Graded Sensor

I am just guessing, but it sounds like the "graded" sensor is a thermo-switch designed to open and close at some given temperature. The ungraded sensor, therefore, must be a thermocouple for measuring the actual temperature.

Mechanical devices are generally less expensive than electrical devices. But because mechanical devices have moving parts, they are subject to failure. All of the open circuit or shorted circuits I have ever seen are mechanical failures.

My guess would actually be the opposite of the first answer. For graded is defined as arranged in a sequence of grades or ranks; "stratified areas of the distribution" http://www.google.com/url?&q=http://wordnetweb.princeton.edu/perl/webwn%3Fs%3Dgraded&ei=GyKDSp-2B6WEmQeTkpWjDg&sa=X&oi=define&ct=&cd=1&usg=AFQjCNGVJrBZeXQnq-mzzIgHtKeF2yaTBA

So the two sensors will measure the input and output relative temperature difference. While the other probe will just know if it's cold or not.