Would the accuracy of the optical tachometer drift over a period of time? If so, why?

 

are you talking about a stroboscope or an encoder?

 

I assume you are speaking of the part of the tachometer that receives the pulses and computes a pulse rate (i.e. rpm, pps, etc.)

It all depends on the tech used.

If it's computing rate via analog components (resistors, capacitors), then yes, the calibration will change over time. There is probably a calibration pot in the circuit somewhere.

If the system is digital, then it is probably crystal controlled, and only the crystal accuracy/drift characteristics apply. In this case, there will be a small amount of drift due to temperature, but little due to time. There may be an adjustable cap in the crystal circuit to calibrate the timing, but my guess is that you don't have any equipment accurate and stable enough to effect a proper calibration.