The multimeter is a measurement device most used by engineers and we use it in this temperature indicator circuit schematic. The temperature sensor is LM335 wich has a linear characteristic of 10mV / K . In the production process this electronic device is calibrated so can provide 2.73V at 0°C.
LM336 is used as a very precise 2.5V Zener diode; CA3040 amplification can be adjust between 1.08 and 1.10 times thru P1. This temperature measurement circuit can be used for Fahrenheit measurements, in this case the freezing point is adjusted with P1 at 32°F. At 212°F, P2 is adjusted to obtain 0.9V at output. So 1°F correspond to 5mV.
For Celsius degrees the output voltage of IC2 is set with P1 at 2.73 V when the temperature of IC1 is 0°C. The calibration of the temperature indicator at 100°C is done by comparing it with a precision termometer. When LM335 is at this temperature adjust P2 so you read a 1 V difference at the output of the circuit.
The total current consumption is 10mA.