CHT EGT Installation

[lwhitlow]

Hi All

I'm just starting the install of the engine sensors (EGT / CHT) (Jab 3300) and have started thinking about the harness passthrough on the firewall

I've got the pre-made Dynon (Stein) CHT / EGT harness and I know you can't shorten the wires SOOOO...

Where has everyone been putting the extra wire?  Wrapping it up and stuffing it behind the panel?, or bundling it forward of the firewall

And man that wire is heavy!  Must be a pound or so of extra there

Any help you can offer   THANKS!!!

Larry Whitlow
601XL
90% done  103% to go

 

[dynonsupport]

You can shorten the wires no problem. Thermocouple wire can be shortened or lengthened, it's just that when you lengthen it you need to use the same kind of thermocouple wire, not just normal copper wire.

 

[lwhitlow]

Thanks,

Ive already got the cutters out.......  Look out for the blood. :

Larry

 

[G3WGV]

You can shorten the wires no problem. Thermocouple wire can be shortened or lengthened, it's just that when you lengthen it you need to use the same kind of thermocouple wire, not just normal copper wire.

Out of interest, why is that? What's special about "thermocouple wire"?

John.

 

[dynonsupport]

Thermocouple wire depends on the wires being made of different material. If the material changes before it gets to the instrument, there will be an error.

For much more detail, here's the wikipedia article:
http://en.wikipedia.org/wiki/Thermocouple

 

[G3WGV]

Ah yes, that makes sense. I just installed the EGT sensors today (though I have yet to pluck up courage to drill holes in my exhaust pipes!).

Apropos of which, is it recommended to use any sort of sealant around the EGT thermocouple, to ensure a gas-tight fit? This seems especially apposite as there are no straight sections on the rear exhaust pipes of a Rotax 912, so getting a good seal may be tricky.

Thanks.

John.

 

[dynonsupport]

We don't have specific recommendations, but you might give Lockwood Aviation Supply a call - they know everything about Rotaxes, and they do sell Dynon products too.

 

[GigG]

Apropos of which, is it recommended to use any sort of sealant around the EGT thermocouple, to ensure a gas-tight fit? This seems especially apposite as there are no straight sections on the rear exhaust pipes of a Rotax 912, so getting a good seal may be tricky.

Thanks.

John.

Though I had plenty of straight places on my pipe I asked an A&P who stumbled into my hanger when I was getting ready to mount the EGT probes the same question. His answer was, "I don't know but YOU will the first time you start it up."

 

[vinodwahi]

I am installing EMS on RV9A with Lycoming 160 PS engine. Can you please guide the location of bore to be made on Exhaust on Lycoming . Documentation says to bore between 2" and 8".
Thanks:
Vinod :  

 

[edemalleray]

On mine it is fitted on a straight portion of the exhaust, not too close so that it does not burn the sensor, at roughly the same distance from the cylinder. My readings are relative values, not absolute, and that is good enough for what we are looking for, the peak.
Hope it helps

 

[pbennett]

I am in the process of replacing my Rocky Mountains Micromonitor with a Dynon EMS 10. It's nearly, but not quite, a wire for wire replacement, and the RMI has worked happily and apparently accurately for 8 years with the thermocouples extended with copper wire. However the Dynon installation instructions are adamant that the thermocouples must be extended with thermocouple wire. The Wikipedia article referred to by the Moderator (this thread, July 2009) shows that the parasitic EMFs generated by the wire junctions will cancel if they are at the same temperature, as mine are. Is there some soundly based reason for excluding this option when connecting to the EMS 10?

Peter

 

[dynonsupport]

As long as both ends of your copper extensions are at the same temperature, you'll be fine. But any temp difference from one end to the other on the extension will cause an error.

We don't mention this in the manual because it's always easier and more accurate to use thermocouple wire as an extension, and it's not expensive. Your two ends are never at the same temperature, so there's always some error, and characterizing this can be hard. Are you sure the heater in the plane doesn't blow on one end more than the other? Does one end terminate right at the EMS, which generates it's own heat and can throw the readings off by 10 degrees?

Too many factors for the copper extension to be a common recommendation, but if you understand the issues it's fine to use.

 

[pbennett]

Um.... sort of but not quite.
Referring back to the Wikipedia article, there are three laws at play here.

The first law says there is no voltage difference between the wire ends due to temperature difference

The second law says changes of wire in a single lead will have no effect provided the wire stays at the same temperature along its length.

The third law says that if there is a change of temperature along its length, the thermal voltages will sum algebraically. This means that if the same joins are in the same place on both wires of the thermocouple, the unwanted voltages will cancel out.

So the theory says it will all work OK. My question is , are there any practical issues I don't know about that might upset the theory?

Peter

Quote from Wikipedia
Law of homogeneous material
A thermoelectric current cannot be sustained in a circuit of a single homogeneous material by the application of heat alone, regardless of how it might vary in cross section. In other words, temperature changes in the wiring between the input and output do not affect the output voltage, provided all wires are made of the same materials as the thermocouple.

Law of intermediate materials
The algebraic sum of the thermoelectric forces in a circuit composed of any number of dissimilar materials is zero if all of the junctions are at a uniform temperature. So If a third metal is inserted in either wire and if the two new junctions are at the same temperature, there will be no net voltage generated by the new metal.

Law of successive or intermediate temperatures
If two dissimilar homogeneous materials produce thermal emf1 when the junctions are at T1 and T2 and produce thermal emf2 when the junctions are at T2 and T3 , the emf generated when the junctions are at T1 and T3 will be emf1 + emf2 .

 

[dynonsupport]

The practical issue is that the EMS self heats, and you need to make sure that the two ends of the copper wire are really at the same temp as one another. So if the copper terminates into the EMS, the EMS is hotter than the cabin, and it doesn't work.

The whole point is that the EMS measures the temperature at the connector, where we expect the thermocouple to change to copper. This is the "cold junction." By moving the cold junction to somewhere else, by extending the wires from the back with copper out to thermocouple somewhere else in the cabin, the difference between the EMS connector temp and the place where the copper and the thermocouple come together will be an error in the measurement. So you cannot just run copper wire out of the the back of the EMS. You have to go thermocouple-copper-thermocouple, while making sure the two ends of the copper are the same temp.

 

[pbennett]

You have prompted me to read a lot more deeply to find out "why", rather than simply "what".

You are correct that where the thermocouple wire changes to copper, the temperature must be the same as the EMS terminals.
The chromel/copper plus copper/constantan couples, by the third law, provide the opposing emf to the chromel/alumel junction. For this reason they form the reference or cold junction.
The EMS will measure the temperature difference between the thermocouple junction and cold junction. However the EMS is getting its reference temperature from the EMS terminals, not the cold junction location. Provided the EMS is at the same temperature as the copper wire junctions, it will read accurately. Each degree of temperature difference between EMS and copper wire junctions will introduce one degree of error in the reading.

I now have eight thermocouple leads to rewire, but at least I understand why. Thanks.

Peter

 

[mmarien]

I have the EMS-D10 harness and I noticed that the ends are not attached. I suspect that this is so the thermocouple wires can be cut to length.

Do they have to all be equal length? or can they be trimmed to fit with cyl 1 a different leghth then cyl 2 or 3?

Murray

 

[dynonsupport]

You can cut thermocouples to any length you want, and they do not need to match one another.