This Standard specifies requirements which it is necessary for a measuring instrument for vibration severity of machines to meet if inaccuracies of measurement made on the casing of machines, particularly when making comparisons between one machine and another, are not to exceed a specific value. The instruments covered by this Standard give direct indication or recording of root-mean-square (r.m.s.) vibration velocity. Subject to limitation of the measurement frequency range, these instruments can be used for other applications where similar accuracy of measurement is required, measurement of vibration velocity of structures, tunnels, bridges, etc. Optionally phase measurements may be included.