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Temperature compensation, UMA CHT

 
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gilles.tatry(at)wanadoo.f
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PostPosted: Fri Feb 17, 2006 6:14 am    Post subject: Temperature compensation, UMA CHT Reply with quote

Hi Bob and all,

After examining carefully the TC probe sold with the UMA CHT, the situation
appears even worse than I expected. The probe has only 2 ft TC lead wires,
and then copper extension wires. So with my in-line engine, cylinder 2 the
hottest, the "cold" junction will be INSIDE the engine compartment, where
temperature can rise somewhat higher than the 75F the instrument is
calibrated for!

So, I bought TC connectors and extension wires from Omega to move the cold
junction close to the instrument.
Jean-Pierre Castiello, who designed the A320 brakes temp monitoring, is to
craft for me an external compensation box based on the same AD594. Input is
the J extension, output will directly drive the CHT air core movement.

I will let you know the result...

Thanks to all of you for the so precious informations I could gather from
the list!

Gilles


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nuckollsr(at)cox.net
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PostPosted: Sat Feb 18, 2006 2:49 pm    Post subject: Temperature compensation, UMA CHT Reply with quote

At 03:12 PM 2/17/2006 +0100, you wrote:

Quote:

<gilles.tatry(at)wanadoo.fr>

Hi Bob and all,

After examining carefully the TC probe sold with the UMA CHT, the situation
appears even worse than I expected. The probe has only 2 ft TC lead wires,
and then copper extension wires. So with my in-line engine, cylinder 2 the
hottest, the "cold" junction will be INSIDE the engine compartment, where
temperature can rise somewhat higher than the 75F the instrument is
calibrated for!

So, I bought TC connectors and extension wires from Omega to move the cold
junction close to the instrument.
Jean-Pierre Castiello, who designed the A320 brakes temp monitoring, is to
craft for me an external compensation box based on the same AD594. Input is
the J extension, output will directly drive the CHT air core movement.

I will let you know the result...

Thanks to all of you for the so precious informations I could gather from
the list!


Gilles. I've not forgotten our earlier conversation about a
temperature compensated driver for the UMA T/C driven gages.
You need to hook a resistor in series with a variable power
supply and tell me how much CURRENT through the instrument
and VOLTAGE across the instrument to get 0 to 100%
of full scale reading and at 3 to 5 points between 0 and 100%.

I understand these are powered instruments. I'll need to know
what resistance is measured between the signal input pins of
the instrument and power ground.

I've got a design for a thermocouple signal conditioner driving
a constant current generator that will provide the adjustability
you need for calibrating along with cold-junction compensation.

Bob . . .


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ainut(at)hiwaay.net
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PostPosted: Tue Feb 21, 2006 6:41 am    Post subject: Temperature compensation, UMA CHT Reply with quote

The AD594 already has all that built in. The output from the 594 is
0-5vdc. The 594 also helps linearize the output curve from the TC.

David M.

Robert L. Nuckolls, III wrote:

Quote:


At 03:12 PM 2/17/2006 +0100, you wrote:


>
><gilles.tatry(at)wanadoo.fr>
>
>Hi Bob and all,
>
>After examining carefully the TC probe sold with the UMA CHT, the situation
>appears even worse than I expected. The probe has only 2 ft TC lead wires,
>and then copper extension wires. So with my in-line engine, cylinder 2 the
>hottest, the "cold" junction will be INSIDE the engine compartment, where
>temperature can rise somewhat higher than the 75F the instrument is
>calibrated for!
>
>So, I bought TC connectors and extension wires from Omega to move the cold
>junction close to the instrument.
>Jean-Pierre Castiello, who designed the A320 brakes temp monitoring, is to
>craft for me an external compensation box based on the same AD594. Input is
>the J extension, output will directly drive the CHT air core movement.
>
>I will let you know the result...
>
>Thanks to all of you for the so precious informations I could gather from
>the list!



Gilles. I've not forgotten our earlier conversation about a
temperature compensated driver for the UMA T/C driven gages.
You need to hook a resistor in series with a variable power
supply and tell me how much CURRENT through the instrument
and VOLTAGE across the instrument to get 0 to 100%
of full scale reading and at 3 to 5 points between 0 and 100%.

I understand these are powered instruments. I'll need to know
what resistance is measured between the signal input pins of
the instrument and power ground.

I've got a design for a thermocouple signal conditioner driving
a constant current generator that will provide the adjustability
you need for calibrating along with cold-junction compensation.

Bob . . .


















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nuckollsr(at)cox.net
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PostPosted: Tue Feb 21, 2006 10:20 pm    Post subject: Temperature compensation, UMA CHT Reply with quote

At 08:40 AM 2/21/2006 -0600, you wrote:

Quote:


The AD594 already has all that built in. The output from the 594 is
0-5vdc. The 594 also helps linearize the output curve from the TC.

David M.

Yes, that's what we're discussing. He wants to craft a compensated
signal conditioner to remove some of the uncertainty of the
UMA gages that have no cold-junction compensation. I need to know
the base sensitivity numbers for his instruments before I can calculate
the output characteristics of the signal conditioner.

Bob . . .


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ainut(at)hiwaay.net
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PostPosted: Wed Feb 22, 2006 12:13 pm    Post subject: Temperature compensation, UMA CHT Reply with quote

Ok, maybe I'm being dense here, but here goes:
If the 594 is used, no further cold junction compensation is necessary
from the TC side is necessary. If the UMA instrument needs it, then I
suggest dropping it. Use a simple microprocessor (about $1) and an LED
display ($40?) instead. Some of the micros have an LED display driver
already built-in, IIRC. Check out Freescale's website.

