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 
As I see it the problem is that each of these thermocouples which are 
just like a tiny battery only puts out a very small amount of voltage.  
We're talking millivolts.  In my opinion, when measuring such small 
voltages, it would be better to have a dedicated pair of wires going to 
each thermocouple instead of relying on a common ground.  Additionally, 
there could be some thermocouple-like couplings in the ground bus 
between each of the thermocouples or between all of the thermocouples 
and the measuring instrument.  I think the best way would be a dedicated 
pair for each thermocouple with each pair being of the same type of wire 
and the same length.

Dave Philipsen
Velocity STD FG
N83DP