Abstract: Methods and apparatus for on-line detection and measurement of degradation or undesirable changes in the time constants of sensors used in monitoring a system are disclosed. Methods and apparatus of this invention are particularly suitable for use in complex and high-risk industrial processes or other operations where it is undesirable and expensive or dangerous to allow certain parameters of the operation to change above or beyond a predetermined amount. This invention includes a detector for determining when a signal from a sensor crosses a preselected signal level such as for example a zero level. The output of the detector or zero crossing detector is provided to a counter or other means which determines the rate of the zero crossing by counting the number of crossings per a specific time period provided by a timer.