Question

The instrument used to measure speed

Answer 1

Speedometer is instrument used to measure Speed .

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Answer 2

Let me add to Hardick's comprehensive survey;

Instruments for measuring aircraft speed

• Airspeed indicators use a device called a Pitot tube. This is a small probe with an orifice in the fonward facing end, and another near the fuselage, on the side. The air pressure on the side is taken to be the average local air pressure, and the onward movement through the air produces a higher pressure in tip. The difference between the two can be scaled in a complex non-linear manner to indicate airspeed.
• Groundspeed indicators use Doppler radar to measure speed relative to the ground. They are only useful at low altitudes.
• Groundspeed estimators use navigational data to calculate speed over the ground over long intervals.

Instruments for measuring the speed of ships

• Shaft logs count the turns of the main propeller shaft, and use that to estimate distance run and speed. Even though corrections for slip are applied they are notoriougly inaccurate, but the distance run figure is frequently used over longbperiods to estimate fuel efficiency.
• Trailing logs have a small rotatingbimpellor on a cable, so that the impeller is towed behind the ship. These are far better than shaft logs, but still only accurate to about 10 %
• Doppler logs use sonar techniques to measure speed with sound waves. In shallow water they lock onto the bottom, and give a speed relative to the ground. In deep water they reflect off the deep water layers, and give a speed which might be influenced by the current.
• GPS logs record track, distance, and speed from the GPS system.

Assorted industrial speed measurements

• Hand-held tachometers are supplied with cones that can be pushed into holes in the end of mechanical shafts, or wheels to be run on the circumference. They use an optidal encoder inside to generate pulses, and hence calculate speed.
• Rope and conveyor systems have traditionally used an idler wheel running on the surface, with an encoder or 'voltage tacho to measure speed. In these days of VFDs on the motors, though, speed is normally taken from there.
• Water flowmeters are a whole different subject, and a wide range of techniques have emerged down the years;
• Water flow lifts a metal vane against gravity or a spring, and the angle indicates the speed of the water
• A small impeller is spun by the flow, and sensed either on the shaft, or by using a magnet in one vane and a pick-up outside the tube.
• A pin in the tube causes turbulence, which is measured ultrasonically
• An orifice impedes the flow, and the resultant differential pressure between the sides indicates the speed
• Doppler ultrasonic sensors measure the movement through the tube wall.
• Encoders, as in the hand held tacho example, are slotted discs used to interrupt a light beam, and hence generate a pulse train that can be counted.
• Synchro or 'voltage tachos are a small permanent magnet DC generator. The voltage is linearly related to speed.
• Vehicle speed can be measured with two soft air hoses spaced along the track. The wheels compress the air, and trigger pressure sensors. The time differences measured, and knowing the separation can be converted to speed.

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