An Annubar is a differential-pressure flow element - a type of averaging pitot tube - that measures flow across a pipe with a single insertion probe and very little pressure loss. It offers a low-cost, easy-to-install alternative to an orifice plate for gas, liquid, and steam. This guide explains how an Annubar works, how it differs from an orifice, and its oil and gas fit.
Annubar in one line: An Annubar is an averaging pitot tube: an insertion probe spanning the pipe with several ports that sense flow across the full diameter. It measures the difference between the impact (high) pressure of the flow and the low pressure behind the probe, and that differential pressure relates to flow rate by the same square-root law as other DP meters.
An Annubar is a slender bar inserted straight across the pipe. Facing the flow, it has several sensing ports along its length that catch the impact (stagnation) pressure at multiple points across the diameter and average them - which is why it is called an averaging pitot tube. A second set of ports on the downstream side senses the lower pressure in the wake behind the bar. The difference between these two pressures is the differential pressure, and like any DP flow device it follows a square-root relationship: flow rate is proportional to the square root of the differential pressure.
That differential is piped to a differential-pressure transmitter, which converts it to a flow signal. Averaging across several points makes the reading more representative of the true flow profile than a single-point pitot tube. Because the probe only spans the pipe rather than restricting it, an Annubar creates a very small permanent pressure loss - far less than an orifice plate that squeezes the whole flow through a hole.
Compared with an orifice plate, an Annubar's main advantages are much lower permanent pressure loss (saving pumping and compression energy), low installed cost, and easy insertion into an existing line - some hot-tap types install without shutting the line down. The trade-offs are lower rangeability and generally lower accuracy than a well-installed orifice, sensitivity to a poor flow profile so it needs adequate straight run, and susceptibility to plugging of the small sensing ports on dirty service.
In oil and gas and utilities, Annubars measure gas, liquid, and steam where a low pressure drop and simple retrofit matter more than the tightest custody accuracy - plant and instrument air, fuel and flare gas, cooling water, and steam headers. The DP transmitter outputs the flow signal as 4-20 mA or a digital protocol, usually into a flow computer or PLC that applies the square-root extraction and any pressure and temperature compensation. A cloud SCADA such as Merobix reads the resulting flow tags from that controller over Modbus or DNP3 to trend usage and alarm on abnormal flows.
Both are differential-pressure meters following a square-root law, but an orifice plate restricts the whole flow through a hole, creating a large permanent pressure loss, while an Annubar is a thin insertion probe that barely obstructs the pipe, giving very low pressure loss. The Annubar is cheaper and easier to retrofit; the orifice generally offers better accuracy and rangeability.
A simple pitot tube senses flow at one point, which misreads if the flow profile is uneven. An Annubar has several sensing ports along a bar that spans the whole pipe diameter, so it captures and averages the impact pressure at multiple points across the profile. That averaging makes its reading more representative of the true flow than a single-point pitot.
Its small sensing ports can plug on dirty, wet, or particulate-laden fluids, degrading the reading, so it suits cleaner services. It also offers narrower turndown and typically lower accuracy than a well-installed orifice, and it needs adequate straight pipe run because a distorted flow profile affects it. Its low pressure drop and easy installation are the offsetting advantages.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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