REFLECTOR: CHT monitor

Brian Michalk michalk at awpi.com
Thu Mar 19 10:34:06 CDT 2009


There are two types of thermocouple indicators.  One indicator is 
unpowered, and requires nothing but a thermocouple wire to drive it.  
This gauge is usually of low impedance compared to the other type, and 
is really a very sensitive current measuring device.  The other type is 
a high impedance device, and will required power to run the associated 
electronic circuitry to condition the thermocouple voltage.  They are 1 
Mohms or so of impedance.

The low impedance devices are very sensitive to changes in thermocouple 
wire length.  If you want accurate readings using this method, you have 
to trim your TC's for the proper amount of total resistance.  TC wire in 
the smaller gauges can show a ohm resistance per foot.  Thicker wires 
show less resistance.  That may be the reason for the mismatched wire 
gauge that started this thread.  If using the older low impedance 
devices, it's best to trim the TC's according to the gauge companies' 
specification.  One can compensate by using larger gauge TC wire (the 
correct way), or, bring all the TC ends to the cabin side of the 
firewall to a block.  Trim the TC's appropriately to the gauge here.  
This is the cold junction.  Make sure all terminations are at the same 
temperature here.  Also, the gauge is assuming a cold junction 
temperature of about 70F.  This is not always the case, but it's much 
closer here than in the engine compartment.  From this junction, you can 
run copper wires all of the same chemistry to your switch, then to your 
gauge.  Don't mix and match thus creating other thermocouple junctions.  
Shielded wires here are a waste of weight and money.  The impedance will 
easily kill capacitive coupling, and probably most magnetic coupling 
(depending of the frequency).

The high impedance indicators do not care about overall TC resistance.  
The amplifier is already at 1Mohm or more, so a few tens of ohms in the 
TC wire means nothing.  No trimming required to make the TC's identical 
in ohms.  However, now there is no damping, so shielding is an important 
thing if the wires run close to high dv/dt wires --- like strobes, or 
mag P-leads, or alternator wire, or transponder cable.  So, separate 
your ducts into noisy and quiet.  Try as much as possible to not split 
grounds.  Run high current devices with two pairs of wire.  One power, 
one return, and if possible, twist them together to minimize magnetic 
coupling.

Another thing.  With sensors, do not confuse "ground" with signal 
return.  Ground is a power term used to define how current returns to 
the battery.  Never mix signal return references with power ground.  You 
will have problems unless you are lucky.  Easily 75% of the wires in our 
planes do not require shielding if you follow this rule.


Scott Derrick wrote:
> Yes,  coupling is possible with millivolt signals. I designed and build
> a well drilling rig monitor system and had to use shielded singles for
> the millivolt signals. The coupling was about 10-20% for a 30ft run.
>
> Scott
>
> Dave Philipsen wrote:
>   
>> 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
>>
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>
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