Wedge Flow Meter: Working Principle, Applications & Selection Guide

A wedge meter consists of a V-shaped restriction with a 60° or 90° included angle, mounted in the pipe such that flow passes through a single segment-shaped opening. The defining dimension is h — the gap height from the wedge apex to the pipe wall opposite it. The beta ratio β is defined as the equivalent diameter ratio derived from the segment open-area ratio (ISO 5167-6:2022, §3.3). For a wedge with gap height h in pipe diameter D, β² = A_open/A_pipe where A_open is the segment open area. This is fundamentally different from the orifice-plate β = d/D. As fluid approaches the wedge, it is forced into the converging gap above the apex. The streamlines separate at the apex itself — not at an unpredictable location along a rounded edge — creating a single, sharp separation point that remains geometrically fixed regardless of Reynolds number. This is the wedge's defining physical advantage. A sharp-edged orifice creates a separation bubble at the upstream edge corner; if that corner rounds through wear, the separation point moves downstream and the measured ΔP changes. A wedge apex, even if slightly worn, remains a geometric discontinuity that forces separation at the same location. The pressure taps are located upstream of the wedge face and at the apex or slightly downstream in the low-pressure wake. The measured differential pressure follows the same square-root flow relationship as any DP meter: q_m ∝ √(ΔP × ρ). The discharge coefficient converts the measured ΔP to mass flow, accounting for the three-dimensional flow contraction through the segment-shaped opening.

ISO 5167-6:2022 standardizes wedge meters with a 60° or 90° apex angle and the following key parameters. (1) Beta ratio: β is derived from the segment open-area ratio per ISO 5167-6:2022 §3.3, valid range h/D = 0.2–0.6, corresponding to β ≈ 0.377–0.791. (2) Reynolds number floor: ReD ≥ 1×10⁴. The wedge ReD floor of 1×10⁴ is higher than the orifice floor at low β (4,000 for β ≤ 0.45), but at medium-to-high β the orifice ReD requirement far exceeds the wedge (e.g. β = 0.65 requires approximately 50,000 or more for an orifice versus 10,000 for a wedge), giving the wedge its advantage at low ReD conditions. At ReD = 10000, the discharge coefficients of concentric elements become strongly ReD-dependent. The wedge's fixed separation geometry makes its Cd much less sensitive to ReD in this regime, which is precisely why it earned its own ISO part. (3) Discharge coefficient: C = 0.77 − 0.098 × β^0.8. This is the ISO 5167-6:2022 formula. Note: some older EMCO (original wedge patent holder) datasheets reference a linear C = 0.77 − 0.09β. This is not the ISO 5167-6 formula and differs by 1–2% in Cd at high β. Always use the ISO formula for new designs. (4) Uncertainty: ±4% (k=2) on Cd without calibration. This is higher than the ±0.5–0.8% of a standard orifice plate, reflecting the more complex 3D flow field and the smaller validation database. (5) Expansibility factor (ε): ISO 5167-6:2022 provides a thermodynamic isentropic expansibility equation for compressible fluids. For gas/steam service, ε is calculated using the isentropic exponent and Δp/p₁ ratio. (6) Pipe diameter range: DN 50–DN 600, though most installations are 50–300 mm. (7) Material: the wedge body and apex are typically 316L, duplex, or Hastelloy depending on corrosion and erosion requirements. The apex may be hard-faced with Stellite or tungsten carbide in abrasive service.

The wedge Cd equation is empirical, derived from calibration data across a range of β values, apex angles, and Reynolds numbers. Unlike the orifice-plate Reader-Harris/Gallagher equation — which is a multi-term physics-based correlation involving β, ReD, pipe roughness, and tap location — the wedge equation is a simple function of β only, with a single exponent (0.8) chosen from regression of the available calibration database. At β = 0.50 (h/D = 0.25), C ≈ 0.77 − 0.098 × 0.574 = 0.714. At β = 0.80 (h/D = 0.64), C ≈ 0.77 − 0.098 × 0.836 = 0.688. The variation across the usable β range is only about 0.026 — the wedge Cd is relatively flat. This flatness is what gives the wedge its ReD insensitivity: because the separation location is geometrically fixed, the Cd does not undergo the systematic shift that characterizes orifice-plate behavior in the transitional regime. The tradeoff is that the absolute uncertainty (±4% (k=2)) is higher than an orifice plate's at any given β. This is inherent: a flatter, less-sensitive Cd necessarily carries higher baseline uncertainty because the available experimental data cover a wider range of conditions with a single simplified correlation. For custody transfer, calibration is mandatory — a calibrated wedge can achieve ±0.5–0.8% uncertainty when the calibration covers the as-installed Reynolds number range, fluid, and upstream piping configuration. Process control and internal accounting applications can use the uncalibrated ISO Cd with the standard uncertainty.

