VansAirForceForums  
Home > VansAirForceForums

- POSTING RULES
- Donate yearly (please).
- Advertise in here!

- Today's Posts | Insert Pics

  #11  
Old 05-27-2020, 05:38 PM
Greg Arehart's Avatar
Greg Arehart Greg Arehart is offline
 
Join Date: Jul 2007
Location: Delta, CO/Atlin, BC
Posts: 2,369
Default

I have 1000 hours on my engine with Dynon probes and D120. I replaced one EGT maybe 100 hours ago and am starting to see flashes of "shock cooling" on one CHT that probably indicates it is about to go bad. I'm happy with the longevity. For reference, my EGT probes are about 4" down on the exhaust pipe.
__________________
Greg Arehart
RV-9B (Big tires) Tipup @AJZ or CYSQ
N 7965A
Reply With Quote
  #12  
Old 05-28-2020, 05:17 PM
plehrke's Avatar
plehrke plehrke is offline
 
Join Date: Sep 2006
Location: Defiance, MO
Posts: 1,619
Default

JPI probes still functioning at over 900 hours with them 2” from flange to have them all equal distance. That gives me peak EGTs of 1475 to 1550 deg F, which is were I run 90% of the time (I know I am old school not running LOP). Talked to JPI at OSH one year and asked about the high EGTs and he said only issue is probes may go bad early than normal. 900+ hours and doing fine.
__________________
Philip
RV-6A - 13 years over 900 hours
Based at 1H0 (Creve Coeur)
Paid dues yearly since 2007
Reply With Quote
  #13  
Old 06-03-2020, 04:51 AM
Rainier Lamers Rainier Lamers is offline
 
Join Date: May 2007
Location: Somerset West
Posts: 1,027
Default

We have been producing a variety of EGT probes over the last 20 years or so. At a guess, probably well over 200.000 so far.

On some installations the probes last "forever". on others not - even if similar installations.
There are a whole host of possible failure causes.

Yes, quality plays a role - up to a point. In principle you look for the quality of the thermocouple cable - the materials are dictated as far as the cores go. Electrical insulation - some probes I have seen use fiberglass, others a mineral fiber. The mineral fiber withstands higher temps - but is brittle and damages easily. The fiberglass can work - but only if the probes construction inside does not rely on it (probes can use ceramic sleeves inside which also makes them much stronger at high temps).
Typically 316 stainless is used - that's perhaps the best overall material. Wall thickness is again a compromise between strength and speed of reaction. Typically 0.4 to 0.5mm is the norm.

Most failures we have investigated are down to the cable itself - mechanical damage usually due to the cable flexing with engine movement. Just like bending a piece of fence wire repeatedly - the internal cores do not like that and break. Can also affect the insulation - that breaks and the cores short out or to the usual metal sheath of the cable. Most often the break is around the entry point to the probe itself (usually a spring is used to assist this area).

Then there are other issues. Fortunately more rare: Complete probe disintegration. When we get one of those - first thing we do is see if a magnet sticks to the probe. If it does - you have rust in the fuel. Rust particles instantaneously melt in the combustion flame (in particular with turbo changed engines), loosing the oxygen in the process so you have tiny liquid blobs of iron exiting the combustion chamber - usually by the time they get to the EGT probe they are ready to melt onto the surface. Layers of this get added over time (the probe diameter gets bigger). Trouble with this is iron has a very different thermal expansion compared to 316 stainless. The iron actually rips the probe to pieces over time. I have seen this a few times over the years - the probes totally disintegrate or if caught early enough you see them bent (sometimes at crazy angles) - this due to the iron coating not being even on all sides.
The "magnet check" shows quickly what you are dealing with - also on any of the iron you can scrape off the probe.
So that is one to watch out for - it will destroy any probe - no matter how much you paid for it.

Probes typically can go up to 1200 degrees C for short times - at this temperature they glow white. Very high flow velocities can conceivably bend the probes as they will be weak. I have not seen this happening with our probes though (but it's not impossible).
At normal combustion temperatures (non turbo, low revving engines) the probes are pretty much bullet proof - they barely even glow a dark red.
High revving engines (like Rotax's 912 range) can have much higher temps (the probes glow a nice satisfying red at up to 850 degrees C redline). Above that it gets orange and eventually white hot.

If you have a probe fail - investigate why it failed - first look at the cable very carefully (in particular where it enters the probe). If that all looks good - take a dremel with a cutting disk and cut the probe open lengthwise - the thermocouple itself should be close to the tip. Anything interesting to see there ? Of course if the probe is mechanically damaged then look elsewhere.

Rainier
CEO MGL Avionics
Reply With Quote
  #14  
Old 06-03-2020, 06:18 AM
Carlos151's Avatar
Carlos151 Carlos151 is offline
 
Join Date: Jan 2008
Location: Lebanon, TN
Posts: 226
Default

Quote:
Originally Posted by Rainier Lamers View Post
We have been producing a variety of EGT probes over the last 20 years or so. At a guess, probably well over 200.000 so far.

On some installations the probes last "forever". on others not - even if similar installations.
There are a whole host of possible failure causes.

