Brumby: Do you think the temperature control of this particular iron is to be trusted?
I can't answer that from personal experience. Sorry.
If it doesn't measure the actual tip temperature I don't mind, as long as it measures consistently from somewhere else.
That is actually one of the problems. There is a temperature gradient between the sensor and the solder puddle - and it is dependent on how good the thermal coupling is in this region (as mentioned before) and how much heat is being drawn. From just sitting in the stand, to soldering an SMD resistor, to soldering a metal tab to a ground plane, the heat flow increases and so does the difference between what the sensor is measuring and what the temperature of solder puddle actually is. This difference is due to the construction of the iron and unless the sensor is something like a thermocouple you stick into the solder puddle, there will always be a difference - and that difference
will change depending on the amount of heat flowing. What only ever matters is the actual temperature of the solder puddle.
You might get a better feel for things if you look at what is happening as a thermal resistance exercise - with the element being analogous to the die in a transistor, the joint analogous to the heatsink with all the intermediate boundaries that exist (element/barrel/tip/solder/joint/ambient as a minimum) and paying particular attention to where the temperature is being measured by the sensor.
I really have no idea what to set the temperature dial at for "normal" PCB/component soldering for instance, so I would just set it at a low setting, then go up from there until it melts the solder sufficiently and take a note of that.
We all go through a 'familiarisation' stage when we get a new iron - so what you are saying here is pretty normal thinking. After a time, you will get to know how your iron deals with different jobs and how to adjust it to perform properly in different situations.
Or have I misunderstood what you're saying? I take it that a good soldering station with a digital readout actually outputs 250C at the tip when it says 250C on the display.
That is an "ideal" soldering station. There will always be a temperature drop - especially during soldering - but the really good stations will have a more accurate control system.
But regarding the wattage, do I understand it correctly that this 70W iron is suitable for delicate work (through hole, or even SMD components to PCBs) when the temperature dial is set to a low temperature, and at the same time being suitable for soldering larger surfaces, cables etc. when the temperature dial is cranked up for a higher temperature?
This is where the "ideal" gives way to the real world.
A 70W iron doing SMD is not too far from the ideal. You set a temperature and because there is not a lot of mass to suck away heat, the actual temperature is not too far away. As such, you set the temperature closer to the actual melting point of your solder and the temperature control system is switching the element on and off to keep it there.
However, when you are soldering heavier joints, there is a lot of heat getting sucked out - so you need more heat in. Setting your iron to the same temperature as you would for SMD means that the temperature at the tip would drop very quickly and the temperature control circuitry would switch on when the sensor dropped below the set point. What happens next is important...
Once the element is turned on, it heats up, then it's surrounds heat up, the sensor heats up and eventually the tip heats up - followed by the solder. The temperature control system monitors the sensor - so when the
sensor reaches the set point, the control system switches off the element. The problem with this is, that the tip and the solder haven't been heated up enough - because the joint is sucking away heat big time.
The simple response to this is to do exactly what you have said - crank up the temperature. This allows the element to be kept on long enough to deliver the heat required to get the solder and joint up to temperature. The risk is that
you now have to take responsibility for the actual temperature at the joint - something that will come only with experience.