The ELE-SB-4L Steel Test Block offers four precision calibration points for fine tuned accuracy from four hundredth inch with a ratio of twenty hundredths of an inch (1.016, 1.524, 2.032, 2.540 mm). Steel calibration test blocks allow you to conveniently check the accuracy of ultrasonic thickness gauges. All blocks are made from ultrasonically tested material and include NIST traceable Certification Reports and hardwood storage cases.