Abstract: A method of and apparatus for determining when an unknown quantity is within a desired range of a reference quantity. The apparatus includes a dual slope integrator which integrates the reference quantity and the unknown quantity and discharges the resultant integrated levels. By integrating the reference quantity at least once and the unknown quantity at least twice for three different time periods which differ from one another by amounts which are proportional to the desired range, a comparison of the discharge times of the resultant integrated levels will provide an indication of whether the unknown quantity is within the desired range, when the desired range is a percentage of the value of the reference quantity.