def rh_dwyer(raw_value):
    """Returns Dwyer sensor relative humidity (RH) from a raw register value.

    Range is 0-100%.

    """

    # Humidity linear calibration = 100 / (2^15 - 1)
    RH0 = 0.0
    RHs = 100.0 / (2 ** 15 - 1)

    return (RH0 + RHs * float(raw_value), "percent")