What happens when stack or duct flow falls below EPA thresholds?

Under EPA reference methods, most flow measurements require a minimum differential pressure of 0.05 inches of water column. When flow is too low to meet this threshold, standard inclined manometers become unstable or unreadable, small pressure fluctuations appear as noise rather than measurable signal, temperature and gas composition variations increase uncertainty, and some flow devices lose calibration accuracy in low-velocity conditions.

What alternative devices can be used for low flow emissions testing?

Several alternative devices may be considered depending on gas characteristics and source configuration: vane anemometers for moderate-temperature clean gas streams, hot-wire anemometers for low-velocity low-particulate environments, critical orifice systems where predictable flow control is possible, direct plumbing to dry gas meters when stable volumetric measurement is feasible, and tracer gas injection techniques for complex or inaccessible stacks.

What validation steps are needed before low-flow emissions testing?

Before testing, several validation steps are required: instrument compatibility checks for temperature ranges, gas composition, and particulate resistance; sensitivity and accuracy verification against the required test method; assessment of system back-pressure implications before deploying any device that restricts flow; calibration traceability to ensure EPA compliance; and field validation under actual operating conditions before beginning regulated testing.

What are the benefits of proper low-flow measurement strategy?

A properly engineered low-flow measurement strategy delivers accurate and defensible flow data suitable for EPA reference methods, avoiding 2-4 week retesting delays and $10,000-$40,000+ remobilization costs. It reduces uncertainty in emission rate calculations, prevents measurement errors that can invalidate test results, minimizes operational impact to the facility by avoiding costly shutdowns, and provides compliance confidence during audits, reviews, and regulatory submissions.

A Technical Guide to Low Flow Emissions Testing and Alternative Measurement Methods

Q: How do you accurately measure emissions when a source operates at extremely low flow rates?

Low-flow emissions measurement is addressed through four approaches: upgrading to digital manometers with higher resolution, deploying alternative devices like vane anemometers or tracer gas systems, using engineering calculations based on fuel data, or modifying the sampling system. Method selection depends on gas temperature, particulate loading, and regulatory requirements. Proper planning avoids the $10,000-$40,000+ costs associated with failed testing and remobilization.

Alliance Technical Group
Dec 4, 2025
3 min read

When a stack operates at very low flow rates, differential pressure often falls below the readable range of standard EPA instruments. Emissions still need to be measured, but conventional methods lose accuracy under these conditions, creating risk for both data integrity and regulatory compliance.

This scenario is increasingly common as facilities optimize processes, reduce throughput, or cycle equipment. Low-flow conditions challenge even experienced emissions professionals and demand advanced technical evaluation.

This article outlines the key issues that arise during low flow emissions testing and the alternative methods available to address them.

What Happens When the Stack or Duct Flow Falls

Below EPA Thresholds

Under EPA reference methods, most flow measurements require a minimum differential pressure of 0.05 inches of water column. When the flow is too low to meet this threshold:

A standard inclined manometer becomes unstable or unreadable.
Small pressure fluctuations appear as noise rather than measurable signal.
Temperature and gas composition variations increase uncertainty.
Some flow devices lose calibration accuracy in low-velocity conditions

Compounding this, low-flow sources can present additional complications such as:

Elevated or fluctuating temperatures
Gas streams with non-air composition
Particulate loading that interferes with sensors
Restrictions that can cause unacceptable back-pressure to the process

In these environments, conventional measurement techniques are insufficient, and a more tailored engineering approach is required.

Alternative Measurement Methods for Low Flow Emissions Testing

When low flow prevents the use of standard EPA methodology, technical teams must evaluate alternative strategies. A structured solution framework typically includes:

1. Instrument Sensitivity Adjustment

Upgrading to a lower-range or digital manometer with higher resolution can resolve many borderline low-flow scenarios.

