No machine is used. The intensity of a tornado is determined by a visual survey of the damage.
Fujita intensity scale
The Fujita scale is used to determine how strong a tornado is. It is widely used for two reasons: it was the first scale to assess tornado intensity, and it breaks damage down into six easily recognizable levels.
To rate a tornado, scientists analyze the damage the tornadoes does to estimate wind speed. This is then used to assign a rating, which may range from F0 to F5 (EF0 to EF5 as of 2007).
There is no particular instrument used for measuring tornado intensity. Ratings are based primarily on damage assessment. Occasionally doppler radar has measure wind speeds inside a tornado, but such measurements are rare.
The intensity of a tornado is usually judge based on an analysis of the damage, which is used to estimate wind speed.
It depends on what substance/object you are trying to determine the intensity.
Tornado intensity is determined by damage, which is used to estimate wind speed. These wind speed estimates are used to sort a tornado into one of six categories from EF0 to EF5.
In some cases, Doppler radar is used to measure winds inside a tornado. However, as these measurements are rare and cannot scan ground-level winds, they are not used in ratings. The intensity rating is determined based on analysis of damage.
In most cases the intensity of a tornado is never actually measured. Tornadoes are rated based on the severity of the damage they cause. Mobile Doppler radar can measure wind speeds in a tornado, but since such radar cannot measure ground-level winds the measurements are not used in ratings.
The Fujita scale uses the damage done by tornadoes to estimate their intensity.
Ted Fujita discovered the Fujita Scale to determine how much strength a tornado has based on the damage it produces.
Richter scale