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Air Pressure Flow Meter: Differential Pressure Orifice and Pitot Tube Systems
Quick Answer: For compressed air, combustion air, or ventilation air, a differential pressure flow meter with an orifice plate or pitot tube gives you a repeatable flow reading without moving parts. For most industrial air lines from DN25 to DN300, we quote an orifice plate with a DP transmitter. For large duct or stack air, a pitot tube is often cheaper and easier to install.
Silver Automation Instruments supplies DP transmitters, orifice plates, averaging pitot tubes, and full air flow metering packages. We work with end users and distributors in Southeast Asia, Oceania, Latin America, Africa, and the Middle East. This page covers the two main air pressure flow meter options. It also gives you the specs we need for a quote.
How a Differential Pressure Air Flow Meter Works
All DP flow meters rely on a restriction or a sensing point in the pipe. The restriction creates a small pressure drop. That drop has a known relationship to flow rate. You measure the high side and low side pressure. A DP transmitter converts that signal to flow.
In practice, most engineers skip the full theory and focus on three things. The beta ratio for orifice plates. The pipe inner diameter. The density or pressure and temperature of the air at the meter location. If you give us these values, we can size the primary element and the transmitter.
Orifice Plate Systems for Air Pressure Flow
An orifice plate is a thin metal plate with a machined bore. It bolts between flanges on a pipe. For air, standard orifice plates work well from DN15 up to about DN300. Beyond that, the plate gets heavy and the permanent pressure loss becomes higher. Most compressed air applications sit between DN25 and DN150.
We have seen orifice plates used in a paint plant in Indonesia, a bottling line in the Philippines, and a cement plant in Kenya. The attraction is simple. No moving parts. Low upfront price. Good repeatability. The downside is permanent pressure loss. For air at 7 bar, a typical orifice plate may lose 0.1 to 0.4 bar depending on beta ratio. That loss costs you compressor energy. So you do not want to oversize the meter or pick a very high differential.
For air flow measurement, we usually pair the orifice plate with a DP transmitter that has a range of 0 to 250 mbar or 0 to 500 mbar. The transmitter can output 4-20 mA HART. We can also add a flow computer or use a PLC to calculate the square root. Some customers ask for a multivariable transmitter. That works well if you need pressure and temperature compensation in one device.
Pitot Tube Systems for Air Pressure Flow
A pitot tube senses the dynamic and static pressure of the air. An averaging pitot tube has multiple sensing holes across the pipe. This gives a better average in larger ducts. Pitot tubes create much less permanent pressure loss than an orifice plate. That matters for low pressure air systems like aeration blowers or stack gas air flow.
In duct sizes above DN300, an averaging pitot tube is often the first choice. It installs through a small insertion point. You can mount it hot tapped on some lines. We supply pitot tubes for duct sizes from DN100 up to DN1000 and larger. For a wastewater plant in Mexico, we quoted an averaging pitot tube for an aeration air header at 0.6 bar. The permanent loss was below 2 mbar. The plant saved blower power compared to an orifice plate.
Pitot tubes are also used in natural gas and steam. For air, the main question is dirty or clean air. Dirty air from a kiln or a spray dryer can block the sensing holes. In that case we suggest purge ports or a different meter type. For clean compressed air or ventilation air, a pitot tube with a DP transmitter is a solid option.
Key Specs We Need for a Quote
Do not start with a model number. Start with the process data. We need the pipe size in DN or inches. We need the flow range in kg/h or m3/h. We need the air pressure in bar or kPa. We need the temperature in degrees C. We also need the pipe schedule or inner diameter.
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We also ask for installation details. Is this a horizontal pipe? Is there straight run before the meter? For an orifice plate, you want at least 10D upstream and 5D downstream in most air systems. For an averaging pitot tube, the vendor will give a straight run requirement based on the upstream fittings. If your straight run is short, we can quote a flow conditioner or a different meter such as a vortex flow meter.
Common Air Flow Meter Packages We Supply
We often quote a complete package. That includes the orifice plate or pitot tube, the 3-valve or 5-valve manifold, the DP transmitter, and the impulse tubing. For hazardous areas, we can supply ATEX Zone 1 or Zone 2 transmitters. For outdoor sites, we use stainless steel housings and cable glands.
A compressed air system in a Singapore electronics plant used a DN80 orifice plate with a 0 to 250 mbar DP transmitter. The output was 4-20 mA HART to the building management system. The meter ran for four years with no failure. That is the typical result when the sizing is right. The main failure mode is water or oil in the impulse lines. We recommend a water knockout or a dry air supply for the transmitter manifold.
Pricing and Lead Time
Small air pressure flow meter packages for DN25 to DN100 usually cost less than a Coriolis meter or an ultrasonic meter. A basic orifice plate with a standard DP transmitter can be delivered in 2 to 3 weeks. Averaging pitot tubes may take 3 to 4 weeks depending on the length and material. If you need a rush order, send us the full process data and we will check factory stock.
For distributors in the Middle East and Africa, we can prepare a quote with a discount structure. We also support OEM customers who package our DP transmitters with their own orifice plates. Send us your pressure (bar), temperature (°C), pipe size (DN), and flow range. We will reply with a sizing sheet and price.
FAQ: Air Pressure Flow Meter Differential Pressure Orifice and Pitot Tube Systems
Which is better for compressed air, an orifice plate or a pitot tube?
For pipe sizes up to DN300, an orifice plate is common and economical. For larger pipes or low pressure air, an averaging pitot tube is better because it has less permanent pressure loss. Send us your pipe size and air pressure, and we will recommend one.
Can a differential pressure flow meter measure air flow in a dirty duct?
Yes, but only if you use purge connections or select a design that can handle dust. In many dirty air applications, a thermal mass flow meter or a vortex meter may be easier to maintain.
What accuracy can I expect from an orifice plate air flow meter?
A properly sized and installed orifice plate system can give 1 to 2 percent of rate accuracy. Poor straight run and wrong inner diameter can reduce that to 4 percent or worse. We size each meter for the actual flow range.
What is the typical turndown for a DP air flow meter?
With a standard DP transmitter, turndown is usually 4 to 1 or 5 to 1. If you need wider range, we can use a multivariable transmitter or two transmitters with different ranges. For very wide turndown, an ultrasonic or thermal mass meter may be a better fit.
Do you supply air pressure flow meters for ATEX zones?
Yes. We supply DP transmitters with ATEX Zone 1 and Zone 2 certifications. We need your gas group and temperature class. Some pitot tubes and orifice plates are certified for use with those transmitters.
Silver Automation Instruments supplies industrial flow meters and pressure transmitters. For air pressure flow meter quotes, call us at Tel: +86-135-1213-1526, WhatsApp: 86-135-12131526, or WeChat: +86 13512131526. Send your pressure (bar), temperature (°C), pipe size (DN), and flow range.