Wedge Flow Meter: Working Principle, Applications & Selection Guide

A wedge meter places a defined wedge restriction in the pipe so that flow passes through a segment-shaped opening of height h. The beta ratio is calculated from the segment open-area relation in ISO 5167-6:2022, not from β = h/D or β = √(h/D). The standardized geometry has a wedge plane angle of 90° ±2° and upstream and downstream external angles of 135° ±2°. Its pressure taps and all dimensional details must match the standard or the meter's calibration basis. The measured differential pressure follows the same square-root relationship as other DP devices, while the discharge coefficient accounts for the contraction and three-dimensional flow field created by this geometry.

For an uncalibrated ISO 5167-6:2022 wedge meter, verify all of these limits: 50 mm ≤ D ≤ 600 mm; 0.2 ≤ h/D ≤ 0.6; 0.377 ≤ β ≤ 0.791; and 1×10⁴ ≤ ReD ≤ 9×10⁶. The discharge coefficient is C = 0.77 − 0.09β, with relative expanded uncertainty U(C) = 4% at k = 2. For gases and vapors, use the Part 6 isentropic expansibility treatment only when p2/p1 ≥ 0.75. Material and hard-facing choices are project-specific and must be checked for corrosion, erosion, pressure, temperature, fabrication, and inspection requirements.

The standardized wedge equation is empirical and applies only within the stated geometry and operating limits. At β = 0.50, C = 0.725. At the upper standardized limit β = 0.791, C ≈ 0.699; across the full β range from 0.377 to 0.791, C changes by about 0.037. Do not infer a calibrated uncertainty from this relatively flat curve. The uncalibrated relative expanded uncertainty remains 4% at k = 2. If the required uncertainty is lower, or the device will operate outside the standardized range, specify a calibration whose geometry, Reynolds-number range, fluid basis, installation configuration, uncertainty budget, and traceability cover the intended service.

A wedge may be preferable when its segment opening, erosion-resistant construction, or calibrated low-Reynolds-number performance is better suited to a viscous or solids-bearing fluid than a sharp-edged orifice. The comparison must still be calculated: the standardized wedge range begins above ReD = 1×10⁴, while the orifice limit is geometry-dependent, so neither element can be selected from a universal ReD = 5,000 rule. For service below the wedge limit, a calibration or another measurement principle is required. For slurry, flashing, or cavitating service, assess particle size and concentration, settling behavior, erosion rate, minimum static pressure, vapor pressure, noise, vibration, and maintenance access; the wedge geometry alone does not guarantee trouble-free operation.

For heavy crude, calculate viscosity and Reynolds number across the operating envelope, then determine whether the proposed wedge remains inside the standardized range or needs calibration. For abrasive slurry, document solids concentration, particle-size distribution, hardness, velocity, material compatibility, hard-facing repairability and inspection strategy. For wastewater or fibrous solids, confirm the minimum clear opening, orientation, cleaning access and bridging risk. Final performance depends on the supplied geometry, calibration basis, fluid behavior and maintenance regime.

ISO 5167-6:2022 standardizes a wedge plane angle of 90° ±2° with upstream and downstream external angles of 135° ±2°. The ISO discharge-coefficient equation and its 4% relative expanded uncertainty apply to that standardized geometry within the specified limits. A supplier may offer another geometry for a particular duty, but it must not be presented as covered by the uncalibrated ISO equation. Require a geometry drawing, calibration basis, uncertainty budget, materials, surface treatment, and inspection criteria for any non-standard design.

ISO 5167-6:2022 provides disturbance-specific straight-length requirements for the standardized wedge geometry. Use the actual beta ratio and piping arrangement rather than borrowing an orifice-plate value. Wedge orientation and pressure-tap locations must match the approved design and coefficient basis. For settling solids, entrained gas, condensate or viscous fluids, the orientation should be selected from the expected phase behavior, minimum clear passage, drainage, venting and cleaning requirements. Impulse-line routing, insulation and heat tracing are likewise project-specific; they must prevent unequal heads, trapped phases, freezing or solidification without changing the defined pressure measurement.

Provide every bidder with fluid name, phase and composition; solids type, concentration and particle-size distribution where relevant; minimum, normal and maximum flow; operating and design pressure and temperature; density and viscosity at each operating condition; measured pipe internal diameter; piping material and schedule; available straight run with each disturbance identified; allowable permanent loss; required uncertainty; wetted-material and erosion requirements; and the measurement purpose. State ISO 5167-6:2022 and whether an uncalibrated standardized design or a calibrated design is required. Ask the supplier to return h/D, β, complete wedge geometry, orientation, tap positions, ΔP and permanent loss at each flow point, C and expansibility where applicable, the uncertainty basis, standard-limit check, material certificates, calibration scope if used, and the required straight runs.

Select a wedge meter when its defined opening and available material or calibration options suit the fluid better than a concentric restriction. Confirm that all standardized limits are met; below ReD = 1×10⁴, above ReD = 9×10⁶, or outside the geometry limits, the uncalibrated ISO equation does not apply. Solids do not automatically disqualify every other technology, and custody transfer does not automatically approve a wedge: both decisions require the project uncertainty, maintenance, commercial, and regulatory basis. The wedge is a specialized option whose value comes from a documented match between geometry, fluid, installation, and calibration.

For a wedge-meter selection review, send the process conditions through our Contact page. The engineering response can cover h/D, β, differential pressure, permanent pressure loss, uncertainty basis, material protection and installation orientation as applicable to the stated service.