AbstractWe have developed instrumentation using an internal‐bubble pressure transducer with a data acquisition system to sense LLDPE bubble stability on a commercial blown‐film line. Real‐time bubble instability was also recorded using a video camera at a speed of 30 frames/s. The pressure signal was analyzed using a fast‐Fourier‐transform (FFT) computer program to determine the frequency of bubble oscillation. Since bubble instability often consists of large‐scale oscillation and bubble flutter, and both motions affect gauge and layflat uniformity, the pressure signal was decomposed to determine the frequency of each oscillation mode. To obtain a stable bubble, the amplitudes of both oscillation modes must be reduced to a minimum. Under a single mode oscillation, the period of bubble‐diameter variation analyzed using videotape matches that of the pressure trace. The initial results indicated that the trace of the pressure can provide a warning to the stability of a blown