Thermocouple

Depends on what type of thermocouple you're using. Also whether that's degrees celcius or fahrenheit. Here's a start :

http://www.claustemp.com/an-htp15.htm

Is the gradient, from hot end to cold end, across the thermocouple = 165°C?

Or is the hot end at 165°C and the cold end at some other temperature?

What is the cold end temperature of the thermocouple?

What type thermocouple?

Tables are your friend, as Steve has pointed out. But it's useful to remember the number 40mV per degree C, for a quick estimate. Most types are in that territory at room temperature, decreasing at very low temps, increasing somewhat at high temps.

The thermocouple (T/C) tables are valid only when the cold end is ice point or cold junction referenced.

The T/C tables do not provide a direct value when the 'cold' end is at any temperature other than 0°C, or when ice point or cold junction compensation is not implemented.

For instance, take a type K thermocouple lying on a bench top at an ambient temperature of 30°C. The temperature of both ends of the T/C is identical at 30.0°C.

A thermocouple table gives a value of 1.203mV for 30.0°C. But measuring the output of the T/C will give a reading of 0.0mV, because there is no thermal gradient across the thermocouple, even though the 'hot' end is at 30°C.

The temperature of the cold end junction point must be added to the uncompensated temperature value read from a mV table.