A rocket specific issue - re calibrating a new TIT probe

[aviatoreb]

This summer I was chasing an induction leak issue, which involved finding lots of small leaks and one bigger leak in a metal tube that we sent out for a weld-fix and also replacing some hoses.  

Anyway that's just backround.  Through the process of chasing this problem the TIT probe was displaced and then replaced and these are delicate things and well it broke.  So I got a new one.

Then induction leak fixed but then the TIT was reading higher.  So I suspected a calibration change. 

The rocket manual gives a criterion for calibrating your TIT.  It says go to 8000 to 1200 ft and run 33'' 2400rpm and set to 24gph (this is a very high setting above normal cruise settings), and then start leaning and look for the peak TIT value - and to do it quickly but no longer than 60 seconds since this is a gawd awful deal with the devil setting and it suggests the peak will be near 19gph.  Sure enough my peal is 19.1.  And I can do this seep in under 45 seconds and get useful readings.  The POH declares that this peak will be 1700 degrees and any deviation from that should is cause to recalibrate the TIT measurement device.

So it says if you get say 1650 at the "1700" engine setting then from then on you add 50 to your reading to get the true turbo inlet temp.  

My reading at the ugly setting of 33'' 2400, 19.1gph was 1755.  !  Boy that makes me cross eyed seeing that on my instrument.  But trusting the POH and sweeping it quickly - about 25 sec in the ultra high region, so it is.

Then now I interpret that according to the POH I should be subtracting 55 degrees from all of my normal range TIT readings.  I can even have my EDM830 do the subtraction for me so I will never have to see ugly high numbers.

Even though the POH tells me to do this - it is such an ugly thing to sweep high numbers under the rug I was hoping for some reassurance that this is legit.  Its in the POH... 

[carusoam]

Hmmmmm.....

1) That’s uncomfortable....

2) There is a limit to how high the EGT will actually go... flame temp of fuel and air is.... ?

3) We usually see a number lower Than actual Flame temps... because the exhaust isn’t a constant stream of flame...

4) It is six waves, every two revolutions of the prop... and the reading is an average over time...

5) And the TIT is pretty far away from the actual flame in most cases...

6) So... even while flying in flaming dragon mode, there is a limitation of how high TIT can actually get...

7) We typically calibrate K type thermocouples using ice water on the low end and boiling water on the high end...

8) 1700°F is pretty far out side the range of 32 to 212°F.... leaving quite a bit of an assumption....

9) We could get a K thermocouple chart to list temp vs. microvolts... if that would help?

10) How linear is the instrument’s interpretation of the data? does it have a look up table?

Hmmmmm... Again...

I think I have more questions Than I have Answers for...

 

We have a good EI engine monitor guy that can probably fill us in with an answer to this calibration question... at least how they handle it on their MVP units...

Lets throw the question over to @oregon87 This would give the EI technical guys an opportunity to answer a tough Mooney technical question...
 

Best regards,

-a-

[N201MKTurbo]

I once found the tips of my turbine wheel melted off. That sounds like a good way to do that.

[aviatoreb]

Thank you both.

The calibration step is what is recommended in the POH.  It offers this setting, 33'' 2400rpm, 19gph (there about but whatever that peak is), to be 1700.  And then to consider whatever you read from your device as offset.  So in principle, if I believe the POH then I got 1700 for about 20 seconds as instructed but I read 1755 which in principle is a fake reading.

Book says, I should subtract 55 from all TIT readings from now on, or even better, set this as the offset in my EDM830 which can be done. 

You two seem to find this as creepy as I do.

I am hoping someone who is specifically experienced on this can speak up that this is exactly right, trust my POH and do what it says, or say, no that is specifically wrong.

Edited September 1, 2020 by aviatoreb

[N201MKTurbo]

Thermocouples are rarely off by more than a few degrees. What does vary is the placement of the junction to the fire. I believe there is a huge temperature gradient inside the exhaust pipe, both radially and axially.

[aviatoreb]

1 hour ago, N201MKTurbo said:

Thermocouples are rarely off by more than a few degrees. What does vary is the placement of the junction to the fire. I believe there is a huge temperature gradient inside the exhaust pipe, both radially and axially.

You are surely right on a newly installed sensor.  Differences could likely be due to placement.

