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Rosemount Air Flow Meter: Differential Pressure Ventilation Duct Probes for Ventilation and Process Air
Quick Answer: A Rosemount air flow meter with differential pressure duct probes measures air flow in ducts from 150 mm to 2000 mm. The probe generates a velocity pressure signal and the DP transmitter converts it to a 4-20 mA HART output for the DCS or PLC. Silver Instruments supplies replacement DP transmitters and full duct air flow measurement packages for HVAC, cleanroom, dust collector and combustion air systems.
Many engineers ask us about Rosemount air flow meter setups because the original duct probe is still in good shape but the transmitter has failed or the range no longer matches the air flow. In practice you can keep the existing Rosemount duct probe and install a new Silver Instruments DP transmitter if the impulse lines and probe connections are intact.
Where a Rosemount DP Duct Probe Works Well
Differential pressure duct probes work in ventilation ducts, AHU supply and return air, cleanroom make-up air, spray dryer exhaust, boiler combustion air, paint booth exhaust and dust collector outlet flow. A paint manufacturer in Vietnam asked us last year to replace a transmitter on a Rosemount duct probe in a 600 mm exhaust duct. The probe was 316L stainless steel and still in good condition after five years of service. The original transmitter ranged 0 to 250 Pa but the plant needed 0 to 500 Pa after a fan upgrade. We supplied a Silver Instruments DP transmitter with 0 to 500 Pa range and the same 4-20 mA HART output.
Key Sizing Parameters for Duct Air Flow
You need the duct cross section, operating static pressure, air temperature, velocity range and probe type. Most ventilation ducts run at 0 to 2 bar static pressure and 0 to 60 °C. DP transmitters for these probes usually fall between 0 to 125 Pa and 0 to 2500 Pa. Most engineers skip the square root conversion in the transmitter and leave it in the PLC or DCS. Silver Instruments recommends setting the DP transmitter to a linear 4-20 mA signal and then applying the square root in the control system because field checks are easier.
For a 300 mm duct with air velocity around 10 m/s, the velocity pressure is about 60 Pa. A 0 to 250 Pa transmitter gives enough span. For a 1000 mm duct with 5 m/s, a 0 to 125 Pa span is often enough. Provide the duct size in DN and the normal and maximum air flow in m3/h or Nm3/h when you request a quote.
Installation Notes That Matter
Straight run matters. A Rosemount duct probe should sit 8 to 10 duct diameters downstream of a bend, damper or fan discharge and 2 to 3 diameters upstream of another disturbance. In many real installations this is not possible. Plant engineers then move the probe to the best available location and accept a repeatability loss of 2 to 5 percent. We have seen this many times on customer sites in Southeast Asia and the Middle East.
Dusty or sticky exhaust air can plug the total pressure holes on the probe. Use a purge air system or select a probe with larger sens
Silver Instruments Replacement DP Transmitters
Silver Instruments supplies the SI-3051DP transmitter for air flow probes with ranges from 0 to 100 Pa up to 0 to 2500 Pa. Outputs include 4-20 mA HART, 4-20 mA with Modbus RS485 or 1 to 5 V DC. Wetted parts include 316L stainless steel, Hastelloy C and silicone oil fill. Housings are IP67 and available for ATEX Zone 1 or Zone 2 when the duct is in a classified area.
We do not sell Rosemount parts directly. We sell Silver Instruments DP transmitters that connect to existing Rosemount duct probes in most cases. The impulse tubing connections are 1/4 inch NPT or 6 mm tube fittings. Check the threads on your probe manifold before ordering.
Typical Applications for Duct Air Flow Measurement
The same Rosemount air flow meter and duct probe concept fits pharmaceutical cleanroom ventilation, data centre cooling air, heat recovery ventilation, combustion air to burners and flue gas recirculation flow. In desalination plants, building pressurization air and toxic gas dilution air also use this approach. Silver Instruments has supplied DP transmitters for HVAC and process air projects in Vietnam, Indonesia, the Philippines, Saudi Arabia and Chile.
FAQ
1. Can I keep my Rosemount duct probe and replace only the DP transmitter?
Yes. Most Rosemount duct probes use standard impulse lines or manifold connections. Silver Instruments DP transmitters work with these probes. Check the probe connection size and the DP range before ordering.
2. What DP range do I need for ventilation air flow?
Start with 0 to 250 Pa for ducts up to 600 mm and normal HVAC velocities. For larger ducts or low velocities use 0 to 125 Pa. For process exhaust with fast air speed use 0 to 500 Pa or 0 to 2500 Pa. Send us the duct size and air velocity to confirm.
3. Do I need square root output from the transmitter?
Most plants use a linear 4-20 mA signal and perform square root in the DCS or PLC. This makes field calibration and loop checks simpler. The Silver Instruments SI-3051DP transmitter can provide linear or square root output depending on your order code.
4. What is the typical accuracy of a DP duct air flow system?
A good installation gives 1 to 2 percent of rate when the duct probe is installed with proper straight run and the transmitter span is matched to the velocity pressure. Poor straight run or probe blockage can degrade accuracy to 5 percent or more.
5. How do I order a Silver Instruments DP transmitter for a Rosemount duct probe?
Send us your static pressure in bar, temperature in °C, duct size in DN, air flow range in m3/h and the existing probe connection type. We will reply with price, delivery time and a quotation within 24 hours.
Contact Silver Instruments today. Tel: +86-25-68650347. Whatsapp: +86-25-52155837. WeChat: +86 15365082610. Send us your static pressure in bar, temperature in °C, duct size in DN and flow range in m3/h.
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