REFLECTOR: CHT monitor

Brian Michalk michalk at awpi.com
Thu Mar 19 22:00:10 CDT 2009 

It's a very safe assumption that your digital gauge is high impedance.

You do need to be careful when checking the resistance on equipment like
this, because your ohmmeter puts voltage across the device under test
and measures current ... which is linear with resistance.  An old gauge
might get zapped if it is expecting low voltage thermocouple inputs.  A
thermocouple may generate 5 mA at the most if I recall correctly.
Ohmmeters might put more voltage than that out the probes.

aminetech at bluefrog.com wrote:
> Thanks, Brian.  That's wery helpful and is making me re-think.  Can I assume that all digital CHT gauges are high impedance?  From your description it looks like my RMI monitor is high impedance - uneven thermocouple leads, junction in the engine compartment, lines separated from the noisy wires.  I measured 7 M ohm on the new gauge.  Do I have to have power to it to get the correct resistance reading?
> 
> John
> 
> --- michalk at awpi.com wrote:
> 
> From: Brian Michalk <michalk at awpi.com>
> To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
> Subject: Re: REFLECTOR: CHT monitor
> Date: Thu, 19 Mar 2009 10:34:06 -0500
> 
> 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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