Still - I am looking for affirmation here - POH says defining 33'' 2400 19gph (peak) as 1700 and that the sensor should be calibrated to that.  But I am worried since the example it gives is an under measurement shift rather than an over measurement shift as I observed.

So I am looking for affirmation, or otherwise caution, from folks of what to do here.  Recalibrate my readout as the POH says, or reconsider.

[aviatoreb]

10 hours ago, carusoam said:

 

Lets throw the question over to @oregon87 This would give the EI technical guys an opportunity to answer a tough Mooney technical question...

 

 

Hey Anthony - how do I do what you just did - tag people?  I want to tag some of the rocket posters here for their specific experience if they have done the calibration step and their thoughts on the veracity of doing this calibration step.

Whether calibrating up or calibrating down - the values we read from our digital displays are not necessarily the actual temps as we well know - and wouldn't it be great if we could compare it to a universal standard like freezing ice 0C.  So the rocket poh is the only one of the Mooney turbos that I am aware of that call out a calibration standard.  And yet I am anxious about if it should be trusted.  In the end - we should all be anxious of what we read from our TIT if it is reading too high or too low relative to truth.

[N231BN]

Considering the fact that you are using different equipment to measure TIT than Rocket Engineering did when they developed the STC, I would leave it alone.

Will this new higher reading affect the way you normally run your aircraft?

[Andy95W]

2 hours ago, aviatoreb said:

 

Still - I am looking for affirmation here - 

 

Okay, Erik- here ya go.  You did ask for it...

 

[aviatoreb]

48 minutes ago, Andy95W said:

Okay, Erik- here ya go.  You did ask for it...

 

:-)

I have many times referred to that dude.  Such a great skit.

And who knew Stuart Smalley would become a US Senator?  And oye - the story after that.

[aviatoreb]

2 hours ago, N231BN said:

Considering the fact that you are using different equipment to measure TIT than Rocket Engineering did when they developed the STC, I would leave it alone.

Will this new higher reading affect the way you normally run your aircraft?

Well - yes.  I figure 1560 is a perfectly good setting but if I am seeing 1610 I would say no I don't want to cruise at 1615, so I would enriched a lot until I see something lower.  But that would have me running maybe 1gph greater than I have been at all ROP settings, say even book settings.  I don't want to be running over rich - I just want to enrichen just right. 

Or if I were to use the "lean find mode" every time to enrichen instead of reading book settings, that would put me say what if I am targeting 125 rich of peak (on the EGT right?) but it would sweep me past a reading on my TIT that would now be above redline (continuous - is 1650 and absolute for less than 60 seconds is 1700), and that just hurts to see. And anyway I don't like the idea of lean find to an rop setting each time.

When rocket developed the STC to came with an EDM700 system.  When I upgraded that to the EDM830, it was using those same original probes.  SO I always presumed the readings were the same, but now just shown to me on a larger screen with color too.  So in principle this is the same system but with a color display and a refreshed probe - although the mechanic did tell me today that the probe ended up going much deeper into the hole than the original he pulled out - I guess since it shortened over time?  So it is deeper into the core of that hot flow.

[N231BN]

I didn't realize they had a 700 as part of the mod, that is good.

Was the calibration performed with the old probe when the 830 was installed? It is known that EGT/TIT probes will give lower readings as they erode over time.

Perhaps you have been running your turbo at 1600+ for a while now, that isn't necessarily a bad thing.

[Marcopolo]

Hi Erik,

 Does the TIT line up with the other probes at ambient before engine start?  If so then I would say its good at whatever reading you get.  Sure, there may be some non linear difference as temperatures go up, but I would not suspect a difference of 55 degrees.  I would go with the TIT readings you are getting with this probe before I would expect the calibration routine in the POH to give me a number to trust as there are just too many variables to think that every Rocket engine would be at that temp at those settings.  Just my opinion!

 

Ron

[aviatoreb]

21 minutes ago, N231BN said:

I didn't realize they had a 700 as part of the mod, that is good.

Was the calibration performed with the old probe when the 830 was installed? It is known that EGT/TIT probes will give lower readings as they erode over time.

Perhaps you have been running your turbo at 1600+ for a while now, that isn't necessarily a bad thing.

