Soldering iron temperature from bar graph meter to OLED display

[pyroesp]

Hi all,

So I have this little project where I'd like to "upgrade" my soldering iron.
It's a VTSS20 (http://www.velleman.eu/downloads/1/vtss20vtss30gbnlfresd.pdf) soldering iron from Velleman.
They say you can set the temperature from 150°C to 420°C.

Being a bit bored a couple days ago, I opened the case, measured here and there, and found the temperature sensor signal.
I couldn't find any datasheet on what type of temperature sensor they use and what the output voltage is per °C, so I just turned the pot to it's max value and measured the voltage of the temperature sensor. The temperature sensor signal goes through a x60 amplifier and I measured 1.36V for (I suppose) 420°C.

I'm using a PIC16F877A to write on a winstar 2x16 OLED display. (I was testing my display library on the 16F877A before trying to use it as a temperature meter, I'm probably changing that to a 16F88)
I added a little opamp to amplifie the temperature sensor voltage, because 0V-1.36V with a 5V ref at the ADC isn't that great. Now I have like 0V-4.24V  .

My program just measures the voltage:
voltage = ADC * (5.0 / 1024.0);
And converts that to a temperature:
temperature = voltage * (420.0 / 4.24);

Anyways, the thing I wanted to ask you guys is if anyone knew what the V/°C is on this soldering iron.
Btw, the soldering iron is a 24V 48W model SL-1 or SL-I.

I would like to know the exact V/°C so I can have a precise temperature. I don't have the tool to measure the temperature of the soldering iron either to verify if  the soldering iron is 420°C when the temperature control is set at maximum.

Here are some pics :

EDIT:
I want to "upgrade" it so I can set a temperature and have both the actual temperature and set temperature displayed on the OLED.
If you have other ideas, let me hear them.

[ve7xen]

This is vaguely in the ballpark for a thermocouple (Type-K with 1.36/60 = 23mV is approx 520C), which makes sense for a soldering iron.

You could try measuring the input to the amplifier with the iron turned off but hot and see if the voltage roughly corresponds with what the thermocouple voltage should be as the iron cools. I don't know of an easy way to tell what type it would be if that's the case though, and it sounds like the iron probably doesn't have cold junction compensation.

[pyroesp]

I've measured the input of the amp for the thermocouple and I get 23mV (V average on scope) for, what they claim to be, 420°C.
It seems a bit unlikely to be 520°C @23mV, right ? They say it goes from 150°C to 420°C, so ...

Btw, I saw very little on thermocouple. Isn't there a way to know what thermocouple they use ?

[SeanB]

Simpler would be to buy a SL30 display board and put it in the case. It has the digital set point and display of tip temperature already.

Get a cheap type K reading multimeter and check the tip temperature with the thermocouple, and then you can get the 2 points you need to do a zero and scale for the micro to calculate the tip temperature. You can read the set values at the same time to get the set point voltages at the same time. Add in an extra pin to read the heater on led and you can use the display to show this as well. Use a DAC output and a pin to switch between the original or a MPU generated set point will also be good.