If he's married to the UMA, then enjoy the exercises Cool.

David M.
Robert L. Nuckolls, III wrote:

Quote:


At 08:40 AM 2/21/2006 -0600, you wrote:


>
>
>The AD594 already has all that built in. The output from the 594 is
>0-5vdc. The 594 also helps linearize the output curve from the TC.
>
>David M.


Yes, that's what we're discussing. He wants to craft a compensated
signal conditioner to remove some of the uncertainty of the
UMA gages that have no cold-junction compensation. I need to know
the base sensitivity numbers for his instruments before I can calculate
the output characteristics of the signal conditioner.

Bob . . .


















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nuckollsr(at)cox.net
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PostPosted: Thu Feb 23, 2006 6:55 am    Post subject: Temperature compensation, UMA CHT Reply with quote

At 02:11 PM 2/22/2006 -0600, you wrote:

Quote:


Ok, maybe I'm being dense here, but here goes:
If the 594 is used, no further cold junction compensation is necessary
from the TC side is necessary. If the UMA instrument needs it, then I
suggest dropping it. Use a simple microprocessor (about $1) and an LED
display ($40?) instead. Some of the micros have an LED display driver
already built-in, IIRC. Check out Freescale's website.

I don't think the UMA bothers to use dynamic cold-junction
compensation. Their cold junction is at the back of the instrument
and the calibration is optimized at a 20C cockpit. So
ranges of temperatures that most pilots are willing to
endure in the cockpit is assumed to introduce insignificant
error.

Possibly true for most enclosed cockpit/pilot combinations.
This is an open cockpit a/c where the owner says his
motivations to fly outweigh other pilot's inhibitions
(maybe he has a heated flight-suit). In any case, the
stated accuracy of the stock gage is found deficient for
his needs.

The idea is to apply EXTERNAL signal conditioning using
the 594 and drive the instrument with whatever combination
of constant current/voltage seems best. This allows us
to provide offset/scale-factor pots that will permit
calibration to number probably better than the off-the-shelf
instrument. Dynamic cold-junction compensation comes with
the package.
Quote:
If he's married to the UMA, then enjoy the exercises Cool.

That's the major rub. He has the instruments, they're both
physically attractive for their size (tiny panel) and round
dials but a tad short on performance. Just ONE of life's
little challenges . . .

Bob . . .


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gilles.tatry(at)wanadoo.f
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PostPosted: Sun Feb 26, 2006 1:24 am    Post subject: Temperature compensation, UMA CHT Reply with quote

You are absolutely right, Bob!
Except that I am not concerned with cold, but hot weather flying, when an
open cockpitt is delightful...
It is the worst case for engine temp, and also the worst for CHT reading.
With the cold junction at the back of the instrument, CHT can read about 20
C less than reality. If the cold junction was in the engine compartment,
reading might be somewhere in the middle of the scale, or lower, instead of
close to, or even higher than the engine CHT limitation...
Too late indeed to change the instrument, wich size is very specific: the
panel is built and wired, I don't want to do it again.
Moreover, I am highly interested indeed in this intellectual challenge, and
learn a lot...
Thank you for the help!

Gilles

---


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nuckollsr(at)cox.net
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PostPosted: Sun Feb 26, 2006 11:36 am    Post subject: Temperature compensation, UMA CHT Reply with quote

At 10:13 AM 2/26/2006 +0100, you wrote:

Quote:

<gilles.tatry(at)wanadoo.fr>

You are absolutely right, Bob!
Except that I am not concerned with cold, but hot weather flying, when an
open cockpitt is delightful...
It is the worst case for engine temp, and also the worst for CHT reading.
With the cold junction at the back of the instrument, CHT can read about 20
C less than reality. If the cold junction was in the engine compartment,
reading might be somewhere in the middle of the scale, or lower, instead of
close to, or even higher than the engine CHT limitation...
Too late indeed to change the instrument, wich size is very specific: the
panel is built and wired, I don't want to do it again.
Moreover, I am highly interested indeed in this intellectual challenge, and
learn a lot...
Thank you for the help!

Have you the tools and where-with-all to get
the measurements we need?

Bob . . .


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gilles.tatry(at)wanadoo.f
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PostPosted: Sun Feb 26, 2006 12:26 pm    Post subject: Temperature compensation, UMA CHT Reply with quote

I will probably be able to send you the measurements shortly, thanks to
well-equipped friends.

---


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nuckollsr(at)cox.net
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PostPosted: Sun Feb 26, 2006 7:28 pm    Post subject: Temperature compensation, UMA CHT Reply with quote

At 09:24 PM 2/26/2006 +0100, you wrote:

Quote:

<gilles.tatry(at)wanadoo.fr>

I will probably be able to send you the measurements shortly, thanks to
well-equipped friends.

Great!

Bob . . .


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gilles.tatry(at)wanadoo.f
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PostPosted: Wed Mar 01, 2006 10:38 am    Post subject: Temperature compensation, UMA CHT Reply with quote

Hi Bob,

Please find hereafter Jean-Pierre's analysis of my CHT compensation issue.
He got the instrument, did some bench testing, and proposes an interface
architecture as follows.
Any comment?

Gilles
---


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