Instruction manual

C-1
APPENDIX C. ADJUSTING THE CA27 ZERO OFFSET
A zero velocity anechoic chamber can be made by lining a 5-gallon bucket with foam. The foam lining
prevents acoustical reflections from the bucket walls. Two small dish cloths can be used to close off the
opening of the bucket.
Place the CA27 head inside the foam-lined bucket. Cover the opening with the dish cloths. Connect the
CA27 to the electronics box and the Signal/Power cable to the appropriate channels on the datalogger.
Remove the cover of the CA27 electronics box by loosening the four screws to expose the circuit board.
Use Instruction 2 with a multiplier of 1 and an offset of 0 to measure the wind speed voltage. Slowly turn
the “offset” potentiometer (Figure C-1) until the voltage measured by the 21X is approximately zero. A
±20 mV fluctuation is normal.
FIGURE C-1. CA27 Electronics Box