Temperature Question

delta means "change or difference", so you tell us? what are both readings, I'll give you the difference

The need is bizarre, and possibly unique.

• Please explain what is to happen with this information.
• Please explain what is to happen were the temperatures at the two locations to be the same and the readings taken to be different.

1. You could use two platinum 100 ohm resistance temperature detectors [RTD] in a bridge arrangement with two fixed resistors. With suitable fixed resistors (117.5 ohm approx) the bridge output would be zero at 45 Celsius, but the signal amplitude double.
2. Note that the error of a class 'B' RTD is about +/- 0.5 degrees (half that for class A), so your differential error would be +/- 1 degree without balancing the bridge at 45 degrees on both RTD. The error of thermocouples is about +/- 3 degrees, again without zeroing the indication with both TC at 45 degrees. It depends how accurate you need to be!
3. You have not indicated the temperature difference between the two sensors.
   
4. At 45 degrees you could use transistors as sensors (with base shorted to collector) in a bridge. The differential sensitivity is about 2 mV per degree with 1 milliamp emitter-collector current. I believe Δt of 0.01 degrees can be measured with that technique (compare sensitivity about 40 microvolts per degree for TC and 0.4 mV/degree for RTD).
5. Transistors are cheap compared to the usual sensors, but you would have to calibrate a sensor pair individually around 45 degrees and design a mounting and lead-out.
6. There are some integrated circuits which give a current or voltage output proportional to temperature, but again these would need a mounting and some calibration/balancing at about 45 degrees.
7. Except possibly for technique 6., you will need a good high gain amplifier to get a useable readout.
   

If you want a diagram, I could do one for your preferred technique.

67model

What type of temperature element do You have?

As pointed out already by others, You can have analog circuit, giving voltage proportional to temperature difference. And You can see it directly on analog or digital indicator. Properly made analog part should allow reading to 0.2deg +-1% of temperature difference.

You want to see it or to log it?