Learn all about Differential Pressure Flow Meters: what they are, how they work, the types of primary elements like orifice plates and venturis, integrated meters versus built systems, advantages and disadvantages of this technology, accuracy, gas and steam capability, how they compare to other flow meter technologies, installation considerations, pricing, available outputs such as switches and transmitters, and more.
What Is a Differential Pressure Flow Meter?
A differential pressure flow meter (often shortened to “DP flow meter”) measures how fast a fluid is moving through a pipe by measuring a pressure difference instead of measuring the flow directly. The meter places a precisely engineered restriction in the flow path, measures the pressure before and after that restriction, and calculates the flow rate from the difference between the two readings. It is one of the oldest, most common, and most versatile flow measurement methods in industry, and it works with liquids, gases, and steam.
A complete DP flow measurement system has up to three parts: a primary element (the engineered restriction), a secondary element (the differential pressure transmitter or sensor that measures the pressure difference), and a tertiary element (a display or flow computer that turns those pressure readings into flow data you can use). Some DP flow meters combine everything into a single instrument, while others are built from separate components, an important distinction we cover below. You can browse KOBOLD’s differential pressure flow meter offerings to see both styles.
How Do Differential Pressure Flow Meters Work?
If you have ever put your thumb over the end of a garden hose, you already understand the physics. When you squeeze the opening smaller, the same amount of water has to get through a smaller space, so it speeds up. Something less obvious happens at the same time: the pressure of the water drops as its speed increases. Speed and pressure trade off against each other. That trade-off is Bernoulli’s Principle, and it is the entire basis of differential pressure flow measurement.
A DP flow meter puts a controlled “thumb over the hose” inside the pipe. As the fluid accelerates through the restriction, its pressure drops, and the size of that pressure drop is directly related to how fast the fluid is flowing. Specifically, the pressure drop is proportional to the square of the flow velocity. By measuring the pressure on each side of the restriction, the flow rate can be calculated using a well-established relationship: flow rate is proportional to the square root of the differential pressure, often written as Q ∝ √(ΔP). Modern transmitters like the KOBOLD PAD perform this math internally and output a direct flow signal.
What Is a Primary Element in Flow Measurement?
If you spend any time researching DP flow measurement, you will run into the term “primary element”. The primary element is the physical component in the pipe that creates the pressure drop: the orifice plate, venturi, pitot tube, nozzle, or other engineered restriction. It does not measure anything by itself; it simply creates the measurable condition. The measuring is done by the secondary element, the differential pressure transmitter, which senses the pressure on both sides of the primary element and converts the difference into an electrical signal. A tertiary element: a display, flow computer, or control system turns that signal into flow readings, totals, and alarms. Keeping these three roles straight makes every product page and datasheet in this technology far easier to read.
What Are the Types of Differential Pressure Flow Meters?
DP flow meters are usually categorized by the type of primary element that creates the pressure drop. Each element makes the same basic physics work in a different package, with different strengths:
- Orifice plates — A plate installed in the line with a precisely machined opening. Orifice plates are the most common and cost-effective primary element: they produce highly repeatable pressure drops, install easily, and come in a range of materials and opening styles (concentric, eccentric, segmental, quarter circle, conical entrance, and bi-directional) to suit everything from clean liquids to slurries and fluids with entrained gas. KOBOLD’s KPL orifice plates cover line sizes from 2″ to 24″. To learn more about orifice plates, see KOBOLD’s article “All About Orifice Plates and Orifice Flow Meters”.
- Venturi elements — Instead of an abrupt plate, a venturi gradually narrows the flow, passes it through a short throat, and gradually widens it again. That smooth transition means excellent accuracy with only a low permanent pressure loss, which makes venturis ideal where energy efficiency matters. KOBOLD’s RCD uses a venturi element in a compact integrated meter for liquids and gases in ½″ to 3″ lines.
