Isokinetic Versus Nonisokinetic Sampling Explained

A stack test can produce precise laboratory results and still misrepresent actual particulate emissions if the sample was not collected correctly at the stack. The central issue in isokinetic versus nonisokinetic sampling is whether the gas entering the sampling nozzle moves at the same velocity as the surrounding flue gas. For particulate matter testing, that distinction directly affects whether the sample represents the emissions stream or introduces collection bias.

This is not simply a field technique preference. The required approach depends on the pollutant, the applicable test method, the purpose of the data, and the physical behavior of the material being measured. Facility personnel reviewing a test plan should understand the difference because it affects method selection, field setup, schedule, cost, and the defensibility of reported results.

What is isokinetic sampling?

Isokinetic sampling means the velocity of gas entering the sampling nozzle is matched to the local velocity of the gas traveling through the stack or duct. If the stack gas at a traverse point is moving at 60 feet per second, the sample must enter the nozzle at 60 feet per second at that point.

This velocity match matters when sampling particles that have enough mass and inertia that they do not perfectly follow changing gas streamlines. Under isokinetic conditions, particles approaching the nozzle continue into it in proportions that are representative of the stack gas at that location. The collected material can then support a defensible calculation of particulate concentration and mass emission rate.

Many particulate source test methods require isokinetic collection. EPA Method 5 is a familiar example for filterable particulate matter. Related methods may use the same core principle while adding requirements for metals, condensable particulate matter, organic compounds, or other target analytes. The governing method, permit condition, or regulatory program determines the exact equipment configuration and procedural requirements.

Achieving isokinetic conditions requires active control during the test. Technicians measure stack gas velocity and select a nozzle size that permits an appropriate sample flow range. As the probe moves through the required traverse points, the sampling rate is adjusted to account for changes in local velocity, temperature, static pressure, and gas density. The nozzle must also be properly aligned with the direction of gas flow.

Why velocity matching affects particulate results

The consequence of a poor velocity match depends on particle size, density, shape, and the magnitude of the mismatch. Larger or denser particles are generally more susceptible to inertial effects than very fine particles.

When sampling is subisokinetic, gas enters the nozzle more slowly than the surrounding stack gas. The nozzle can collect a disproportionately high amount of larger particles because their inertia carries them into the nozzle rather than allowing them to follow the diverted gas streamlines. This can bias the reported particulate concentration high.

When sampling is superisokinetic, gas enters the nozzle faster than the local stack velocity. The accelerated intake can result in undercollection of larger particles, creating a low bias. Neither condition is acceptable when the test method requires isokinetic sampling within a specified tolerance.

The issue is especially significant where particulate loading is near a permit limit, where control equipment performance is being evaluated, or where emissions data will be used in enforcement-sensitive reporting. A result that is biased high can trigger unnecessary investigation. A result that is biased low can create a more serious compliance and reporting risk if the data cannot withstand regulatory review.

Isokinetic versus nonisokinetic sampling: when each applies

Nonisokinetic sampling does not attempt to match the nozzle intake velocity to the stack gas velocity. This does not make it inherently incorrect. It is appropriate for certain pollutants and methods where particle inertia is not the controlling concern, or where the sampling system is designed for a different measurement objective.

Gas-phase pollutants are common examples. For compounds that are present as gases and are well mixed in the flue gas, a fixed sampling rate may be acceptable if the prescribed method allows it. Flue gas characterization for oxygen, carbon dioxide, carbon monoxide, nitrogen oxides, sulfur dioxide, hydrogen chloride, volatile organic compounds, and similar gaseous constituents often uses nonisokinetic extraction systems. Continuous emissions monitoring system evaluations and instrumental methods may also rely on sample conditioning, analyzer response, calibration gases, and flow controls rather than isokinetic nozzle velocity matching.

Nonisokinetic sampling may also be used for screening, process troubleshooting, or specialized measurements where the objective is to identify the presence of a contaminant rather than establish a representative particulate mass rate. However, a screening result should not be assumed to satisfy a compliance requirement unless the applicable regulation or permit explicitly permits that approach.

The practical rule is straightforward: sample the pollutant in the form in which it exists in the stack, using the method specified for the compliance decision being made. Particle-bound material typically demands isokinetic collection. True gas-phase constituents generally do not. Some analytes can exist in both particulate and gaseous phases, which makes method selection more complex.

Method selection requires more than identifying the contaminant

A request to test for “particulate” or “metals” is not enough to establish a field method. The testing team must confirm the relevant regulatory requirement, emission source, operating scenario, target reporting basis, and whether filterable, condensable, gaseous, or total emissions are required.

For example, metals may be associated with particulate matter, vapor-phase compounds, or both, depending on source temperature and chemistry. A method designed only for filterable material may not answer a requirement for total recoverable emissions. Likewise, particulate testing performed at an incorrect location or outside representative operating conditions may not support permit compliance, even if isokinetic rates were maintained correctly.

Sampling location also affects the ability to obtain reliable data. A compliant traverse requires adequate access, safe working conditions, and a flow profile suitable for the method. Cyclonic flow, stratification, insufficient straight-run duct length, obstructed ports, or unstable process operation can complicate the work. These conditions should be evaluated during test planning rather than discovered after equipment is mobilized.

Field controls that support defensible isokinetic data

Isokinetic testing is a controlled measurement process, not a single flow-rate setting. Before testing, the team evaluates stack dimensions, port configuration, expected temperature, moisture, gas composition, velocity range, and anticipated particulate loading. That information supports nozzle selection, equipment preparation, safety planning, and a realistic estimate of test duration.

During the test, technicians document traverse-point velocities, nozzle diameter, sampling rate, temperatures, pressures, meter volumes, and the calculated isokinetic ratio. They also perform required pre- and post-test leak checks and verify the calibration status of critical equipment. Sample recovery is equally important because material deposited in the nozzle, probe liner, filter assembly, impingers, and other specified components may need to be recovered and analyzed.

Quality assurance review should examine more than the final concentration. It should confirm that the selected method was appropriate, the process was operating representatively, the sampling train met temperature and recovery requirements, the traverse was completed as required, and all applicable acceptance criteria were met. When a field issue occurs, it should be documented promptly with an assessment of its effect on data validity.

Common planning errors to avoid

The most frequent mistake is treating isokinetic sampling as a universal requirement. Applying it to every gas measurement can add unnecessary complexity without improving the result. The opposite mistake is more consequential: using a convenient fixed-rate gas sampling setup for particulate or particle-bound contaminants that require isokinetic collection.

Another common error is relying on historical stack velocity to set the sample rate. Stack conditions can change with fuel, load, draft, control equipment operation, ambient conditions, and production rate. Field adjustments are necessary because the relevant condition is the local velocity at each traverse point during the actual test.

Facility teams can reduce risk by providing accurate process information before the test and confirming the intended compliance use of the results. This allows the testing provider to align the method, sampling train, analytical requirements, and schedule with the actual regulatory need.

For facilities managing permit limits or emissions reporting, the useful question is not whether one approach is generally better. The question is whether the chosen method produces representative, method-compliant data for the pollutant and decision at hand. Early technical review of that question helps prevent a test that is expensive to perform but difficult to defend.