Calibration Issue

[MUHAMMAD NOUMAN RAFIQ]

Dear Dr. William,

I’m using PIVlab for a BOS (background-oriented schlieren) analysis with one static reference image and a sequence of dynamic images. I form pairs as (A+B), (A+C), …, (A+end).

Issue: With a time separation of Δt = 1 s per pair, the measured displacements are very small, so the velocity vectors end up numerically equal to the displacements. When I recompute the same data assuming 1000 fps (Δt = 1 ms), PIVlab reports a maximum velocity magnitude of about 0.2 m/s, which is still much lower than the actual flow speed (~1.7 m/s). How can I resolve this discrepancy?

Setup details

• Calibration: 388.77 pixels = 40 mm

• Multigrid interrogation windows: 128 → 64 → 32 → 16 px, 50% overlap each pass

• Correlation robustness: tested both “standard” and “extreme”

Clarification about timing:
Is the issue that PIVlab assumes a constant Δt between all image pairs? For example, from A to B the time separation is 1 ms, but from A to C it is 2 ms, from A to D it is 3 ms, and so on. However, PIVlab may be treating all pairs as if they have the same Δt. Could this be the cause of the error in the velocity profile?