- Pitot tubes — A probe-style sensor that measures the difference between the total and static pressure, yielding the dynamic pressure, which is used to calculate the flowrate. Pitot tubes are used primarily for gas and air flow and shine in large ducts where a full-bore meter would be impractical. KOBOLD offers the ANU pitot tube for these applications.
- Flow nozzles — A hybrid with orifice plate and venturi characteristics, flow nozzles offer excellent accuracy in high-velocity applications and are particularly common in steam service and power generation.
Other engineered restrictions exist on the market for specialized applications such as V-cone and wedge elements, for example — but orifice plates, venturis, pitot tubes, and flow nozzles cover the vast majority of DP flow measurement in industry.
Integrated DP Flow Meters vs. a DP Transmitter with a Primary Element: What’s the Difference?
Here is where DP flow measurement offers a choice most other flow technologies don’t: you can buy the whole system in one integrated instrument, or you can build it from components.
An integrated DP flow meter, like the KOBOLD RCM or RCD, combines the primary element and the readout or transmitter in a single instrument. It arrives as a pre-calibrated unit which eliminates compatibility guesswork and installation errors. If you want a flow reading on the pipe with minimum engineering effort, an integrated meter is the fast path.
A component-built system pairs a separate primary element (like the KPL orifice plate) with a differential pressure transmitter (like the PAD). The payoff is flexibility: each component can be selected specifically for your application rather than accepting a fixed combination, either component can be serviced or replaced independently without disturbing the other, the primary element can be inspected and cleaned without touching any electronics, and you can upgrade one side of the system as technology advances without replacing everything. Component systems also reach where integrated meters can’t: larger line sizes, higher pressures, and higher temperatures.
Can You Build a Flow Meter with a Differential Pressure Transmitter and an Orifice Plate?
Yes, and this is the possibility many people miss. Differential pressure transmitters are famous as all-purpose workhorses for pressure and tank level measurement, and plenty of plants already have them on site. What is less widely appreciated is that the same instrument becomes a complete, high-capability flow meter the moment you pair it with an orifice plate.
The recipe is simple. The orifice plate is installed in the line and creates the pressure drop. Pressure taps on each side of the plate, often built into pre-tapped orifice flanges, route those two pressures to the transmitter. The transmitter measures the difference, applies the square-root calculation that converts pressure into flow, and outputs a standard industrial signal. In KOBOLD terms, that is a KPL orifice plate paired with a PAD differential pressure transmitter. The PAD amplifies the small pressure signals, linearizes the square-root relationship internally, and outputs a 4-20 mA signal for the flow.
The capability ceiling of this combination is remarkable. The KPL covers line sizes from 2″ all the way to 24″, in stainless steel, carbon steel, and other materials.
On the measurement side, the PAD delivers ±0.075% of calibrated span accuracy. Add a 3-way or 5-way manifold valve and you can isolate, calibrate, and service the transmitter without shutting down the process.
For large lines, extreme conditions, or plants that want a flow solution built on an instrument type their technicians already know, a DP transmitter plus an orifice plate is one of the most capable and economical flow measurement systems available. To see just how versatile the transmitter side of this system is across flow, level, and pressure duties, visit our article on differential pressure transmitters for flow, level, and pressure.
What Are the Advantages and Disadvantages of DP Flow Meters?
DP flow meters have held their place at the center of industrial flow measurement for good reasons:
- No moving parts — Nothing in the flow stream rotates, oscillates, or wears out. That translates to exceptional reliability, minimal maintenance, and a long service life with proper installation, a real advantage in remote or hazardous locations.
- Versatility — One technology measures liquids, gases, steam, and even corrosive chemicals with appropriate material selection.
- Extreme condition capability — DP systems can handle process conditions that most other electronic flow meters cannot.
- Cost-effectiveness — Especially in larger pipe sizes, DP measurement frequently offers lower costs than other technologies.
- Proven, predictable technology — The measurement principle is grounded in fundamental fluid dynamics with an enormous base of field experience, so performance is well understood and predictable.