Yes, quality plays a role - up to a point. In principle you look for the quality of the thermocouple cable - the materials are dictated as far as the cores go. Electrical insulation - some probes I have seen use fiberglass, others a mineral fiber. The mineral fiber withstands higher temps - but is brittle and damages easily. The fiberglass can work - but only if the probes construction inside does not rely on it (probes can use ceramic sleeves inside which also makes them much stronger at high temps).
Typically 316 stainless is used - that's perhaps the best overall material. Wall thickness is again a compromise between strength and speed of reaction. Typically 0.4 to 0.5mm is the norm.

Most failures we have investigated are down to the cable itself - mechanical damage usually due to the cable flexing with engine movement. Just like bending a piece of fence wire repeatedly - the internal cores do not like that and break. Can also affect the insulation - that breaks and the cores short out or to the usual metal sheath of the cable. Most often the break is around the entry point to the probe itself (usually a spring is used to assist this area).

Then there are other issues. Fortunately more rare: Complete probe disintegration. When we get one of those - first thing we do is see if a magnet sticks to the probe. If it does - you have rust in the fuel. Rust particles instantaneously melt in the combustion flame (in particular with turbo changed engines), loosing the oxygen in the process so you have tiny liquid blobs of iron exiting the combustion chamber - usually by the time they get to the EGT probe they are ready to melt onto the surface. Layers of this get added over time (the probe diameter gets bigger). Trouble with this is iron has a very different thermal expansion compared to 316 stainless. The iron actually rips the probe to pieces over time. I have seen this a few times over the years - the probes totally disintegrate or if caught early enough you see them bent (sometimes at crazy angles) - this due to the iron coating not being even on all sides.
The "magnet check" shows quickly what you are dealing with - also on any of the iron you can scrape off the probe.
So that is one to watch out for - it will destroy any probe - no matter how much you paid for it.

Probes typically can go up to 1200 degrees C for short times - at this temperature they glow white. Very high flow velocities can conceivably bend the probes as they will be weak. I have not seen this happening with our probes though (but it's not impossible).
At normal combustion temperatures (non turbo, low revving engines) the probes are pretty much bullet proof - they barely even glow a dark red.
High revving engines (like Rotax's 912 range) can have much higher temps (the probes glow a nice satisfying red at up to 850 degrees C redline). Above that it gets orange and eventually white hot.

If you have a probe fail - investigate why it failed - first look at the cable very carefully (in particular where it enters the probe). If that all looks good - take a dremel with a cutting disk and cut the probe open lengthwise - the thermocouple itself should be close to the tip. Anything interesting to see there ? Of course if the probe is mechanically damaged then look elsewhere.

Rainier
CEO MGL Avionics
Rainier, thanks for the detailed analysis you provided.
The probe in question is still providing intermittent false indications that last about 7 seconds after starting up so I'm not in a hurry to inspect or replace it anytime soon. I will however remove it and inspect it as you suggested along with all of the wiring to that probe. I have a new probe in hand to replace the questionable one and I strongly suspect that it is a wiring issue from the probe. I'll be inspecting the spring area at the entry point closely. These wires are the most likely to catch on the cowling Skybolt fasteners when installing or removing for access so it's possible that may be the cause. Now I'm curious to cut the probe open. Will report back if I find anything.

Once again, thanks for the detailed insight into the life of an EGT probe.
__________________
Karl Richcreek
RV-8
N151TK "La Otra Chica"
Flying as of August 14, 2015
TN26
Reply With Quote
  #15  
Old 06-03-2020, 08:53 PM
RV10inOz's Avatar
RV10inOz RV10inOz is offline
 
Join Date: May 2008
Location: Brisbane Qld. Aust.
Posts: 2,247
Default

I have six Dynon supplied probes. One failed at about 1100 hours the rest are still going fine at 1800.

Probes can be damaged by being pushed into holes with insufficient clearance or at a funny angle. They fail some time later, and nobody ever knows why. I suspect they fail from mechanical damage at installation. Just a hunch.
__________________
______________________________

David Brown

DYNON Authorised Dealer and Installer


The two best investments you can make, by any financial test, an EMS and APS!
Reply With Quote
  #16  
Old 06-03-2020, 09:16 PM
Carlos151's Avatar
Carlos151 Carlos151 is offline
 
Join Date: Jan 2008
Location: Lebanon, TN
Posts: 226
Default

Quote:
Originally Posted by RV10inOz View Post
I have six Dynon supplied probes. One failed at about 1100 hours the rest are still going fine at 1800.

Probes can be damaged by being pushed into holes with insufficient clearance or at a funny angle. They fail some time later, and nobody ever knows why. I suspect they fail from mechanical damage at installation. Just a hunch.
Thanks David. When you say “sometime later” what timeframe does that fit?
I installed all four probes using the same Dynon approved and published instructions 375+ hours ago. Now, one probe is acting up, so when you say “sometime later”, is that 300 hours, 3000 hours, or 30,000 hours? With that kind of statement, IMHO, you’re pretty much saying the probe can fail at any time. Well, that’s pretty much true of any man made mechanical or electrical item.
__________________
Karl Richcreek
RV-8
N151TK "La Otra Chica"
Flying as of August 14, 2015
TN26
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 04:15 AM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.