Stack Flow Monitors Compared

Measuring pollutant concentration tells you what is present in flue gas. Measuring stack flow helps determine how much of that pollutant is being emitted.

For facilities reporting emissions in kilograms per hour or tonnes per year, reliable flow measurement is an important part of the monitoring system. However, different stacks present different challenges, including variable velocities, moisture, dust, corrosion and restricted installation access.

Thomson Environmental Systems (TES) can help compare ultrasonic, differential-pressure and optical flow technologies for Australian and New Zealand applications.

Four options include the Teledyne Monitor Labs UltraFlow 150, DXG SCD-90, DXG PGA-91 and OSI OFS-2000.

Why Does Stack Flow Measurement Matter?

Volumetric flow is determined from gas velocity and the stack’s internal cross-sectional area. Combined with pollutant concentration on a consistent reporting basis, it allows calculation of pollutant mass emissions.

For example, a concentration in mg/Nm³ must be combined with flow expressed at matching normal conditions and moisture basis. Multiplying a dry-normal concentration by an uncorrected actual wet flow produces an inconsistent result.

Selecting the flow monitor therefore involves both measuring velocity reliably and defining how the system will calculate and report flow.

How Do These Flow Monitors Compare?

The best choice depends on the operating range, flow profile, installation geometry and reporting requirements.

Teledyne UltraFlow 150: Ultrasonic Measurement for Continuous Stack Monitoring

The Teledyne Monitor Labs UltraFlow 150 measures the travel time of ultrasonic signals passing through the gas in opposite directions. The difference between upstream and downstream transit times is used to calculate gas velocity.

Its transducers are mounted outside the main gas stream and protected by continuous purge air. This reduces direct exposure to particulate and corrosive gas.

The instrument supports single-path and dual-path arrangements. A dual-path “X” configuration can reduce sensitivity to certain non-axial flow components, making it worth considering where the flow profile is challenging.

A further advantage is its full-system calibration-check functionality. The UltraFlow is suitable for 150 for US 40 CFR Part 75 applications and provides diagnostic information, reference-test linearisation and plant communication options.

Use it when: continuous stack flow measurement, automated checks and configurable acoustic paths are priorities. Installation still requires suitable port geometry, access and a reliable purge supply.

DXG SCD-90: Cross-Duct Ultrasonic Flow Measurement

The DXG SCD-90 ultrasonic flowmeter measures velocity across a duct using ultrasonic signals. Its cross-duct approach is useful where a path measurement is preferable to relying on a single insertion point.

It measures a velocity range of 0–50 m/s, measurement distances of 1–50 metres and a response time below five seconds.

The system includes ultrasonic transducers, a controller and a purge-air arrangement. Outputs include an analogue signal and serial interfaces for integration with monitoring equipment.

Use it when: the stack dimensions and gas conditions suit the specified ultrasonic configuration, particularly where cross-duct measurement is desirable. Confirm performance at the lowest operating velocity, rather than selecting solely on maximum range.

DXG PGA-91: Pitot-Based Flow Monitoring with One-Sided Access

The DXG PGA-91 belongs to DXG’s S-type Pitot flowmeter range. It measures differential pressure associated with gas movement and uses this information, together with the necessary gas-density information, to determine velocity.

The probe installs from one side of the stack. This can simplify access where mounting equipment on opposite sides would be difficult.

It has variable ranges catering for full-scale velocities from 10 to 50 m/s. The PGA range also measures dynamic pressure, static pressure, ambient pressure and temperature. It is designed for gas temperatures below 500°C, but special configurations can operate at higher temperatures.

An insertion probe introduces maintenance considerations: pressure openings must remain clear, and deposits, corrosion or erosion can affect performance. It is however a more CAPEX effective solution than an ultrasonic solution.

Use it when: one-sided access is valuable, the process provides a suitable pressure signal, and probe inspection and cleaning can be accommodated. Very low velocities require particular attention because differential pressure falls approximately with velocity squared.

OSI OFS-2000: Optical Flow Measurement for Challenging Gas Streams

The Optical Scientific OFS-2000 uses optical scintillation. It observes fluctuations in transmitted light associated with moving turbulence in the gas stream and processes them to determine path-averaged velocity.

Its optical heads sit outside the flow, behind windows. Unlike an opacity monitor, the instrument uses fluctuations in the received light to measure flow rather than simply reporting how much light is obscured.

The measurement path can be installed perpendicular to the flow. This can simplify platform arrangements compared with an angled measurement path.

The OFS-2000W incorporates automatic gain control for changing opacity associated with particulate or moisture. Applications include use in wet scrubbers, baghouses, incinerators and thermal oxidisers. Optical transmission, window protection and suitable turbulence must still be assessed.

Use it when: external optical sensing and a perpendicular cross-stack path offer practical advantages, particularly for an application suited to the wet/high-opacity variant.

Does a Cross-Stack Monitor Automatically Measure the Stack Average?

A path measurement samples more of the duct than a local point measurement, but it does not automatically equal the area-weighted average velocity.

Bends, fans, dampers and changes in duct shape can create uneven or swirling flow. Measurement location, path arrangement and reference testing remain important for every technology.

A well-positioned instrument with validated correction factors can be more useful than a higher-specification instrument installed at an unsuitable location.

Which Flow Monitor Is Best for Compliance Reporting?

Start with the applicable monitoring specification. Identify the required operating range, uncertainty, availability, verification procedures and reporting basis.

Manufacturer statements about overseas standards provide useful supporting information, but site acceptance also depends on the exact configuration, installation and validation.

Discuss reference velocity traverses, testing across operating loads, temperature and pressure measurements, moisture correction and data-system integration when requesting a quotation.

Choosing a Stack Flow Monitor with TES

Provide TES with the stack’s internal dimensions, normal and minimum velocities, gas temperature, pressure, moisture and particulate loading. Include drawings showing nearby bends, fans, dampers and available access platforms.

This information helps determine whether the UltraFlow 150, SCD-90, PGA-91 or OFS-2000 offers the most appropriate balance of measurement performance, installation practicality and maintenance requirements.

Contact Thomson Environmental Systems to discuss stack flow monitoring for your emissions reporting or CEMS project.

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