REFLECTOR: CHT monitor

[Dave Philipsen]

Douglas Holub wrote:
> It's the minus (-) wire, and it might get tied to ground inside the 
> RMI (although that's not the way I'd do it.)
>
> A thermocouple is like a tiny battery whose voltage changes with 
> temperature. I've been thinking about it, and if I wanted to measure 
> six batteries to see how their voltage changes with temperature, it 
> would be fine to tie all their negative terminals together and measure 
> each positive terminal individually. I wish I had given it some 
> thought before I wired up two double-pole rotary switches for EGT and 
> CHT. It was a pain.
>
> The RMI wiring diagram calls for a double-pole rotary switch, so I'd 
> call Ron at RMI first if I were you. I'd be curious to know what you 
> find out.
>
> Doug 
Just thinking a little more on this (and I'm sorry to jump in in the 
middle of this thread without having read everything) but another 
possible reason for using a DPDT switch if you're going to be switching 
thermocouples in and out:  Think of the switch junction as a 
thermocouple itself since somewhere inside of it there will invariably 
be two dissimilar metals forming a junction.  If you put an extra 
thermocouple (SPST switch) in the line (in series) it could be just like 
stacking an extra battery with an increase (or decrease) in voltage.  A 
DPDT switch, however, would introduce the same "thermocouple" on BOTH 
sides of the circuit and it would be sorta like stacking two batteries 
together with opposite polarities.  They sort of cancel each other out 
if you get my drift.

Dave Philipsen
Velocity STD FG
N83DP