How to Measure the Speed of a Turbine?

Oh there are so many ways:

• Direct methods (Adding instrumentation to the turbine)
• A relative encoder tachometer attached to the turbine shaft can trigger a Frequency to Voltage converter that gets read by a suitably scaled voltmeter
• A measurement generator attached to the turbine shaft. The voltage out (peak, RMS or DC depending on generator type) will be proportional to the angular velocity by Lenz's law.
• Painting half of the shaft black and using a reflective opto-coupler and a one second counter circuit to add up the number of rotations in a second. (The basis of a relative encoder)
• Indirect methods
• Measure flow rate out of pump (time the filling of a known volume, vane flowmeter, paddle flowmeter)
• Use a microphone to sample sounds emanated by engine. (one "chuff" per engine cycle) FFT analysis and complicated (but fun) interpretation for a relative value.
• Listen for squeal of dazzled children or irate wife.

I'm sure that I did not exhaust all of the possibilities, but I think you get the general idea. Besides knowing the logistics of where, if and how to post a sensor on your engine, the questions of resolution, accuracy, continuous or discreet readings must be set for a detailed answer.

Combination Digital Hand-Held Tachometer.

What's the measurement thingy say?

In the good ole pre-electronic days, we used to have these laying around. Some were even permanently attached to their turbine. If interested, search for 'vibrating reed tachometer'. But be warned, they can be tricky to read if your turbine is running at 1/x speed. And they come in differing ranges.