Company Profile

Air Velocity Sensors: Continuous Duct Exhaust and Draft Monitoring Wall Modules

Quick Answer: Air velocity sensors for continuous duct exhaust and draft monitoring wall modules measure air speed in m/s, duct static pressure in Pa, and temperature in many cases. They send a 4-20 mA HART signal, a Modbus RS485 output, or a relay alarm to a BMS or PLC. Silver Instruments supplies these wall mount transmitters with insertion probes for ducts from DN80 to DN600 and larger.

What These Wall Modules Actually Do

These devices sit on a duct wall or a panel and give plant operators a live value of air speed. They do not assume that the fan is running. The sensor confirms it.

In a continuous exhaust duct, air velocity changes when filters load, dampers move, or a fan belt slips. A wall module for draft monitoring catches this before smoke, dust, or solvent vapor leaks into a work area.

We have seen this on customer sites many times. A paint booth in Vietnam used a wall module to monitor exhaust velocity. The sensor showed 6.2 m/s during normal operation. After two weeks the reading dropped to 4.1 m/s. The maintenance team found a blocked intake filter. That is the value of continuous monitoring.

Key Parameters Engineers Check Before Quote

Duct size matters. For round ducts we ask for DN100, DN150, DN200, DN250, or larger. For rectangular ducts we need width and height in millimetres.

Air velocity range is the next point. Clean room exhaust often works between 0.5 m/s and 10 m/s. Industrial dryer exhaust can run from 5 m/s to 30 m/s. Combustion air lines may need 0 to 40 m/s.

Temperature is part of the selection. Standard wall modules handle up to 60 °C or 80 °C. High temperature probes with stainless steel heads go to 120 °C or 200 °C for oven exhaust.

Duct pressure is often integrated. Some users want one device that shows velocity and static pressure. We supply draft monitoring wall modules with ranges from -250 Pa to +250 Pa, -1000 Pa to +1000 Pa, or -5000 Pa to +5000 Pa depending on the fan system.

Output signal is a dealer breaker. Many Southeast Asian plants use 4-20 mA HART. Some Australian sites prefer Modbus RS485 for BMS integration. We can also provide two relay outputs for low flow and high flow alarms.

Sensor Type and Installation Reality

For duct exhaust, hot wire and thermal dispersion probes are common. They are accurate at low velocity and respond fast. Differential pressure based sensors with a pitot tube work better in dirty air or high temperature lines.

Wall modules typically include a probe, a mounting flange, and a transmitter. The probe length must match the duct insertion depth. For a round duct, the sensor tip should sit near the centre line. For a rectangular duct, the tip should be at one third of the duct depth.

Most engineers skip this part. If the probe is too short, the reading is biased. If it is too long, vibration may damage the sensor. We ask for the duct diameter and the insertion length to avoid this.

Some wall modules have a small digital display and three buttons for zero and span. Others are blind transmitters with just a 4-20 mA loop. We have supplied both versions to food packaging lines in Latin America and water treatment plants in Africa.

Outputs and Plant Control Integration

Standard output is 4-20 mA two wire. The signal is linear to air velocity or duct pressure. For example, 4 mA equals 0 m/s and 20 mA equals 20 m/s. A PLC or BMS reads the loop and logs the value.

HART is useful for calibration and diagnostics. A technician can read the sensor from the loop without opening the panel. Modbus RS485 gives a digital value with units and status bits. Relay outputs let you wire a buzzer or a beacon directly without a PLC.

Power supply is usually 24 V DC. Some wall modules accept 85 to 264 V AC with an internal power supp