REFLECTOR: Blue Mountain CHT/EGT

[Bob Jackson]

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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