Today's Message Index:
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1. 10:16 AM - Re: was "Contactor Questions" now battery in the trunk (Robert L. Nuckolls, III)
2. 10:46 AM - Thermocouples (Finn Lassen)
3. 03:34 PM - Re: Try These Ideas to Help with Accounting Homework (OttilieMccall)
4. 06:18 PM - Re: Thermocouples (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: was "Contactor Questions" now battery in the |
trunk
At 11:57 PM 5/19/2021, you wrote:
>This is kinda related to contactors...
>
>On the BD4 that I am rebuilding, the battery is
>behind the baggage compartment, with a really
>fat wire running up to the firewall.=C2
>
>IO 470 engine so lots of amps required for start.
>
>Is there a recommended contactor
>placement/battery disconnect=C2 protocol out there?
>I'm thinking safety in the event of an incident
>which might short out the fat wire to the fuselage.
Battery contactor located as close as practical
to battery; starter contactor as close as practical
to starter. FAT wire connects battery contactor
to starter contactor and main bus feeder.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
Message 2
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Hi Bob,
I re-read your chapter on thermocouples.
The question I have is: what voltages are introduced at
copper/thermocouple wire junctions?
You show them in an ice bath. Does that mean that no voltages are
generated at 0 degC at copper/chromel and copper/alumel junctions?
I made a probe using K-type wire in a 5/32" brass tube, squeezed shut
over the twisted wires and then silver-soldered to make it water tight.
When I connected it to my engine monitor it read as much as 220 deg F in
boiling water. In other words 8F too high.
Now I'm wondering if it's contamination of the joint by brass or silver
solder, or something in the engine monitor.
The Data Acquisition Module for the engine monitor have 16 (x2) inputs
going into MAX4558 8-to-1 multiplexers and from there to a MAX6675
(Type-K to 12 bit digital with cold-junction compensation). They are on
the same PCB, so in theory the temperature should be the same at the
screw connectors and the MAX6675 pins. Terminal block (ED555/DS) is CuZn
with wireguard (the metal under the screw pressing down on wire) is CuSn.
An issue is that the Data Acquisition Module is mounted where it gets
hot (seen as much as 160F). The MAX6675's compensation should be good to
185F, but wondering about the other junctions on the PCB. Since each
thermocouple wire has same path (terminal, PCB cu traces, solder,
multiplexers) any junction voltages should balance out, or?
I have been using LM35 (or similar) temp sensors but they appear to be
failing.
Finn
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Message 3
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| Subject: | Re: Try These Ideas to Help with Accounting Homework |
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Message 4
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| Subject: | Re: Thermocouples |
At 12:45 PM 5/23/2021, you wrote:
>
>Hi Bob,
>
>I re-read your chapter on thermocouples.
>
>The question I have is: what voltages are
>introduced at copper/thermocouple wire junctions?
I don't have specific numbers but they
are known. ANY two DISSIMILAR metals
will demonstrate the Seebeck effect.
>You show them in an ice bath. Does that mean
>that no voltages are generated at 0 degC
>at=C2 copper/chromel and copper/alumel junctions?
No, it only means that the voltages generated
are known and STABLE due to the inherent stability
of an ice bath.
>I made a probe using K-type wire in a 5/32"
>brass tube, squeezed shut over the twisted wires
>and then silver-soldered to make it water tight.
The ideal way to bring two thermocouple wires
together is by welding. We used to have a t/c
welder that was nothing more than a 50vdc
power supply fitted with a high resistance
inductor to help stabilize the arc. I.e.
a miniature, low power arc welder.
Before we got the welder the techs would
silver solder the junctions . . . I don't
know if they had done any validation of
the technique.
>When I connected it to my engine monitor it read
>as much as 220 deg F in boiling water. In other words 8F too high.
Do you have other probes that are closer
to 212?
>Now I'm wondering if it's contamination of the
>joint by brass or silver solder, or something in the engine monitor.
You've got several variables there. Try
building up some 'test' junctions with
your stock wire. Make 2 or 3 junctions
simply twisted and compare with your cited
results. Then silver solder; repeat. Finally,
add your housing and check for consistency.
>The Data Acquisition Module for the engine
>monitor have 16 (x2) inputs going into MAX4558
>8-to-1 multiplexers and from there to a MAX6675
>(Type-K to 12 bit digital with cold-junction
>compensation). They are on the same PCB, so in
>theory the temperature should be the same at the
>screw connectors and the MAX6675 pins. Terminal
>block (ED555/DS) is CuZn with wireguard (the
>metal under the screw pressing down on wire) is CuSn.
The nature of the various junctions in
each t/c loop is not critical as long as
they occur in thermally matched pairs.
>An issue is that the Data Acquisition Module is
>mounted where it gets hot (seen as much as
>160F). The MAX6675's compensation should be good
>to 185F, but wondering about the other junctions
>on the PCB. Since each thermocouple wire has
>same path (terminal, PCB cu traces, solder,
>multiplexers) any junction voltages should balance out, or?
If the DAS designer built the signal
conditioning with T/C requirements in mind,
you should be okay . . . but it's worth
confirming. Sort out the probe issues first
then perhaps test the same probes with
the DAS multiplexer in an elevated temperature
environment. (cardboard box with 100W bulb
makes good po' boy's environmental oven).
>I have been using LM35 (or similar) temp sensors
>but they appear to be failing.
What's the highest temperatures of interest
to be explored? If this is a 'scratch built'
DAS system, have you considered RTDs? Much
larger delta-volts/degree and no parasitic
junction issues.
A 100 ohm RTD changes about 0.38ohms/C.
Bias the critter up at 1mA gives you about 380uV
per C. That's about 10x bigger signal than
a K thermocouple. You can run 2 wires for short
runs . . . but if you're really picky, a 3-wire
connection will let you do a direct copper drop
read on the lead wires or a 4-wire for
separate excitation/sense paths.
A down side of little RTDs is that they're
a bit tricky to mount. I usually tack them
onto the end of a shielded trio for a 4-wire
connection then dip the assembly in JB
Kwik for a rugged encapsulation.
Perhaps the simplest thing to do first is to
identify root cause for errors in your
thermocouples.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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