Isokinetic Sampling: Theory, Probe Design, and When You Need It
Engineering guide to isokinetic sampling for particulate-laden gas and liquid streams. Theory, probe nozzle sizing, velocity matching, EPA Method 5, ASME PTC 38, and design tradeoffs.
What Is Isokinetic Sampling?
Isokinetic sampling is the practice of withdrawing a sample from a flowing stream at exactly the same velocity as the bulk fluid at the sampling point. The motivation is the inertial behavior of suspended particles: when the sample velocity differs from the bulk velocity, particle concentration in the sample is biased — often by an order of magnitude.The single-sentence rule:
Iso-kinetic = same velocity = correct particle distribution.
Why It Matters
For dissolved gases or fully homogeneous liquids, the velocity match is irrelevant — diffusion equalizes concentration faster than sampling distorts it. For particle-laden streams (fly ash, catalyst fines, droplets in steam, asphaltene in crude), the result is dramatic.
| Sample velocity vs. bulk | Coarse particle bias |
| Sample slower (sub-isokinetic) | Over-sampled — large particles enter preferentially |
| Sample matched (isokinetic) | Representative |
| Sample faster (super-isokinetic) | Under-sampled — large particles deflect away |
The Stokes Number
The dimensionless parameter that decides whether a stream needs isokinetic sampling is the Stokes number:
Stk = (ρp × dp² × U) / (18 × μ × L)
Where ρp = particle density, dp = particle diameter, U = bulk velocity, μ = fluid viscosity, L = characteristic dimension of the probe.
When Stk >> 1, particle inertia dominates and isokinetic conditions are essential. When Stk << 1, particles follow the gas streamlines and any reasonable sampling velocity will give the correct result.
Governing Standards
| Standard | Scope |
| EPA Method 5 | Particulate emissions from stationary sources |
| EPA Method 17 | In-stack particulate (no impingers) |
| ASME PTC 38 | Determining the concentration of particulate matter in a gas stream |
| ISO 9096 | Stationary source emissions, manual gravimetric |
| ISO 12141 | Low concentration mass concentration |
These standards prescribe sharp-edged, thin-walled sampling nozzles aligned to the flow with traverse points across the duct cross-section.
Probe Design Implications
An isokinetic probe is not the same as a standard sample probe. Distinguishing features:
1. Sharp, thin-walled inlet nozzle — leading edge < 50 µm thick to avoid disturbing the streamlines
2. Heated transfer line to keep particles airborne
3. Right-angle bend inside the probe to allow inlet-axis alignment with flow
4. Pitot tube mounted parallel for real-time velocity measurement
5. Flow controller that adjusts pump suction continuously to maintain velocity match
The bulky standard sample probe geometry generated by our configurator is not isokinetic; it is appropriate for dissolved-gas custody transfer sampling where particle inertia is negligible.
When to Specify Isokinetic vs Standard
| Service | Sampling Type |
| Natural gas custody transfer | Standard probe (no particles) |
| Refinery fuel gas | Standard probe |
| Stack flue gas (boiler) | Isokinetic |
| Cement kiln gas | Isokinetic |
| Crude oil pipeline (no wax) | Standard |
| Crude oil pipeline (waxy) | Isokinetic or jet mixer + standard |
| Catalyst slurry | Isokinetic |
| Hydrogen pipeline | Standard |
Anisokinetic Error Estimation
The classic Belyaev-Levin correction:
Cs/Co = 1 + (U₀/Us − 1) × β(Stk)
Where Cs = sampled concentration, Co = true concentration, β is a function of Stokes number that approaches 1 at high Stk and 0 at low Stk. For coarse particles (Stk > 1) and a 2× velocity mismatch, the sampled concentration error can exceed 50%.