Here is what worked: Type K thermocouples are being used as the temperature sensor. We need a T287-0000. The T287-0000 is a head mount, PC configurable, temperature transmitter. It can be configured for a differential thermocouple input. The 4/20 mA dc output represents the differential temperature. When both temperatures are equal the output will be at 12 mA. For explanation's sake we will refer to the thermocouple signals as A and B. The controller will respond to heat the A side of the product if the 4/20 mA signal is less than 12 mA, and the B side if the 4/20 mA signal is greater than 12 mA.

The indication of excessive variance is accomplished by using a Q108-0000 (or G108). The Q108-0000's configurable input is set for the 20 mA input range One set point is configured as a 'Hi' trip, the other as a 'Lo' trip. The set points are adjusted for the specified temperature differential. When A exceeds B by the specified temperature differential the 'HI' relay will provide the indication. Similarly, when B exceeds A by the specified differential the 'Lo' relay will trip.

The Q108-0000 has an isolated 24 Vdc @ 20 mA source to power the T287-0000. The T287-0000 has a measly 9 V drop allowing for 15 V drive remaining in the current loop, more than enough to accommodate the input of the controller.

If the T287-0000 is not mounted in a thermocouple head junction assembly it can be DIN rail mounted next to the Q108-0000.