The wedge meter's advantages over an orifice plate are concentrated in five application domains where the orifice plate is either non-compliant, unreliable, or uneconomical. (1) Low Reynolds number: if ReD at minimum flow falls below 5,000, ISO 5167-2:2022 declares the orifice-plate Cd equation invalid. The wedge remains valid down to ReD = 1×10⁴ — lower than the orifice-plate floor for many β ratios. For a heavy crude at 15°API flowing at 0.5 m/s in DN 150, ReD is approximately 2,000 — non-compliant for any orifice β above 0.50, and below the wedge floor as well; a calibrated wedge would be required for such extreme low-ReD service. (2) Solids-laden flow: a sharp-edged orifice concentrates the flow into a single high-velocity jet that accelerates abrasive particles, causing edge rounding and progressive under-reading. The wedge's segment-shaped opening distributes the flow over a larger cross-sectional area with lower peak velocity, and the apex — even if worn — remains a geometric discontinuity that forces separation. (3) Slurries and non-settling solids: the single large opening passes solids without the plugging risk of a multi-hole balanced plate or the dead zones of a nozzle throat. The wedge can be mounted with the opening at the top for settling solids (they pass below the measurement zone), at the bottom for gas with condensate (self-draining), or at the side for horizontal liquid flows. (4) High-viscosity fluids: polymer melts, heavy fuel oil, bitumen, and molasses can have viscosities exceeding 1,000 cP. At these viscosities, ReD is below 10,000 even in large pipes at moderate velocity, and only a calibrated or empirically validated device can operate. The wedge's fixed separation means its Cd behavior is more predictable in this regime than any concentric element. (5) Flashing or cavitating service: the wedge's gradual contraction minimizes the static-pressure depression compared to a sharp-edged orifice, reducing the risk of cavitation inception at the measurement plane.

Case 1 — Heavy crude oil, DN 200, 12°API, 120°C, viscosity 350 cP, 15–60 m³/h. ReD at minimum flow: approximately 1.2×10⁴. An orifice plate at β = 0.60 would be non-compliant (ReD < 20,000 for β = 0.60 per ISO 5167-2). A wedge with h/D = 0.30 (β = 0.55) produced ΔP = 18 kPa at normal flow with permanent loss of approximately 14 kPa. The customer installed the wedge with the opening at the top to allow sand and formation fines to pass below the measurement zone. After 18 months of continuous service, the apex showed minor polishing but no dimensional change exceeding 0.1 mm. The calibrated Cd remained within 0.3% of the initial value. Case 2 — Lime slurry, DN 100, 15 wt% solids, 25°C, 20–80 m³/h. Previous metering attempts: a magnetic flow meter failed after 4 months when lime scale coated the electrodes. An orifice plate lasted 6 weeks before the bore diameter increased by 1.2 mm due to abrasion. A wedge meter with 316L body and Stellite 6 hard-faced apex was installed with the opening at the side. After 12 months of service, the apex showed no measurable wear and the flow signal remained stable within 1% of the calibration. The key design decision was the hard-facing: the base 316L wedge would have worn at approximately 0.3 mm/year in this service; Stellite reduced the wear rate to below 0.02 mm/year. Case 3 — Wastewater sludge, DN 150, 3–5% solids, 30–120 m³/h. The sludge contained fibrous material (rags, wipes) that wrapped around the bluff body of a vortex meter within days. A wedge meter with a 90° apex angle (wider opening than 60°) and the opening at the top passed the fibrous material without accumulation. The wider apex angle was chosen to maximize the gap for solids passage at the cost of slightly lower differential pressure — an acceptable tradeoff for this non-custody application.

The wedge apex angle affects the Cd, the generated ΔP for a given h/D, and the susceptibility to solids accumulation. A 60° wedge produces a sharper contraction, higher ΔP for the same flow and h/D, and a Cd that is approximately 2–3% lower than a 90° wedge at the same h/D. The sharper apex also creates a more defined separation point, which improves Cd stability at very low ReD. A 90° wedge has a wider opening angle, lower ΔP, and a Cd closer to 0.77–0.80. It is preferred when solids passage is the dominant concern — the wider angle reduces the risk of fibrous material bridging across the apex. ISO 5167-6:2022 provides Cd equations for both angles; verify which angle applies to your supplier's quoted Cd before comparing bids. Some manufacturers offer a 60° apex as standard for liquids and a 90° for gas and steam applications where lower permanent loss is desired, but this is a manufacturer convention, not an ISO requirement. The apex surface finish and hardness specification should be separate from the angle selection: for abrasive service, specify a hard-facing material (Stellite, tungsten carbide, or ceramic coating) applied to the apex only, not the entire wedge body, to control cost.

