Photoelastic analysis of a large fused sandwich seal, composed of two glasses with slight composition differences, permits reliable measurement of the thermal expansion difference to 0.0002 ppm/ degrees C. If the expansion of one of the glasses is known, the method becomes absolute. For the titania-silica system, it is shown that for any two compositions with small titania differences the expansion coefficient differential remains constant from -195 degrees C to 925 degrees C. A direct correlation between ultrasonic velocity and thermal expansion has been established for this glass system and has led to successful nondestructive measurements. With an experimentally defined relationship, the measurement of the ultrasonic velocities yields absolute or differential expansions. Excellent agreement with seal testing data is shown to exist with differential data taken by both methods.