..ok I know nothing.  

All of the rockets I thought included the 700.  Mine had a 700 installed when the rocket stc was installed and I assumed it was a package.  The original 231 only came with a cdt which is still there.

I think the recalibration was done at the time of the 830 installation but that was like 5 or 6 years ago, so certainly they could have drifted and anyway ....

You are quite right - my TIT may well have been degrading over time giving me a false sense of cool.  But besides degrading the new probe is longer the mechanic said by about 1/4'' so the material may have been eroding away and so the new one is sticking deeper into that hot flow perhaps more toward the hotter center.

Whatever I am reading I do want it to be relative to truth.  So be it if I have been running 1600 all along and just didn't know it?

Nonetheless rocket engineering does call out a set point in the poh - the define 33'' 2400 and peak (19gph) as 1700 and I should take the difference from what I read on my instrument and 1700 as a defined offset.  If the is the case, i would offset my read out by 55F.

Where's the rest of the rocket-team on here.  Hey rocket dudes - please pipe up!

Actually I would think this is generally a turbo issue we would all be interested in - which is instrument calibration.  Anyone here from NIST?

Edited September 1, 2020 by aviatoreb

[dzeleski]

Thermocouples do wear over time and can produce lower voltage as time goes by. Lower voltage == Lower temp. So depending on how long that old one was in service this could be expected.

[aviatoreb]

1 hour ago, Marcopolo said:

Hi Erik,

 Does the TIT line up with the other probes at ambient before engine start?  If so then I would say its good at whatever reading you get.  Sure, there may be some non linear difference as temperatures go up, but I would not suspect a difference of 55 degrees.  I would go with the TIT readings you are getting with this probe before I would expect the calibration routine in the POH to give me a number to trust as there are just too many variables to think that every Rocket engine would be at that temp at those settings.  Just my opinion!

 

Ron

By line up do you mean does it read the same value?  I don't know - I will need to check.

Even if they are reading the same at room temp, I don't know if that means the sensors would vary at the same rate or not.  Even if not a nonlinear effect, they could simply have a different linear ramp between them so that by the time a EGT reads 1400 say the TIT might be reading 1600 say.  Even if they both read 75F before start up.

So you are asserting that the rocket POH thing is junk?

I am undecided.  I agree it might be junk.  But I am wondering - it might be brilliant.

Do any other engines in any other airplanes have a similar calibration procedure?

E

[carusoam]

12 hours ago, aviatoreb said:

Hey Anthony - how do I do what you just did - tag people?  I want to tag some of the rocket posters here for their specific experience if they have done the calibration step and their thoughts on the veracity of doing this calibration step.

Whether calibrating up or calibrating down - the values we read from our digital displays are not necessarily the actual temps as we well know - and wouldn't it be great if we could compare it to a universal standard like freezing ice 0C.  So the rocket poh is the only one of the Mooney turbos that I am aware of that call out a calibration standard.  And yet I am anxious about if it should be trusted.  In the end - we should all be anxious of what we read from our TIT if it is reading too high or too low relative to truth.

Use the @ then spell their name @aviatoreb and watch, as you are spelling the list that gets auto generated... don’t forget to click on the right one...  

It is an imperfect system, but it works...
 

 

The eerie-ness comes from how sensitive the turbine blades are to the actual temperature...

a few degrees too hot, the blades soften and stretch with the centrifugal force Of tens of thousands of rpm... grinding the tips against the case... or lopping them clean off...

Running too hot over a long period of time and erosion is a big challenge... Leaving nubs in place of blades...

What we know as a safe temperature... is the actual temperature reading before it gets to the blades...

The blades are at an unknown temperature... because we can’t easily measure them in flight...

So... we use the exact known TC, mounted in the exact known location, to get the exact reproducible results that our POH/STC gives us...   Because this safe TIT was tested and well defined... we can only duplicate the same procedure to get the same result....

Make sure you are using Rocket Engineering procedures for the calibration... or at least make sure the Mooney procedure applies to the RE engine...

We can also invite the Knowledgable @M20Doc and ask him how this gets done in the field... (TIT calibration)
 

When it comes to knowing engine temps in M20Ks @kortopates Paul is a natural guy to ask this question to...

PP thoughts only, not a mechanic...

