature to operating temperature. Do not calculate beta from nominal pipe size. Actual internal diameter depends on schedule, manufacturing tolerance, lining, corrosion allowance and measurement location. A small error in d or D propagates strongly because the flow equation contains the opening area and the velocity-of-approach factor. Procurement should define measurement method, tolerance and as-built dimensional reporting. ## Effect on differential pressure A lower beta means a smaller opening and higher velocity through the bore. For the same flow, it creates a larger differential-pressure signal. This can improve low-flow resolution and move the transmitter away from zero-related uncertainty. The cost is greater permanent pressure loss, higher local velocity and potentially more noise, erosion or cavitation risk. A higher beta creates less differential pressure and generally lower permanent loss, but the signal at minimum flow may become too small for the required uncertainty. High beta also tends to increase sensitivity to upstream velocity-profile distortion. Selection is therefore an optimization, not a search for the maximum or minimum allowed value. ## Reynolds number and validity The discharge coefficient depends on pipe Reynolds number, beta ratio and tapping geometry. ISO 5167-2:2022 defines validity limits for use of its correlation. Check Reynolds number at minimum flow, because that is where a proposed design most often falls outside the standard range. Fluid viscosity at actual temperature must be used. Operating outside a standardized limit does not make flow physically immeasurable, but it means the standard coefficient and stated uncertainty cannot simply be claimed. Options include changing bore or pipe size, improving the process range definition, using a device suited to lower Reynolds number, or performing representative flow calibration. ## Uncertainty and installation sensitivity Beta affects the uncertainty contributions from bore and pipe-diameter measurement and the discharge-coefficient correlation. As beta increases, the velocity-of-approach factor rises and dimensional effects become more pronounced. Installation requirements also generally become more demanding because a large opening exerts less conditioning action on a disturbed profile. A sizing program should report component uncertainties rather than only a final accuracy label. Review the effect of measured d, measured D, transmitter span, density, expansibility for gases, and discharge coefficient. If the installation needs a high beta to reduce loss, the engineer may need better dimensional inspection or more upstream pipe to preserve uncertainty. ## Pressure loss and energy Lower-beta plates normally impose greater permanent loss at a given flow. Convert the predicted loss at each operating point into annual pumping or compression cost. An apparently inexpensive plate can consume many times its purchase price in energy when installed in a large, continuously operated line. Increasing beta reduces loss but also reduces the differential-pressure signal approximately according to the governing flow equation. Do not optimize energy by making the bore so large that minimum-flow uncertainty becomes unacceptable. If both low loss and strong rangeability are essential, compare a Venturi, nozzle, cone, multiple ranges or another suitable technology. ## Worked selection logic Suppose candidate calculations at normal flow produce 100 kPa, 40 kPa and 15 kPa for three increasing beta ratios. At one-tenth normal flow, the approximate differentials become 1 kPa, 0.4 kPa and 0.15 kPa before property changes. The largest beta may save energy but place minimum flow near the practical noise and zero limit of the measurement system. Now check maximum flow: differential pressure rises with flow squared and may exceed the transmitter span or allowable process loss. The selected beta must keep maximum differential and velocity within limits, minimum differential measurable, and all points inside ISO 5167-2:2022 Reynolds and geometry limits. This multi-point check is more reliable than sizing only at normal flow. ## What to require from the supplier Request calculated beta using the measured pipe bore, plate bore at reference and operating temperatures, Reynolds number, discharge coefficient, expansibility where applicable, differential pressure and permanent loss at minimum, normal and maximum flow. The report should identify tapping type, pipe roughness assumption, straight-run basis and the exact standard edition. Also require plate flatness, thickness, edge, surface finish and concentricity inspection appropriate to ISO 5167-2:2022. The final beta choice should be documented with its tradeoffs: signal margin, energy use, standard validity and installation sensitivity. That record helps commissioning teams distinguish a design decision from an arbitrary bore size. ## Engineering support For project-specific selection, send the fluid, minimum/normal/maximum flow, pressure, temperature, pipe size, viscosity, allowable pressure loss and available straight run through https://www.shizhongflow.com/contact.