REFLECTOR: Blue Mountain CHT/EGT

[Tom Martino]

Damn!  I was just composing a reply for Brett when your excellent
response showed up in my email box.  I could not have said it better.
 
Yooo-da-man.
 
-----Original Message-----
From: Bob Jackson [mailto:bobj at computer.org] 
Sent: Saturday, February 12, 2005 10:30 PM
To: Velocity Aircraft Owners and Builders list
Cc: Bob Jackson (Home)
Subject: RE: REFLECTOR: Blue Mountain CHT/EGT
 
Tom,
 
The direct answers are:
 
a)  Resistive CHT Probes.  The Aerosance (thermistor) resistive probes
can be run directly into Blue Mtn. EFIS One channels 18-25 using regular
copper wire, provided you configure these channels with the circuit
board jumpers in the 500-5000 ohm resistive range positions.  I think
you will have to add the jumpers because I believe the default Blue Mtn.
shipping configuration is for these channels as voltage inputs (no
jumpers installed).   Blue Mtn. provides a built-in look up table for
the Aerosance CHT senders when used this way, so you can then just load
this table in order to display the CHT temperatures properly scaled on
one of the EFIS engine panel instruments.  The temperature range that
you will be able to display will be about:  232 to 572 deg F.
 
b)  EGT Thermocouple Probes.  It is best to use thermocouple extension
wire (that use the same wire as the thermocouple itself) for the run
from the EGT thermocouples all the way to the EFIS input connectors.
There are also built-in look up tables provided for both Type J and Type
K thermocouples on the EFIS sensor calibration/set-up screen.
 
The 'geek' answers:
 
1)  Resistive CHT Probes.  If for some reason you think you need it, you
can achieve greater 'dynamic range' with the same Aerosance thermistor
CHT probes if you configure the EFIS 18-25 A/D channels for 0-1.67 volt
input, rather than as resistive channels as described above.  By using a
simple resistor divider network and by entering your own custom A/D
calibration look up table for those channels, it is possible to achieve
a displayed CHT temperature range of something like:  41 to 500 deg F.
If you're really interested in this approach, I can provide you with
some resistor values and the associated custom look up tables.  And of
course if you're really into accuracy (or need it for something you're
doing), you can calibrate out various errors along the sensor chain with
your own custom look up tables and the EFIS' ability to accept them.
 
2)  EGT Thermocouple Probes.  Since most people are not looking for
absolute accuracy with their EGT temperatures, but only indications for
the mixture settings where EGT peaks for each cylinder', it is also
possible to skip the 'official' thermocouple extension wire and just use
regular copper wire (preferably twisted and shielded, since these are
low level signals).  However, if you do this it is best to make the
transition from the thermocouple wire to the copper wire on the cabin
side of the firewall so that the temperature at this junction is
approximately the same as that at (or inside) the EFIS box.  
    For those who might need better absolute thermocouple temperature
accuracy (like for turbocharger TIT temperatures, for example), it's
probably best to run official thermocouple extension wire all the way up
to the EFIS connector to avoid cold junction temperature issues.  The
EFIS provides 'cold junction compensation' in it's input circuitry for
thermocouple sensors.  However, I'm not sure how accurately it
compensates for the difference between the temperature at the Analog 1
EFIS connector, and the temperature where the cold junction compensation
is accomplished (at the integrated circuit inside the EFIS, which I
think would be quite a bit hotter).  Using thermocouple extension wire
all the way to the EFIS connector, and the built-in EFIS look up tables
for Type K thermocouples, and given any inaccuracies of the EFIS' low
level A/D input channels, my guess is that you will still have a 100 deg
F error or larger (and/or difference between the readings of the various
EFIS thermocouple channels).  
     This is another area where the EFIS' ability to accept custom look
up tables might be used to improve accuracy or take out errors.  It
might also be used to give you better accuracy even though you use
copper wire on the run from just inside the firewall up to the EFIS box.
Unfortunately, the best way to improve accuracy along the entire sensor
chain using a custom look up tables is to create them 'the old fashioned
way' -- with a torch, a good digital multimeter, and a whole lot of
patience!  There are few situations that will justify this approach.
 
As Greg Richter would say -- 'Hope this helps!'
Good luck -- the Blue Mountain EFIS is incredibly capable, but sometimes
with power comes complexity!
Bob Jackson
	-----Original Message-----
	From: reflector-bounces at tvbf.org
[mailto:reflector-bounces at tvbf.org]On Behalf Of Brett Ferrell
	Sent: Saturday, February 12, 2005 6:01 PM
	To: reflector at tvbf.org
	Subject: REFLECTOR: Blue Mountain CHT/EGT
	Tom & other BMA folks,
	 
	I'm getting ready to hook up my engine probes, and I've been
reading the discussion group and then manual, and I think I've got this
sorted out but wanted to check.  The CHT probes are 'resistive'
Aerosance units from BMA, and I think I can run these straight to the
E/1 with standard aircraft wire.  However, I think the EGT's should be
run with thermocouple wire, but I wasn't sure if you could just take the
thermocouple wire and crimp it directly into the E/1's analog connector
or not??  Thanks for any input.
	
	Brett
	www.velocityxl.com
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