2. Technology Assessment for Alternative Flow Devices

Depending on gas characteristics and source configuration, several devices may be considered:

Vane anemometers for moderate-temperature, clean gas streams
Hot-wire anemometers for low-velocity, low-particulate environments
Critical orifice systems where predictable flow control is possible
Direct plumbing to dry gas meters when stable volumetric measurement is feasible
Tracer gas injection techniques for complex or inaccessible stacks

3. Engineering Calculations Using Plant Data

If reliable fuel flow, fuel composition, and combustion efficiency data are available, EPA methodologies allow deriving stack flow via calculation rather than direct measurement.

4. Mechanical or System Modifications (When Necessary)

Some cases require temporary or permanent adjustments, such as:

Installation of an in-line dry gas meter
Construction of a temporary test stack
Additional sampling personnel and instrumentation for tracer-based methods

Each option must be evaluated for compatibility, safety, impact on the process, and regulatory acceptance.

Quick Answer: How do you accurately measure emissions when a source operates at extremely low flow rates?
Our teams address low-flow conditions through four approaches: upgrading to digital manometers with higher resolution, deploying alternative devices like vane anemometers or tracer gas systems, using engineering calculations based on fuel data, or modifying the sampling system. Method selection depends on gas temperature, particulate loading, and regulatory requirements. Proper planning avoids the $10,000-$40,000+ costs associated with failed testing and remobilization.

Quick Answer: How do you accurately measure emissions when a source operates at extremely low flow rates?

Our teams address low-flow conditions through four approaches: upgrading to digital manometers with higher resolution, deploying alternative devices like vane anemometers or tracer gas systems, using engineering calculations based on fuel data, or modifying the sampling system. Method selection depends on gas temperature, particulate loading, and regulatory requirements. Proper planning avoids the $10,000-$40,000+ costs associated with failed testing and remobilization.

Before the Test: Validation and Verification Steps

Implementing the selected strategy requires detailed technical planning:

Instrument compatibility checks (temperature ranges, gas composition, particulate resistance)
Sensitivity and accuracy verification against the required test method
Assessment of system back-pressure implications before deploying any device that restricts flow
Calibration traceability to ensure EPA compliance
Field validation under actual operating conditions before beginning regulated testing

Experienced emissions professionals must balance measurement feasibility, cost, and regulatory requirements to avoid invalidating test results.

What Proper Low-Flow Measurement Delivers

A properly engineered low-flow measurement strategy yields:

Accurate and defensible flow data suitable for EPA reference methods- avoiding the 2-4 week retesting delays and $10,000-$40,000+ remobilization costs that result from invalid data.

Reduced uncertainty in emission rate calculations- preventing significant errors like the 20%+ velocity increase that results from misreading 0.02 delta P as 0.03, which invalidates test results and requires costly retesting.

Minimized operational impact to the facility- solutions beyond stack modifications or fabricating spool pieces can be evaluated with your technical team, avoiding facility shutdowns or process modifications that can cost $10,000 - $50,000+ per day in lost production.

Compliance confidence during audits, reviews, and regulatory submissions – eliminating the risk of failed demonstrations that trigger enforcement actions, penalty assessments, or permit delays

By selecting the right combination of instrumentation and methodology, even difficult low-flow sources can be measured reliably.

Alliance TA Team: Expertise for Complex Emissions Testing

Low-flow conditions present some of the most technically challenging scenarios in emissions testing. They require specialized knowledge, careful technology selection, and a deep understanding of both equipment limitations and EPA methodology.

Alliance’s Technical Advisory (TA) Team, led by Glen Capra, has the expertise to analyze these situations, recommend the most effective measurement techniques, and guide field teams and clients through implementation. Our specialists evaluate each source on a case-by-case basis to ensure accurate, compliant, and cost-effective results.

If you are facing low-flow measurement challenges—or want to proactively plan for them—reach out to Alliance’s TA Team for expert guidance and customized technical solutions.

Glen Capra

Technical Advisor Manager

glen.capra@alliancetg.com

Reach Out to Alliance's TA Team →