Like every flow technology, DP measurement also has trade-offs to keep in mind:
- Permanent pressure loss — The restriction that makes the measurement possible also consumes some pressure that the fluid never gets back, unlike non-intrusive technologies.
- Straight pipe requirements — DP meters need stable, fully developed flow to read accurately, which means minimum straight runs of pipe before and after the meter (more on this in the installation section).
- Fluid property sensitivity — Density changes shift the reading, which matters most for gases and steam, and high viscosity can push the flow out of the range where the calibration holds. Compensation or a different technology may be needed in those cases.
Can Differential Pressure Flow Meters Measure Gas and Steam?
Yes. Gas and steam are regular applications for DP flow measurement, and it is often the technology of choice for them, particularly in large pipe sizes where it is highly cost-effective.
There is one important obstacle to understand: unlike liquids, gases and steam change density significantly as temperature and pressure change, and density directly affects the flow calculation. A DP flow meter must be configured for your specific operating conditions, and if those conditions vary in service, the reading needs real-time compensation like external temperature and pressure sensors that determine the actual fluid density and correct the measurement. If your process operates with variable density, KOBOLD’s DVE and DVH multivariable vortex flow meters build that compensation in, with optional integrated temperature, pressure, and density readings.
How Accurate Are Differential Pressure Flow Meters?
Accuracy in a DP system depends on the quality of the primary element, the transmitter, and the installation. KOBOLD’s integrated RCM and RCD meters deliver ±3% of full scale, which is well suited to industrial monitoring duties like cooling water, chillers, and lube oil systems. The accuracy of a component-built system, such as the PAD transmitter and the KPL Orifice Plate, depends on three things: the precision of the plate, the quality of the installation, and the transmitter reading it. Each KOBOLD KPL orifice plate will have unique accuracy specifications as they are custom manufactured for your specific pipe diameter and flow conditions, with sizing calculations and flow coefficient determination included. Paired with the PAD transmitter at ±0.075% of calibrated span, a properly installed orifice system according to the DIN EN ISO 5167-2 standard delivers precision suitable for demanding process control. The total combined accuracy is generally in the 1.5-5% range but can deviate significantly if process conditions differ from the calibrated conditions.
How Do DP Flow Meters Compare to Other Flow Meter Technologies?
Versus magnetic flow meters: magnetic meters only work with conductive liquids. DP meters have no conductivity requirement at all and measure gases and steam, which magnetic technology cannot handle. Magnetic meters, on the other hand, impose no pressure loss and no restriction in the line.
Versus ultrasonic flow meters: both technologies share the no-moving-parts advantage. Ultrasonic meters avoid pressure loss entirely and clamp-on models install without cutting pipe, but DP systems handle extreme temperatures and pressures that ultrasonic sensors cannot, and they are capable of measuring steam.
Versus mechanical meters (turbine, positive displacement, variable area): mechanical meters have parts in the flow stream that wear over time, while a DP system’s stationary design eliminates that failure mode. The place mechanical technology wins decisively is high viscosity as thick fluids undermine DP accuracy, and positive displacement meters like the DON and OVZ are purpose-built for exactly those fluids.
If your application involves large lines, high temperatures, high pressures, or steam, DP flow measurement is a great choice. If your priority is zero pressure loss or if the media is very viscous, another technology may serve you better.
How Do You Install a Differential Pressure Flow Meter?
Installation quality makes or breaks DP measurement accuracy. The theme running through every guideline below is the same: the meter needs to see smooth, stable, fully developed flow, and the pressure signals need a clean path from the pipe to the transmitter.
- Always start with the manual for your exact model as requirements vary by meter and primary element type.
- Respect straight-run requirements. The RCM requires at least 10 pipe diameters of straight run upstream, with valves recommended downstream of the meter. The KPL orifice plate requirements follow DIN EN ISO 5167-2. The RCD requires between 15 to 50 pipe diameters upstream depending on the installation and 5 pipe diameters downstream. The pitot tube (ANU) requires between 8 to 30 pipe diameters upstream depending on the installation and 4 pipe diameters downstream. If you don’t have the space, flow conditioning plates (ZUB-FCZ) or flow straighteners (ZUB-FSB) can substitute for pipe length. Our article on straight run requirements for flow meters explains why this matters.
