fat16lib:
ADC_DELAY is for high impedance sensors. If you have low impedance sources set ADC_DELAy to zero.If ADC_DELAY is nonzero, analogRead is called and the value is discarded to switch the ADC MUX to the channel then delay(ADC_DELAY) is called and finally analogRead() is called again to get the value.
This allows the allows the ADC sample and hold to stabilize with high impedance sensors.
Thanks for the explanation 