REFLECTOR: CHT monitor

[Douglas Holub]

Good thinking. That could be the reason.
----- Original Message ----- 
From: "Dave Philipsen" <velocity at davebiz.com>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Wednesday, March 18, 2009 10:40 PM
Subject: Re: REFLECTOR: CHT monitor


> 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
>
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