I have not used RTDs before. For the project I am working on I have been given a set of drawings from an 'equivalent' project; which has a note "Winding thermistors connected in series" (but no detail)
Are RTDs simply a 'known' resistance/temperature profile? If you connect two in series would the resistance be the 'sum' of the two? (i.e. if you halve the reading you get the 'average' temperature?)
> Are RTDs simply a 'known' resistance/temperature profile?
Yes, an RTD is an element that has a very well defined temperature/resistance profile. The main reason RTDs are used rather than thermocouples is inherent accuracy. But to achieve that accuracy, the resistance of the lead wire must be taken into account, because a 2 wire RTD temp measurement includes an error due to the resistance of the lead wire and the RTD element. A 3 wire RTD temp measurement uses a circuit (in the analog input) that compensates for the lead wire resistance.
I am unaware of any commercial RTD circuits that run RTD's in series, but that doesn't mean that none exist. Conventional lead wire compensation would go out the window. RTD's are the most expensive of the sensor elements and I'm not clear on why one would wire RTD's in series, it would be cheaper to use T/Cs to get an average.
> If you connect two in series would the resistance be the 'sum' of the two?
It would be the sum of the two plus the lead wire resistance. Without looking up resistance values on a chart, I don't know much error the non-linearity contributes. Lead wire resistance, as low as it is, can add considerable error.