Does a 0.3 mm sensing film change the interface it measures?

A sensing film is not a passive observer. It sits in the load path, adds a layer of known thickness and unknown compressive stiffness, and reports the pressure of an assembly that now differs from the one you set out to measure. That is not a reason to avoid it. It is a reason to know which numbers survive the perturbation.

What the film changes

Two things. It opens the interface by its own thickness, so a displacement-controlled process — vacuum bag, fixed-gap press — spends part of its closing travel on the film instead of on the joint. And it adds stiffness: where the film is stiffer than the interface it takes load and shows a peak that is partly an artefact; where it is softer it spreads load and returns a flatter map than the joint really has.

The sealed edge is where the error concentrates

The sensing area is 0.3 mm thick. The seal around it is not, and on a clamped joint that seal is a step in stiffness sitting at a location where the surface is already discontinuous. Edge readings therefore trend high on hard interfaces. Trimming the sheet so the seal falls outside the contact patch removes most of it — the same geometry problem as a flat film on a curved blade surface.

How to keep it usable

Compare, do not reproduce. One film build, one placement, one press cycle, then read the difference between two assemblies. That difference survives the perturbation because both sides carry it. For absolute values, pair the map with a load cell on the same test and check that the integrated total matches the applied force — if it does not, the film is telling you about distribution, not about magnitude.

When the effect is small

On soft interfaces — rubber pad, uncured adhesive, breather — a 0.3 mm layer is a small fraction of the total compliance and the reading sits close to true. On steel against steel at high preload the same layer is a large fraction, and above 6 MPa the standard build is out of range in any case. The honest claim is not that a thin film measures an interface exactly. It is that the film is the only way to see the shape of the contact, and the shape is what separates a good part from a marginal one.