Help understanding 4-20mA current loop temperature sensor

Hi I am interested to a circuit that is a 4mA-20mA current loop temperature sensor but
I don't understand how the circuit is working.

Here is the circuit:
Circuit - TEMPERATURE SENSOR WITH 4 TO 20mA CURRENT LOOP - Circuits designed by David A. Johnson, P.E.

How is it working? And more precisely what is the use of the schottky diodes? How the part with the temperature sensor and the shunt voltage reference is working?

thanks alot,

Jenifer

The op amp is configured in a Howland circuit to generate a constant current independent of loop resistances and the supply voltage in the loop (within it's operating limits), with the current value proportional to the temperature of temp sensor U1.

The diodes put the circuit inside a diode bridge so that it works for either supply polarity.

Ok thanks

And what does the transistor Q1 does?

It's an emitter follower to boost the output current of the op amp (which likely can't deliver 20mA).