- Orient the primary element correctly as flow direction matters. Ensure proper gasket sealing at the flanges.
- Design impulse lines with care: ¼″ to ½″ tubing, kept as short as practical, sloped up for gas service and down for liquid service so bubbles and condensate migrate away from the transmitter rather than into it, and made of material compatible with the process fluid.
- Install a manifold valve (3-way for block and equalize, 5-way to add venting) so the transmitter can be isolated, zero-checked, and serviced without a process shutdown.
- Mount the transmitter close to the taps and where it can actually be reached for calibration and maintenance. For liquid service, mount the transmitter below the tapping points.
- Commission properly: leak test, perform a zero calibration, and a span verification before trusting the readings.
One of the perks of integrated meters: the RCM and RCD need no impulse lines at all, and the RCM installs in either horizontal or vertical pipes.
How Much Do Differential Pressure Flow Meters Cost?
DP flow measurement spans a wide price range because the technology bridges everything from a compact monitoring meter to a custom-engineered system for a 24-inch steam line. Keep in mind that a component-built system needs both a primary element and a transmitter (plus a manifold), while an integrated meter is one purchase.
Where DP economics truly shine is large pipe. Technologies that require a full-bore flow body get expensive fast as diameters grow, while an orifice plate scales up affordably. This is why DP measurement so often delivers the lowest total cost of ownership in big lines.
Do Differential Pressure Flow Meters Have Switches?
Yes. If your application needs an alarm or a control action at a specific flow rate, low-flow protection on a pump, for example, KOBOLD’s integrated DP meters have you covered without adding a separate device. The RCM is available with optional alarms and output signals including analog, switching, frequency, and HART outputs, and the RCD is available with analog or switching outputs alongside its digital display.
What Is a Differential Pressure Flow Transmitter?
You will see the terms “differential pressure transmitter,” “DP transmitter,” and “DP flow transmitter” used almost interchangeably, so here is the distinction. A differential pressure transmitter is the measuring instrument itself. It senses two pressures and reports the difference. It becomes a flow transmitter when it is paired with a primary element and configured to apply the square-root extraction that converts pressure difference into flow rate. The same transmitter, plumbed differently, measures tank level or filter condition instead. Which is exactly why DP transmitters like the PAD are considered the industrial workhorse of process instrumentation. Beyond the flow math, the PAD provides continuous self-diagnostics, remote configuration via HART, and a standard 5-digit local display. For the full picture of what one instrument type can do across flow, level, and pressure, see our differential pressure transmitter guide.
Differential Pressure Flow Meter Manufacturers
KOBOLD is proud to not only manufacture differential pressure flow measurement solutions, from KPL orifice plates and PAD transmitters to integrated RCM and RCD meters, but to sell and support them directly, unlike many manufacturers. Because our philosophy is that the customer matters as much as the product, we are well known in the industry for exceptional and consistent customer service. Plenty of companies build DP flow instrumentation; we believe the engineering expertise and support behind ours make it one of the best values in the market.
How Do I Choose a Differential Pressure Flow Meter?
The short answer: we would love to do it for you! Selection comes down to your flow range and accuracy needs, your fluid (liquid, gas, or steam - and its density, viscosity, corrosiveness, and cleanliness), and your piping (line size, available straight runs, and how much pressure drop you can afford). As a rough map of the KOBOLD line: KPL orifice plates for large pipes and general applications, the RCM for high flow rates and industrial monitoring, the RCD for small lines where low pressure drop matters, and the ANU pitot tube for large ducts. The PAD is the transmitter that completes any component-built system.
Rather than working through sizing calculations alone, let us save you the time, for free. Schedule a free engineering consultation, call us at 1-800-998-1020, or email info@koboldusa.com, and our application engineers will match a DP flow solution (or tell you honestly if another technology fits better) to your exact process.