ISO 5167-6:2022 specifies straight-run requirements that are generally comparable to orifice plates at equivalent β, with two important differences. First, the wedge's asymmetric flow field makes it more sensitive to swirl than a concentric element — a single elbow in the plane perpendicular to the wedge opening creates a swirling component that the wedge geometry does not cancel. The standard specifies longer straight runs for disturbances that generate swirl (two elbows in perpendicular planes, upstream control valves) than for disturbances that only distort the axial velocity profile (a single elbow, a reducer). Second, the wedge opening orientation matters. For liquids with settling solids, mount the wedge with the opening at the top. For gases with condensate, mount with the opening at the bottom. For clean liquids, the opening can be at the side or top. The opening must not face downward in liquid service — solids will accumulate on the upstream face of the wedge, effectively reducing h and biasing the measurement high. Pressure taps are typically located at the pipe centerline on both sides, with the downstream tap positioned at or slightly downstream of the apex. The tap location is design-specific and must match the Cd correlation — do not relocate taps without re-calibration. Impulse lines should slope downward toward the transmitter in liquid service, and insulation or heat tracing may be required for high-viscosity fluids that would solidify or gel in the impulse lines at ambient temperature.

Provide the following to every bidder. Fluid: name, phase, composition (for oil: °API gravity, sulfur content, sand/solids content, wax appearance temperature; for slurry: solids type, particle size distribution, concentration wt%). Flow rates: minimum, normal, maximum in mass or volume units. Operating pressure and temperature: at the meter location, including minimum and maximum conditions. Density: at operating conditions (kg/m³). Viscosity: at minimum, normal, and maximum operating temperatures (cP or mPa·s) — this is critical because wedge meters are often selected for high-viscosity service. Pipe internal diameter: measured (not nominal), in mm. Pipe material and schedule. Allowable permanent pressure loss: kPa at maximum flow. Available straight run: upstream and downstream, in mm, with fitting description. Apex angle preference: 60° (higher ΔP, better low-ReD stability) or 90° (lower ΔP, better solids passage). Opening orientation: top (settling solids), bottom (gas/condensate), or side (clean liquid). Wetted materials: body and apex material; hard-facing requirement for abrasive service. Hazardous-area classification: if applicable, for the transmitter only — the wedge element is passive. Applicable standard: ISO 5167-6:2022 with or without calibration. Measurement purpose: custody transfer (calibration mandatory), allocation, or process control. Require the supplier to return: h/D ratio, β, apex angle, generated ΔP at each flow point, permanent pressure loss at each flow point, C value and uncertainty basis (ISO 5167-6 uncalibrated or calibration certificate range), apex material and hardness, and recommended straight-run lengths with the uncertainty penalty for shorter runs.

Select a wedge meter when the Reynolds number, the fluid, or the solids content makes every other ISO 5167 element non-compliant or short-lived — and accept the tradeoff of higher baseline uncertainty in exchange for operability. The decision chain is three questions: (1) Is ReD at minimum flow below 5,000? If yes, the orifice plate is out. (2) Does the fluid contain solids, fibers, or abrasive particles? If yes, the sharp-edged elements are out. (3) Is the measurement purpose custody transfer? If yes, budget for calibration. If you answer yes to questions 1 or 2, the wedge is likely your only ISO-standardized option. If you answer yes to question 3, specify the calibration envelope to match your operating range. For all other clean, moderate-to-high-ReD, non-solids applications — the orifice plate, nozzle, Venturi, or cone meter will deliver lower uncertainty at lower cost. The wedge meter is not a universal solution. It is the right solution for a specific and common class of difficult fluids that exist throughout the oil and gas, mining, pulp and paper, and wastewater industries. When you encounter one of those fluids, ISO 5167-6:2022 gives you a standardized path to a working meter with a defined uncertainty — and that is worth far more than a low purchase price.

Need a wedge meter sizing for your application? Send your process conditions through our Contact page. We will return h/D, β, ΔP, permanent loss, and uncertainty at no cost — including hard-facing and opening-orientation recommendations for abrasive or solids-laden service.