Best regards,

-a-

[Guest]

I’m not sure if they’re still available, but  Alcoa used to make a tool for calibrating their probes.  It was an electric heater that you placed the probe in while connected to the gauge, then you turned it up the various temperatures to confirm the accuracy.

I believe that Alcoa probes are made by Tempest now.

Clarence

[aviatoreb]

24 minutes ago, M20Doc said:

I’m not sure if they’re still available, but  Alcoa used to make a tool for calibrating their probes.  It was an electric heater that you placed the probe in while connected to the gauge, then you turned it up the various temperatures to confirm the accuracy.

I believe that Alcoa probes are made by Tempest now.

Clarence

Thanks Clarence,

What is your opinion though of going with that rocket POH calibration concept?

If when the probe itself is calibrated, the temp it senses will be surely dependent on the exact placement in the hot gases so the reading of even a calibrated probe would vary with placement.  Is that right?  So there is some sense with that in flight calibration that my POH offers, if the test itself is valid.

[N231BN]

I would recommend following the attached procedure and see if there is already a correction factor that is skewing your result.

Edited September 2, 2020 by N231BN

[Marcopolo]

15 hours ago, aviatoreb said:

So you are asserting that the rocket POH thing is junk?

I am undecided.  I agree it might be junk.  But I am wondering - it might be brilliant.

  No sir, not junk at all.  I just think it has variables that it doesn't mention that would be relative to the expected TIT temperature.  I think for the time that it was written it was very relevant given that every STC build they did was probably extremely similar when the customer picked them up, but as the engines are used, cylinders changed, turbos overhauled, engines overhauled, exhaust systems rebuilt, and probes replaced, things (variables) change.  I would much rather see a test with a known temperature as Doc has mentioned then you setting the offset based on an expected temperature.  

 

  Individual EGTs  will normally read between 150 and 250 degrees lower than the TIT even though the TIT is further downstream in most cases, this is a difference in actual temperature, not a difference in delta from ambient.  Now this also has variables to consider as EGT probes get installed in a variety of positions relative to the exhaust stream they are responding to which is why we don't treat EGTs as absolute temps.   

 

  Again, I'm not saying the POH calibration is junk, I am only saying that I would hope there is a better way and if not then I personally, as I do on my 231, would trust the probe at operating temperature if it lines up at ambient, also taking into account a smooth curve as all temperatures increase.  The EGTs and TIT should rise at the same time but the TIT should lead the way from start up.

Ron

  

[Guest]

4 hours ago, aviatoreb said:

Thanks Clarence,

What is your opinion though of going with that rocket POH calibration concept?

If when the probe itself is calibrated, the temp it senses will be surely dependent on the exact placement in the hot gases so the reading of even a calibrated probe would vary with placement.  Is that right?  So there is some sense with that in flight calibration that my POH offers, if the test itself is valid.

In the absence of another calibration method it’s what is approved as part of the STC installation.  It seems crude to me to set the engine at a specific power setting and then tell the gauge this is the temperature you should read/display.

Clarence

[aviatoreb]

Thank you all.

So based on the above - I checked this am - the TIT does read essentially the same room temperature as the EGT and CHT probes when the avionics is turned on but the engine is off. Abut 70F +/- maybe 2F.  In flight the other day while I was complaining about high TIT on EDM830 - the factory probe with the needle was showing I think 1550.  Maybe it is worn out?

I am not going to offset anything for now.  I need to get me to an avionics shop.

I am comfortable that we treat EGTs as relative numbers, relative to each other and relative to their values as they increase and decrease together (searching for peak, etc), but it is annoying that we need to interpret TIT as absolute numbers to protect the turbo - and the exhaust system, so it is important that we do have accurate measurements at very high temp values indeed.

E

 

Edited September 2, 2020 by aviatoreb

[oregon87]

I can't speak intelligently as to how to calibrate the TIT function in question, however I can offer that we use type K thermocouple and our systems are calibrated at 23uV/°F (microvolts/ degree F).  Additionally, type K is mostly linear and works well due to its useful range.  The area where type K is not so linear is well outside of normal CHT or EGT/TIT ranges, so it is really irrelevant.    

[carusoam]

Great additional details Oregon!

Thanks for chiming in.

Best regards,

-a-