Category
Stack Air Velocity Transmitters
Measure flue gas and air velocity in stacks and ducts.
A stack air velocity transmitter reads velocity, flow or pressure in a hot, dusty flue or stack, for emission load calculations and draft fan control. The Beck 985V switches between flow, velocity and pressure by jumper, and the Halstrup-Walcher P 26.2 pairs with an S-type pitot tube for demanding stack conditions.
2 Products · Stack Air Velocity Transmitter
Selection guide
Stack Air Velocity Transmitters Guide
How a stack air velocity transmitter works
Stack conditions are harder on instruments than a clean supply duct: gas is hot, often dusty, and at low pressure relative to what the transmitter has to resolve. The Beck 985V connects to a primary element such as an S-type pitot tube and lets you select flow, velocity or pressure output with a jumper, plus a built-in switching contact for alarms. The P 26.2 instead focuses purely on differential pressure, at high accuracy, for a separate flow calculation downstream.
How to choose a stack air velocity transmitter
- Function needed: choose the Beck 985V when you want flow or velocity output plus a switching contact from one device. Choose the P 26.2 when you need the most accurate differential pressure signal and will calculate flow separately.
- Output: the 985V is switchable between 0 to 10 V and 4 to 20 mA, with zero adjustment done in the unpressurised state. The P 26.2 gives a standard analogue output to a PLC or DCS.
- Accuracy: the P 26.2 is rated ±0.2% FS up to 50 kPa, with high overpressure protection for the pressure spikes stacks can produce.
- Dust tolerance: pair either transmitter with an S-type pitot tube, which resists blocking better than a standard tube in dusty flue gas.
- Typical use: stack flow for emission load calculations, induced draft fan monitoring, and extraction or scrubber system control, where confirming operating temperature of the transmitter housing matters before installing in a high-temperature stack.
For duct rather than stack conditions, see air flow and velocity transmitters.
FAQ
Common questions
Why measure air velocity in a stack or flue?
It measures velocity, flow or pressure in a stack or flue for emission load calculations and to monitor or control induced draft fans. Unlike a duct transmitter, it's built to work with the higher temperature and dust levels typical of stack conditions, usually paired with an S-type pitot tube.
What's the difference between the Beck 985V and Halstrup-Walcher P 26.2?
The 985V is a multi-function transmitter: it measures flow, velocity or pressure, selected by jumper, and includes a built-in switching contact for alarms. The P 26.2 is a pure differential pressure transmitter optimised for accuracy, rated ±0.2% FS, for projects calculating flow separately downstream.
Can a stack transmitter cope with dusty, hot flue gas?
Pairing either transmitter with an S-type pitot tube helps, since its larger openings resist blocking better than a standard tube in dusty conditions. Confirm the operating temperature rating of the transmitter housing itself before installing it directly in a high-temperature stack.
How accurate is the Halstrup-Walcher P 26.2 for stack flow?
It's rated ±0.2% of full scale for spans up to 50 kPa, scalable from as low as 10 Pa, with high overpressure protection. That range covers both the low differential pressures typical of natural draft stacks and the higher pressures in forced or induced draft systems.
What is stack velocity used for besides fan control?
It feeds emission load calculations, where regulators require a flow figure alongside pollutant concentration to report total mass emitted. It's also used for extraction and scrubber system monitoring, and for confirming an induced draft fan is pulling the expected volume through the system.
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