Abstract: Measuring device for quickly and accurately determining the thickness of an annulus between two cylindrical members, such as the catalyst retaining screens of a radial flow reactor, at any point along its length or circumference, comprises a wheeled cart having two sets of angularly mounted wheels which straddle the inner screen and a pair of spring loaded plungers with ball shaped contacts which contact the outer screen. As the device is lowered into an annulus with a cable, a pair of ultrasonic transducers positioned inside the cart direct opposed beams radially of the annulus to permit the radial distance between the screens to be measured.