Company Profile

Air Velocity Sensors and Flux Sensors: Continuous Ventilation Performance Monitoring

Quick Answer

Continuous ventilation monitoring needs direct air velocity and mass flow data. Silver Automation Instruments supplies insertion thermal mass flow meters and air velocity transmitters that connect to 4-20 mA HART, Modbus RTU, or pulse outputs. For duct sizes from DN50 to DN300 and velocities from 0.1 m/s to 80 m/s, request a quote with your duct size, temperature, and signal type.

Why Ventilation Monitoring Cannot Rely on Static Pressure Only

Many HVAC and process engineers monitor static pressure. That approach works for basic filter alarms but it misses flow degradation. A fan can stay at its pressure setpoint while flow drops because of damper drift, belt slip, or duct leakage. We have seen this on customer sites many times. In practice, direct flow measurement catches these issues before the room pressure or gas concentration changes.

Continuous ventilation performance monitoring requires an air velocity sensor or a thermal mass flow meter at the duct or stack location. The sensor measures the actual flow, not a proxy signal. That matters in pharmaceutical plants, data centers, battery rooms, and wastewater treatment buildings.

Where Air Velocity Sensors Fit

Air velocity sensors work well in clean ducts and moderate temperatures. A typical insertion probe with a PT100 reference and a heated sensor element can read from 0.1 m/s to 80 m/s. Output can be 4-20 mA HART or Modbus RS485. We recommend the Silver Instruments SI-320 insertion thermal mass flow meter for most HVAC and process air applications. It works at process temperatures from -40 °C to 200 °C and pressures up to 16 bar. The transmitter can totalize in Nm3/h or kg/h for compressed air and gas lines.

Flux sensors are not the same as flow sensors in this context. Some energy audits use heat flux sensors on duct walls or motor enclosures. They help find thermal losses but they do not replace direct air velocity measurement. For ventilation performance, direct measurement is the only reliable method.

Recommended Configuration for Continuous Monitoring

A practical installation uses one insertion flow element per duct run. Install the probe in a straight duct section with a minimum upstream run of 15 times the duct diameter and a downstream run of 5 times the duct diameter. That rule applies to air and flue gas applications. The probe depth should be about 0.5 times the duct diameter for round ducts. For rectangular ducts, place the sensing tip at the point of average velocity, not at the center unless the velocity profile is flat.

Connect the meter to a 24 VDC supply. Use a 4-20 mA HART loop for the DCS or PLC. Add Modbus RTU if the site has an energy management system. The SI-320 outputs both signal types on most models. Accuracy is typically plus or minus 1.5 percent of reading plus 0.3 percent of full scale. That is enough for ventilation verification, demand control, and leak detection.

For hazardous areas, ask for an ATEX Zone 1 or Zone 2 probe option. A battery room in Saudi Arabia, a solvent store in Vietnam, or a gas skid in Mexico often needs this certification. Silver Instruments can supply the probe with flameproof or intrinsic safety approval depending on the zone.

Installation Mistakes Field Engineers Should Avoid

One common mistake is installing the probe too close to a bend or damper. That produces a swirl error that no calibration can fully fix. Another mistake is using a compressed air purge in a dusty duct without checking the purge pressure. The purge air can cool the heated sensor and shift the reading low. In cement plants and wood processing sites, we recommend a filtered purge at 0.2 bar to 0.5 bar above duct pressure.

Last year a customer in Vietnam wired a 4 wire sensor as a 2 wire lo